The Solitary Forager Theory of Autism
Reser, J. 2011. Conceptualizing the autism spectrum in terms of natural
selection & natural history: The solitary forager theory. Evolutionary Psychology.
9(2): 207- 238.
Jared Edward Reser
Brain and Cognitive Science Department
University of Southern California
jared@jaredreser.com
Copyright 2006
Abstract
This article reviews etiological and comparative evidence supporting the hypothesis that some
heritable genes associated with the autism spectrum were naturally selected and represent the
adaptive benefits of being cognitively suited for solitary foraging. The systemizing theory of autism is
extended here and people on the autism spectrum are conceptualized as ecologically competent
individuals that could have been adept at learning and implementing hunting and gathering skills in
the ancestral environment. Upon independence from their mothers, young autistic individuals may
have been psychologically predisposed toward a different life-history strategy, common among
mammals and even some primates, to hunt and gather primarily on their own. Many of the behavioral
and cognitive tendencies that autistic individuals exhibit are viewed here as adaptations that would
have complemented a solitary lifestyle. For example, the obsessive, repetitive and systemizing
tendencies in autism, which can be mistakenly applied toward activities such as block stacking today,
may have been focused by hunger and thirst toward successful food procurement in the ancestral
past. Individuals on the autism spectrum share a variety of behavioral traits with solitary species. Both
solitary mammals and autistic individuals are low on measures of gregariousness, socialization, direct
gazing, eye contact, facial expression, emotional engagement, affiliative need and other social
behaviors. The evolution of the neurological tendencies in solitary species that predispose them
toward being introverted and reclusive may hold important clues for the evolution of the autism
spectrum and the natural selection of autism genes. Solitary animals are thought to eschew social
contact as part of a foraging strategy often due to scarcity and wide dispersal of food in their native
environments. Similarly, it is known that, due to frequent and prolonged dry spells, the human
ancestral environment was often nutritionally scarce as well, and this may have driven human parties
to periodically disband. Inconsistencies in group size must have led to inconsistencies in the manner
in which natural selection fashioned the social minds of humans, which in turn may well be responsible
for the large variation in social abilities seen in human populations. This article emphasizes that
individuals on the autism spectrum may have only been partially solitary, that natural selection may
have only favored subclinical autistic traits and that the most severe cases of autism may be due to
assortative mating. This solitary forager hypothesis of autism is explored in the context of
anthropology, comparative neuroscience, epidemiology, evolutionary biology, neuroethology, and
primatology.
Keywords: Theory of mind, autism, neuroecology, ethology, ecology, systemizing
1) Autism and Natural Selection
2) Autism and Evolutionary Medicine
3) Presentation of the Hypothesis
3) Autism Intelligence and Systemizing
4) Autism and Solitary Foraging
5) Autistics and Orangutans: Solitary Foragers
The Evolution of Autism
6) The Selectivity of Neuropathological Changes in Autism
7) Conclusions
Autism and Natural Selection
Autism is a condition that affects individuals from birth or infancy and is diagnosed on the basis of
three primary symptoms: social deficits, impaired communication and stereotyped and repetitive
behaviors [1]. People with autism largely withdraw from social contact and become absorbed in private
worlds of obsessive interests and repetitious activities [2]. Autism has been shown to be highly
heritable and has been associated with a number of both genetic and environmental risk factors [3]. It
is thought that the genes associated with autism create very selective abnormalities that tend to affect
brain regions associated with social cognition [4]. Unfortunately, the genetics, molecular biology and
neuroscience of autism are still, relative to many other neurological disorders, shrouded with
uncertainty due to their highly complex nature.
A portion of this complexity and uncertainty arises from the relatively large number of distinct
susceptibility genes that have been identified, many of which can be completely absent even in
pronounced autism [3]. This genetic heterogeneity may be responsible for the clinical heterogeneity,
which ranges from debilitating social deficits to minor personality traits. Medical science recognizes
certain phenotypes that are thought to lie on a continuum often referred to as the autism spectrum
disorders (ASD). Of these disorders, the DSM recognizes: autistic disorder (Kanner’s autism),
childhood disintegrative disorder and pervasive developmental disorder not otherwise specified (PDD-
NOS, or atypical autism). [1]. Autistic disorder itself has also been broken into 4 subgroups: Asperger
syndrome the highest functioning form [5,6], and then high, medium and low functioning autism [7,8].
Geneticists report that although the clinical distinctions do not map neatly onto specific genes or
patterns of genes, lower functioning individuals may have a higher total number of susceptibility
alleles [3]. For this reason this article will not make evolutionary distinctions between individual autistic
disorders but will focus on the autism spectrum and the range of related genes as a whole.
A number of very different theories about the causes of autism have attempted to explain the
scattered facts. It has been difficult for theorists to create a grand synthesis and autism has been
reconceptualized many times. At one point, it was thought that autistic children were purely a result of
poor parenting, an idea that was once widely embraced but is now utterly rejected [2]. Autism is now
known to be a biological phenomenon in which a genetic diathesis or susceptibility may interact with
early environmental circumstances to determine the severity of outcome [9]. However, why this genetic
susceptibility to autism is so prevalent and how the genes persisted despite the perceived negative
effects is unclear. It is the author’s view that conceptualizing autism in terms of evolutionary biology will
offer insight into the underlying factors, and help to make sense out of its seemingly incongruous
characteristics.
The risk of developing autism approaches .5 percent in the general population, making it a highly
prevalent “disorder” from an evolutionary perspective [3]. In the past two decades alone, the
incidence of autism has increased from 2-5 to 15-60 per 10,000 children, a dramatic increase, which
is attributed to heightened attention by the medical community as well as the broadening of diagnostic
criteria [10,11]. Disorders, like autism, that are so prevalent that they exceed common mutation rates
are thought to have persisted because the genes responsible for them conferred some advantage in
the ancestral environment [12]. To fully understand the evolutionary provenance of such a syndrome
it is important to attain a rough date of its origins in natural history.
For years autism was thought to be a disorder that affected Caucasians exclusively. If this had in fact
been the case, an evolutionary explanation would have to explain why autism originated and spread in
Europe over the last 20 to 40 thousand years. It is now known, however, that autism affects all human
populations. In fact, autism presents with approximately the same prevalence rate, worldwide, in all
studied races and ethnic groups [13,14] and has no association with immigrant status or
socioeconomic status [14]. That autism has the same prevalence in all ethnic groups strongly
suggests that it existed in a fully developed form well before the first humans left Africa. Autism and
the autism spectrum appear to be very ancient, yet how were the responsible genes selected and
maintained over tens or perhaps hundreds of millennia?
This article will delineate the “solitary forager” hypothesis of autism which proposes that some genes
contributing to the autism were selected and maintained because they facilitated solitary life, an
infrequent but unavoidably persistent condition that must have threatened survival during human
evolution. Individuals on the autism spectrum are described here as having had the potential to be
self-sufficient and capable foragers in hunting and gathering times that would have been predisposed
toward taking up a relatively solitary lifestyle. Certain psychological characteristics of autism are taken
here as a suite of cognitive adaptations that would have facilitated a lone existence. Like other solitary
mammals with similar cognitive adaptations, they were probably not completely and obligately solitary;
rather, they may have done much of their foraging solitarily instead of in groups. The article will use
the perspectives from evolutionary medicine and evolutionary psychopathology to expound upon this
hypothesis in a conjectural and exploratory manner. Moreover, corroborating evidence is sought from
evolutionary medicine, the systemizing theory of autism, anthropology, primatology and comparative
neuroscience.
Autism and Evolutionary Medicine
Many medical conditions that are pathological in the present are known to have been adaptive in the
ancestral environment, and the science of evolutionary medicine is concerned with identifying and
understanding these conditions [15]. It is thought that evolutionary perspectives on disease will
elucidate pathophysiology and ultimately inform treatment strategy [12]. The accomplishments of
evolutionary medicine in explaining disease in terms of adaptation to environment have been
extensive and have provided insights into the origins of atherosclerosis, cardiovascular disease, cystic
fibrosis, diabetes mellitus, obesity, sickle cell anemia and many others [16]. Evolutionary medicine has
also attempted to explain the origins of certain mental disorders. Several well-received theories have
been formulated based on these attempts.
The growing fields of evolutionary psychopathology [17,18], and evolutionary psychiatry [19] are
concerned with the application of evolutionary rationale to the understanding of psychological and
psychiatric disorders. Researchers in evolutionary psychology have concluded that there were
multiple, alternative cognitive strategies used to deal with the problems and obstacles that recurred in
our evolutionary past [20]. Furthermore, they have emphasized that individual differences in
developmental patterns may not always represent the effects of idiosyncratic life experiences but in
fact represent biological, naturally selected responses to pressing environmental concerns [21]. Many
articles in the last two decades have espoused this view and reconceptualized various forms of
psychopathology (e.g., anxiety, depression, obsessive compulsive disorder, psychopathy and post
traumatic stress disorder (PTSD)) as adaptive, cognitive syndromes that have ecological utility
[22,23,24,25,26].
Researchers in evolutionary psychopathology have made impressive strides in explaining the
evolutionary basis of psychological disorders. In fact, several psychological and psychiatric conditions
are now thought to be well understood in terms of the adaptive roles that they played in the ancestral
environment. These disorders include: anxiety, thought to represent a careful, cautious strategy [27];
depression, a socially submissive strategy [28]; psychopathy, a socially selfish and opportunistic
strategy [29] and PTSD, a threat avoidant strategy. Significant comparative evidence supports these
four theories. The symptoms of anxiety, depression, psychopathy and PTSD have been shown to exist
in other animals, serving adaptive purposes. Imagine the plight of a guppy born without the capacity
for predator anxiety or a social monkey born without the instinct to act subordinate to larger, dominant
monkeys. Despite the cultural stigma and psychological pain associated with these four disorders
today, genetic susceptibility to them would have conferred specific adaptive benefits in prehistoric
times. Certainly, a person who meets the criteria for an anxiety disorder takes this “cautious strategy”
to an extreme; however, researchers speculate that such people may have achieved high
reproductive success during highly volatile, violent or unpredictable times [17]. Similarly, self-
subordination, threat avoidance and selfishness all represent important behavioral proclivities that
have strong neurological foundations in a large variety of species. Importantly, social and asocial
tendencies also have neurological bases and vary widely between species. Innate predispositions
toward sociality are also thought to vary within species [29], and it is thought that the severity of
autism, like the severity of these other disorders, can be shown to exist on a continuum. As a final
example of the success and applicability of evolutionary psychopathology to mental disorder, let us
consider the popular reconceptualization of attention-deficit / hyperactivity disorder (ADHD).
Thom Hartmann [30] coined the phrase “hunter in a farmer’s world” to explain the predicament of
individuals with ADHD. He totally rethought the condition, explaining that in the environment of
evolutionary adaptedness, children were not naturally selected to listen for hours on end, to pay
careful attention to lessons, or to concentrate for long periods on assignments. Shapiro and others
[31] have taken this theory further. They explained that the predilection for individuals with ADHD to
be impulsive, hyperactive and scattered could have given them an advantage in prehistoric times. It is
likely that the ability to focus on a single concept for a prolonged period was not necessarily beneficial
in hunting and gathering times. High levels of activity and scanning, though, may have kept humans in
tune with a rapidly changing environment and allowed them to identify and act on both immediate
threats and immediate opportunities. Additionally, Panksepp [32] has concluded that even some
severe cases of ADHD may reflect normal variation in these adaptive tendencies. Likewise, this article
will take the perspective of evolutionary psychopathology and make the argument that autism
represents ‘a lone forager in a social world,’ and, as with ADHD, perhaps a large proportion of the
autism spectrum, maybe even pronounced cases of autism, may reflect normal variation. The autism
spectrum will be conceptualized as representing naturally occurring variation, at one extreme end of
the sociality continuum.
Presentation of the Hypothesis
Today, the cognitive disabilities associated with autism are clear and well documented; however,
modern social, occupational, and mating practices may conceal the evolutionary or adaptive benefits.
From an anthropological perspective, the society we live in is very different from the environment of
human evolutionary adaptedness. Because of the large group sizes seen in modern day cities and the
very social nature of modern employment, social abilities such as congeniality, extraversion and
savoir-faire are highly regarded. Even minimal social dysfunction or awkwardness can be
professionally and socially problematic. In fact, the DSM considers social ability as a critical
component of psychological health [1]. Social ability may have been less critical though in the
prehistoric past, especially during certain contexts. During the six million years since our branch
diverged from that of chimpanzees, our ancestors lived in relatively small groups that probably
fluctuated greatly in size [12]. Under these conditions, the abilities to relate effortlessly to a stranger,
to display affection in a demonstrative manner, or to charm a foraging companion were probably
exposed to inconsistent selective pressures. Inconsistencies in the level of social ability that our
environment demanded may well be responsible for the large variation in social abilities seen in
human populations as well as the extremes of social “dysfunction.”
The adaptive value of social ability may have been inconsistent in the ancestral past for many
reasons. Depending on the period of human evolution, the geographical location, the ecological
setting and the cultural habits of the group in question, ancestral foragers may have foraged
intimately with others throughout the day, may have spent weeks foraging alone, or anything between
these two extremes. Humans are a highly social species and this tells us that, like chimpanzees, we
must have spent much of our formative years congregating together. However, just as our social
abilities are a testament to our social past, our asocial abilities may indicate the opposite. In other
words, during prehistoric times, a certain proportion of breeding individuals may have lived under
solitary conditions or under conditions where group size was very small and this may have led to the
numerous autism genes found in the modern-day gene pool.
Certain eccentricities or impediments peculiar to autism may have precluded past researchers from
considering that autistic individuals may have survived well during prehistoric times. As in neurotypical
individuals, behavior in autism can vary markedly from individual to individual. Some individuals with
autism are awkward and unwieldy while others are graceful and nimble. Some are gentle and kind
while others can become angry and violent. Some are severely mentally handicapped whereas others
are cognitively gifted. These extremes, often barely tempered by social constraints and morays, might
make these individuals appear dangerously unnatural. Given this, and given that a proportion of
individuals with autism cannot be mainstreamed comfortably in elementary school, it may appear that
autism would have been maladaptive. Hypothetically though, it would be much more difficult to place a
young orangutan in a human elementary school classroom than it would be to place the average
autistic child and yet orangutans are capable of reaching full ecological independence before age 10.
Although, some individuals with autism have grave social deficits, many may have been able to
survive and prosper as effectively as other solitary mammals in their natural environment. A great deal
of new research supports this line of reasoning by illustrating that autistic individuals may have great
trouble with social cognition but that their other cognitive abilities are largely intact [33]. The next
section will consider some of the findings in the literature on autistic intelligence, supporting the notion
that autistic individuals can be very competent in areas that do not require social cognition.
Autistic Intelligence and Systemizing
Simon Baron-Cohen and others have changed the way that many researchers think about autism by
pointing out that a social deficit alone may account for most of the symptoms. Previous theories of
autism have characterized autistics as cognitively confused, disorganized or incoherent. Baron-
Cohen, on the other hand, has characterized autistics as having profound social disabilities but
otherwise being coherent and able minded. His theories of autism emphasize that the main deficit is in
the social domain, specifically, in the inability to empathize with and model the minds of others [34]. A
large number of subsequent studies have substantiated that “theory of mind,” or the ability to
empathize with others, is impaired in autism [33]. Baron-Cohen and others have argued persuasively
that impaired theory of mind in autism can be independent of learning ability and general intelligence.
Furthermore, he has documented that some autistic individuals can have superior technical
understanding of physical systems (sometimes referred to as folk physics as opposed to folk
psychology) compared to their age-matched peers and he has termed this ability “systemizing.”
Baron-Cohen has demonstrated that even though autistics are poor at empathizing, they are good at
systemizing. Systemizing is the ability to observe a physical system and make inferences or
conclusions about how it works and what causes it to work the way it does. He explains that
systemizing works well for phenomena that are lawful and deterministic but that systemizing is of
almost no use when it comes to predicting moment-by-moment changes in a person’s behavior.
Empathizing and forming theories about the other person’s mind is required for the latter task.
According to Baron-Cohen, abilities that are associated with empathizing include responding
empathetically to distress, intuiting another’s emotional state, being sensitive to facial expressions,
and demonstrating ability with language. Systemizing, on the other hand, is associated with ability in
domains such as map reading, mental rotation, physics, mathematics and motoric systems [34].
People who are good at empathizing are thought to lean toward jobs in the areas of social work,
psychology, and nursing. High systemizing is thought to lead to jobs in engineering, construction, and
science.
Unlike autism, empathizing seems to be unimpaired in individuals with Down syndrome, William’s
syndrome, and some other forms of mental retardation, despite their decreased ability to systemize
and their lower general intelligence [35,36]. This dissociation between intelligence and empathy
indicates a real genetic and cognitive difference between the two types of disorders. Many children
with autism have IQs well below the average for their age; however, the intelligence tests that correct
for social disability have shown that many autistics may in fact not be intellectually impaired at all [37].
Contrary to speculations, researchers have found little evidence of a deficit of executive function in
autistic children younger than 4 years of age. This suggests that the mild executive function deficits
seen after age 4 are not primary to the disorder but may arise later because of the absence of social
learning [38]. In fact, several “less learned, more innate,” executive functions, including inhibition and
visual working memory, appear to be spared in autism [39].
The present author has theorized as to the evolutionary origins of several syndromic forms of
congenital mental retardation, pointing out that they may represent predictive adaptive responses to
environmental cues predictive of nutritional scarcity [23]. Many forms of mental retardation feature a
suite of traits associated with low metabolic rate and have risk factors, such as low birth weight,
associated with the “thrifty genotype” phenomenon. Autism does not seem to be closely associated
with such risk factors or low metabolic rate. Autism does not seem to be a thrifty foraging strategy and
similarly, most forms of mental retardation do not seem to be solitary foraging strategies. Social and
empathic abilities are usually very high relative to IQ in most forms of mental retardation. In fact,
William’s syndrome appears to be the ecological opposite of autism, featuring high empathy,
surprising strengths in language ability and facial processing and a profound impairment in spatial
cognition. An comparable analysis of William’s syndrome would be interesting.
Baron-Cohen, and many who have followed him, has performed a great deal of research showing that
systemizing is part of the genetic profile of autism and that the autistic spectrum is continuous with
normal human variation. He has demonstrated that the parents and relatives of people with autism are
good at systemizing but not very good at empathizing. He has compiled data showing that fathers and
grandfathers of males with autism are twice as likely to be engineers compared to males in the
general population [8]. He has also shown that students in the natural sciences have a higher number
of relatives with autism than do students in the humanities [8]. Baron-Cohen and others have also
shown that in Asperger’s syndrome and in high functioning autism, individuals can perform at normal
or often superior levels in tasks requiring the systemization of information. People with Asperger’s
performed better on his Systemizing Quotient (SQ) [40] and tests of intuitive physics [41] and can
reach extremely high levels of achievement in systemizing domains, such as mathematics, physics,
and computer science [42]. This has been called the “autism advantage” in popular autism advocacy.
It is thought that these high functioning autistic individuals may be neurologically well suited for certain
jobs that are detail oriented, repetitive, and involve the identification of technical errors [37]. A popular
conceptualization of this purports the idea that quirky computer specialists working in Silicon Valley,
for example, desire jobs allowing them to work alone on computing problems, tend to marry others
who are high on systemizing but low on empathy, and tend to have a propensity to have children that,
like them, are on the autism spectrum. Baron-Cohen pointed out that, “If systemizing talent is genetic,
such genes appear to co-segregate with genes for autism [8].”
Baron-Cohen has extended his theories into the “extreme male brain theory” of autism [33]. He
pointed out that women, when compared to men, are more verbal, more socially adept, and have
more sophisticated capacities for empathy and theory of mind. He asserted that the differences
between non-autistics and autistics could be thought of as analogous to the differences between
women and men. This extreme male brain theory is fascinating, well supported, and may produce
testable hypotheses. Unfortunately, the theory can be difficult to relate back to natural selection and,
in its present state, may not appropriately address ultimate or evolutionary concerns. Baron-Cohen’s
analogies could be expanded if one concludes that autism represents the cognitive style of an
individual who is well suited for living alone. Comparing non-autistic and autistic individuals then may
be more like comparing a dog (a pack animal) to a cat (a largely solitary animal) rather than
comparing a woman to a man.
Baron-Cohen and others have been successful in convincing many researchers that autistic people
are intelligent in a different way and very good at systemizing and understanding non-social natural
processes. Baron-Cohen has even posited that evolution may have maintained the genes for autism
precisely because of their systemizing ability, suggesting also that many cases of autism may be due
to the assortative mating of two high-systemizing parents [8]. The present paper hopes to extend
these arguments by pointing out that these natural systemizing abilities could have predisposed
people on the autism spectrum to be able-minded, able-bodied, self-sufficient, and solitary foragers in
the prehistoric past.
Autism and Foraging
In modern society, due to cultural, historic, and sociological circumstances, many individuals with
autism do not learn to become self-sufficient. Due to their social deficits, many people with autism do
not follow their peers on a path toward social and occupational mobility. Today, children must go
through a prolonged period of education and socialization to reach a point where they can hold a job
and live autonomously. Autonomy and even food procurement depend on social abilities as well as
learning that took place in a particularly social setting, the school classroom. Schooling can be very
difficult for individuals with autism because, as the systemizing theory of autism has demonstrated,
individuals with autism have a proclivity to learn things on their own rather than from others.
Unfortunately, the interests chosen and knowledge acquired by individuals with autism often do not
coincide with financial, professional, or social opportunities. Even when carefully guided by loving
parents, it is difficult for many individuals with autism to teach themselves how to become self-
sufficient in today’s world. However, this may have been much more natural in the ancestral
environment, particularly considering that merely 10,000 years ago, neither food procurement nor the
acquisition of a place to sleep were necessarily contingent on social ability as they are today. The
discipline of behavioral ecology, which may be applicable here, is based on the premise that cognitive
traits and the neural substrates responsible for them are shaped by natural selection and hence are
fine-tuned to respond to a particular environment. The autism spectrum may be explicable in terms of
behavioral ecology in the sense that it represents the adaptive value of being fine-tuned to thrive in a
solitary environment.
Similar to mothers in contemporary foraging societies, mothers in the ancestral past would have
required their children to learn how to forage for food from an early age. Virtually all mammals,
whether of social or asocial species, use their systemizing abilities to learn food procurement
techniques from their mothers. The offspring of asocial mammalian species may not be motivated to
empathize or socialize much with their mother but are nonetheless driven by hunger to attend to her
examples and internalize her tactics. This strategy works well for hundreds of species of asocial
mammals (even ones with very small brains and limited systemizing abilities) and presumably would
have worked well for individuals with autism. Of course, it would be an immoral experiment to place an
autistic toddler and its mother in the wilderness for years in order to see if he or she learns and
thrives. However, would such an experiment be needed if it could be shown that autistic behaviors are
normal relative to those of other solitary animals taken out of the wild since birth (say a pet reptile, a
domestic cat, or a captive orangutan)? Are there reasons to think that an individual with autism, living
in prehistoric times, would not be strongly motivated by thirst, appetite, discomfort, sexual urges, and
other innate instincts to become nutritionally independent and to increase its reproductive success?
The behavior of autistic individuals is often seen as bewilderingly inappropriate in a social context,
because they so often become interested in or obsessed with socially meaningless activities. In a
natural environment though, it is likely that hunger would have motivated them to redirect their
obsessive tendencies toward food procurement. Today, their hunger for food does not drive them to
refine food procurement techniques because their parents feed them every time they are hungry.
Modern humans are responsible for social and academic learning and are rarely given the chance to
be positively reinforced by successful food acquisition. This temporal or causal pairing between
learning and satiety, integral for wild animals, has been artificially taken away from modern children.
Because the compelling and coercing natural instinct of hunger does not actuate or motivate modern
individuals with autism, their efforts and skills are misplaced onto irrelevant stimuli.
The powerful and mobilizing asocial fascinations and preoccupations seen in modern day autism
could have aided their prehistoric counterparts in self-preservation. Humans habituate to things that
they are not interested in and systemize things that they find rewarding, motivating, or intrinsically
interesting. In the ancestral past, activities leading up to the sating of hunger would have been highly
reinforced, and thus food procurement and food processing strategies would have been the primary
variables of the reinforcement schedule for individuals with autism. Perhaps, when children with autism
ignore their parent’s examples of social behavior today, it is because these examples seem boring
and meaningless whereas in the ancestral past they would have been inspired by their parent’s
examples of hunting and gathering activities. Today, because they are not able to forage or to watch
their parents forage and because they can obtain food free of effort, their interests are redirected
toward salient, nonsocial activities, like stacking blocks, flipping light switches, lining toys up in rows,
playing with running water, chasing vacuum cleaners, and collecting bottle tops. This kind of
misapplication of an innate tendency is known in behavioral ecology as “displacement behavior.”
The interests of autistic individuals often lead to the development of islets of aptitude or competence.
Despite obvious deficits, many show that they have refined or even mastered certain skills, often
referred to as splinter skills. These may include eccentric proficiencies, such as penchants for
drawing, music, rote-memory, or puzzle solving. In extreme cases, these skills might include what are
often called savant abilities including things like counting, hyperlexia, pattern finding, and calendar or
mathematical calculation. Some of these skills and abilities in autism were probably acquired because
the individual focused on a particular type of problem solving, they learned to systemize a particular
system, and then they retained and elaborated on the ability. In the past, these elaborate abilities
would probably have mapped onto the acquisition of foraging techniques, which, would have been
honed to proficiency through rote repetition and practice.
Many anthropologists studying hunter-gatherer groups exclaim that the foragers that they are
observing have cultivated amazing naturalistic abilities and are able to sense and perceive things to
which they themselves are virtually blind. These abilities, like tracking, ranging, stalking, food
processing, mapping of terrain, and knowledge of flora and fauna may be areas through which the
potential for autistic or savant-like abilities were channeled in the ancestral past. The same could be
said for the deep stores of specialized or technical knowledge exhibited by people on the autism
spectrum. This knowledge could have been dedicated to understanding edible and inedible species,
the habits of prey items, self-protection, maximization of food-collection efficiency, fashioning tools,
and procuring shelter.
Aside from their asocial preoccupations, individuals with autism also exhibit stereotyped and repetitive
actions. In fact, many mammals engage in perseverative or repetitious behaviors especially when
placed into unnatural or confining environments [43]. Repetitive, stereotypical behavior (including self-
injurious behavior) is very rare in the wild but very common in captive animals. More intelligent animals
seem to have increased capacity for self-injurious behaviors. It has been estimated that 10 to 15
percent of rhesus monkeys living alone in a cage develop self-biting, head banging and self-slapping
[43]. This may indicate that the living conditions that many young individuals with autism experience
are artificial, and possibly inhumane as they are not as stimulating or motivating as the wild
environment that they are born expecting. Surely if you aren’t focused on social interaction, you are
destined to focus on other systems. Recent clinical studies have shown that the intensive practice of
social skills and social subskills, from a very early age, can ameliorate many autistic symptoms. This
demonstrates that individuals with autism, like everyone else, have a wide array of potential talents
available to them from an early age but that because of neurological reasons, social developmental
windows close earlier for them.
The propensity toward strict adherence to routine seen in autism also close analogues in the “master
routines” and “subroutines” of many non-social species (e.g., waking, cautious emergence,
defecation, grooming, basking, foraging, returning, bedding, sleeping). Although such neurologically
mediated rituals can be repetitive and inflexible at times in many vertebrate species, they are
important regulators of adaptive behavior. Perhaps the persistent, stereotyped behaviors
characteristic of autism had ecological utility in the sense that they allowed structure, order, and self-
regulation. Why is there the tendency toward tedium and invariability then? Most of the variety in the
life of a human hunter-gatherer is probably found in its social interactions. There would probably be
much less variety in the life of a lone hunter-gatherer. A lone forager would be forced to engage in
lonely, repetitive, and stereotypic activities, such as scanning repeatedly for threats, picking and
processing fruit, searching for and extracting vegetables, and locating and capturing prey items. An
obsessive desire for sameness, repetition, and ritual makes little sense in the context of a social
setting but seems applicable in a lone setting. In fact, it might be bad to expect variability and to thrive
off unpredictability in a solitary scenario because of the monotony of living alone.
A mind that is highly geared toward using social cognition and forming emotional relationships would
have been disadvantageous in an individual who was forced by circumstance to live in a solitary
scenario. This may help explain why many individuals with autism have an undue preference for their
own company, pay attention to the non-social aspects of people, and treat others as if they were
inanimate objects. Concentrating on empathy or theory of mind would probably have been impractical
and counterproductive. Martin Brune [29] has speculated that theory of mind and social cognition are
probably vulnerable to dysfunction. He pointed out that most people at one time or another have
strong desires to ascribe intentions to non-animals. A solitary forager without autism may be likely to
attribute agency or ascribe intentions to inanimate objects, amounting to displaced and confused
behavior.
In her book “Animals Make Us Human” autistic professor Temple Grandin (who herself engaged in
repetitive, restricted and self-injurious behavior at a young age) points out that even though domestic
dogs and cats both live comfortably with humans, only cats can leave a human family to live solitarily
in the wild. “If you put the family poodle out in the countryside, his chances of surviving are low unless
he finds another family to live with. But abandoned cats do fine.” Here she underscores the fact that
some animals are obligately social whereas others can transition between social and solitary lifestyles.
Her life story is attestation to the fact that individuals with severe autism can make this transition while
maintaining the “cognitive coherency” to contribute profoundly to both industry and academia.
Like asocial animals, some individuals with autism avoid close bodily contact and fail to establish
emotional relationships. It seems that these tendencies might have facilitated solitary life whereas the
inclinations to seek out physical contact and emotional relationships could have made solitary life
miserable or unbearable. Classic works of fiction portraying humans abandoned or marooned from
others emphasize the human need for companionship. The protagonists in such works frequently
have unremitting obsessions with imaginary friends, behavior that would be extraneous and probably
maladaptive for a solitary human. People on the autism spectrum would be less likely to yearn for
companionship and more likely to focus on survival. Solitary species have very limited abilities for
social cognition. This is probably because gregarious predispositions, companionable inclinations,
and social instincts in general are maladaptive in a solitary context.
Autism and Orangutans: Solitary Foragers
Most animals, many mammals, and several species of primates seek out food on their own. It is clear
that such a solitary foraging strategy can even be effective for apes. In fact, orangutans, the species
that is genetically third closest to humans as revealed by molecular studies, live solitarily in the wild.
Orangutans eat, sleep, hunt, and forage on their own [44]. They are often described as cautious and
introverted, and it has been estimated that Bornean orangutans spend at least 95% of their time
alone [45]. Orangutans have low interaction and association rates, and only infrequently meet up with
conspecifics, often only to mate [46]. They have been reported to congregate in small groups
temporarily, but only to eat from a particularly fruit-laden tree. Several specialists emphasize that
orangutans have limited social aptitude and tend to prefer solitude [45]. Van Schaik [45] has
concluded that, unlike all other species of apes, well-defined communities do not appear to exist in
any orangutan population studied so far.
In orangutans, inculcation in infancy and juvenility consists of learning about food procurement from
the mother. In stark contrast, chimpanzees are indoctrinated into learning about both food
procurement and socialization [44]. Orangutan mothers teach their babies to recognize edible food
species, implement foraging techniques, find shelter from the elements, protect themselves from
predators, and develop effective ranging capabilities [48]. Compared to chimpanzees, young
orangutans do not learn much from their mothers about communication or socialization; however, they
do learn everything that they need to survive and reproduce.
Unlike their orangutan counterparts, chimpanzee adolescents reliably stay with (or at least frequently
visit) their mothers for months or years after they have been weaned [49]. Their strong affective ties,
emotional attachments, and gregarious nature keep mother and child in contact into adulthood or, at
times, throughout life [50]. Orangutans on the other hand, exhibit very different behavior. Adolescent
orangutans reliably leave their mother and go off on their own to forage very soon after weaning. The
bond between the mother and child is based largely on the provision of milk [48]. One of the most
frequent complaints of parents with autistic children is that they find it very difficult to build an
emotional bond with the child [6]. One might imagine that in an ancestral setting the bond between a
mother and her autistic child could have been as delicate (and yet at times, as ecologically
appropriate) as the bond seen in orangutans. Despite this relative delicacy, the vast majority of
babies and children with autism do develop true psychological attachments to their caregivers. This
attachment would probably have facilitated the acquisition of foraging techniques as it does in
orangutans.
Aside from orangutans, all apes, including gibbons, gorillas, chimpanzees, and bonobos are social
species, much like humans [51]. The ability to make inferences about the mental states of others
probably first emerged in a clear form during early primate evolution due to heavy selection from the
social environment [52]. Apes, relative to monkeys though, have elaborated greatly upon this ability
presumably because their social environment was even more important. To some degree, it seems
that orangutans regressed in their social abilities relative to other apes because, as their group size
decreased, pressure from the social environment must have decreased along with it.
Primatologists generally agree that orangutans live solitary lifestyles primarily because the islands of
Borneo and Sumatra, where they are found, have poor food density. Relative to the lush African
regions that chimpanzees and gorillas inhabit, these islands cannot support large groups of social
apes [44]. If orangutans lived in larger social groups, they would have to travel extremely far (up to 20
miles) each day simply to get enough food to sustain themselves [53]. Because orangutans are large
animals, and because the trees and bushes in their non-seasonal habitat go through various phases
of producing buds, shoots, and fruit, the orangutans must forage through many plants daily to ferret
out the right dietary foodstuffs [44]. Because they must traverse so much land everyday in order to
obtain a sufficient number of calories, orangutans must define large territories for themselves. It is
thought that in Sumatra, each orangutan needs nearly a square kilometer and that in Borneo each
requires an even larger area of spatial isolation [54]. Fascinatingly, Borneo is thought to be less food-
dense, and Bornean orangutans are thought to be less socially inclined when compared to Sumatran
orangutans [45]. Food density of the habitat is actually known to be an important determinant of group
size in a number of animals [55]. In fact, when food becomes scarce, even chimpanzees split up to
search for food alone [56].
Harsh and unpredictable climates marked much of the period of human evolution [57]. The
Pleistocene and Pliocene ice ages caused frequent aridity of the African plains, which would have
dried up many sources of food for our foraging ancestors [58]. These conditions would have rendered
our habitat nutritionally scarce [59], perhaps forcing groups of our ancestors to disband. Humans may
have been forced to do this habitually in the past despite the fact that we are an inherently social
species. In sum, environmental forces prominent during our evolution caused nutritional scarcity that,
at least in some geographic regions, may have created the same conditions that selected orangutans
to become solitary. Another condition that influences selection for sociality is the distribution of food
resources. It is known that when food is approximately evenly dispersed in an environment, solitary
foraging yields higher energy returns but when food is clumped, group foraging becomes more
efficient [55]. One may not even need to invoke scarcity or food distribution patterns to conclude that
many pre-social species, including humans, may have experienced historical disruptions (either
punctuated or gradualistic) in their social continuity that left lasting traces in the genome.
Adaptations particular to orangutans, which are thought to reflect their life-history strategies, may
have implications for autism. Orangutans show large sexual dimorphism [47] and much of this is
thought to be due to high testosterone levels in the males. Autistics are also thought to exhibit
elevated testosterone levels, which may be associated with increased fetal exposure to testosterone
[60]. It might make sense that a solitary forager strategy is better suited to males given that female
animals of many species have an increased need for social support and parenting proficiency. Male
orangutans actually spend a greater proportion of their time alone compared to females (most of the
time spent between two males is thought to be attributable to random, unintended encounters) [45].
Interestingly, autism is four times more prevalent in men compared to women, is accompanied by more
severe anomalies in brain structure in men, and is known to have a worse prognosis in men [6].
Higher testosterone levels in autistic males may have increased their sexual aggressiveness as well
as their sexual attractiveness. Mating practices of individuals on the autism spectrum may have been
similar to those of the orangutan. Females with autism or on the autistic spectrum probably had little
difficulty in procuring receptive sex partners. Males, on the other hand, could have procured partners
using the same tactics used by male orangutans, such as propositioning, perseverative soliciting,
provisioning/food sharing, and others. Like the association with testosterone, orangutans have other
biological predispositions that might elucidate enigmas in autism. Relative to chimpanzees,
orangutans have tendencies toward late weaning, prolonged lactational amenorrhea, and long inter-
birth intervals [48]. It might be interesting and instructive to look for tendencies like these, as well as
other peculiar life-history phenomena, in autism births. Moreover, specific tendencies seen in autism,
such as atypical eating, food consumption rituals, food faddism, and oral fixations may prove to have
ecological significance.
It would be amiss to neglect to mention two important things. First, there is some contention over
whether orangutans have been selected to be solitary or simply have relocated in the last 10,000
years to an environment that requires solitary foraging. If future comparisons demonstrate that
orangutans are relatively socially adapted, the present argument would be forced to abandon the
evolutionary (yet not necessarily the behavioral) comparison with orangutans. Regardless,
evolutionary comparisons with known solitary primates and other solitary animals that lack biological
instincts for gregariousness, affection, and strong social relationships should be informative. It is also
important to affirm that no offense is intended toward autistic individuals in this comparison with
orangutans. Only traits related to sociality are compared here, and individuals with autism are not in
any way equated with apes in general. In fact, individuals with autism are only compared to
orangutans here to the same extent that neurotypical individuals are compared to chimpanzees.
Orangutans were selected as a basis of comparison for this article simply because they are the
species of solitary foragers that is most closely related to humans.
The Evolution of Autism
The evolution of social tendencies in animals is a poorly understood topic of research. It is clear that
there are costs and benefits to living in groups and that the pressures from the social environment
can change abruptly [52]. Surely, in the past, individuals on the autism spectrum were sometimes born
into the arms of supportive and cohesive social groups, much like today. If autism does represent a
solitary forager strategy then this circumstance could have constituted a mismatch that may have
been negatively selected against. However, most adaptations have risks associated with them and
instead of representing a panacea; they reflect tradeoffs that, over evolutionary time, produce on
average more benefits than costs. This mismatch may not even have decreased fitness. Given the
propensities of modern day people with autism, such individuals might have chosen to journey off
alone nomadically or hermitically. Alternatively, they might have chosen to forage alone but return to
their group infrequently in order to interact and travel with group members. This may have depended
on “how autistic” the individual was. Even individuals on the far side of the autism spectrum may not
have even been truly solitary during prehistory. Social chess abilities or Machiavellian intelligence are
not well developed in people on the autism spectrum. Their speech and mannerisms can be
monotonous and mechanical, yet this may not have kept them from offering valuable skills and
forming functional relationships with other humans. In many cases, autistic individuals exhibit
pragmatic competence that could have made them valuable attributes to their foraging companions.
Today, a large proportion of individuals with Asperger’s have been assimilated into elite social and
professional spheres despite the fact that they are not adroit social conformers [34]. It is reasonable
to imagine that a proficient hunter with autism could be not only tolerated but highly valued by a small
band of wild men who were each eccentric themselves, not having been exposed to our modern,
relatively conformist, post-industrial society. This would be akin to the situation today where millions of
mainstreamed autistic children around the world have become accepted on the playground, despite
their differences. It is well accepted that despite valuing their time alone, many individuals with autism
enjoy friendships and have a documented predilection for valuing quality of friends over quantity [6].
Similarly, when in captivity, or when gathering around a large, fertile fruit tree, orangutans can be
observed to have social competencies, even if they are not as facile or effusive as chimpanzees.
As a rash speculation about the origins of autism, the present author would like to point out that
autism could be a remnant of genetic introgression that took place before humans were the lone
species in our genus. Perhaps the genes for autism evolved not in our direct ancestral line but in an
asocial subspecies or hominin which later merged genetically with our line of descent through gene
flow. Demes, or subpopulations of relatively solitary humans, could have been assimilated into our
gene pool after a fair amount of interbreeding following the migration. Such autistic subpopulations
could have arisen in ecologically or climatologically different geographic regions of Africa at anytime in
the Plio-Pliestocene.
The evolution of human populations with short stature may shed light on this issue. The pygmy
phenotype, characterized by small body size, appears to have evolved independently in South
American, African, and Southeast Asian populations in the last 200,000 years. The phenotype is
mostly restricted to rainforest environments and is thought to represent a response to the selective
pressures associated with living in a tropical rainforest [61]. This powerful climatological association
has motivated adaptationist hypotheses as to how small size would increase reproductive success in a
warm, wet environment with dense foliage. It now seems clear to physical anthropologists that short
stature may have increased the capacity to cope with one or more of the following: caloric limitations,
high temperature, high humidity, extensive forest undergrowth, increased mortality, or vitamin
deficiencies [61]. Multiple genetic disruptions of the growth hormone and insulin-like growth factor I
pathway are likely to be involved etiologically but no specific DNA mutations have been uncovered
[62]. It is clear though that when pygmies intermarry with non-pygmies, the result is a spectrum;
children tend to be intermediate in stature to those of the two parental populations [62]. Pygmy
populations show us a few things that are interesting in the present context: 1. isolated pockets of
humans can remain reproductively insulated for long enough to evolve discrepant ecological
strategies; 2. such populations can quickly (less than 40,000 years in the South American and Asian
pygmies [62]) develop features that vary markedly from the norm; 3. these traits can involve multiple
genes at different loci; and 4. interbreeding can result in either continuous or polymorphic variation in
subsequent generations. It is interesting to note that, as these indigenous people become assimilated
into other gene pools, the genes for short stature will persist and may affect phenotypic variability in
sporadic and unpredictable ways for a long time to come.
The genes that make up a polygenic trait usually all have different natural histories. They appeared
as mutations at different periods, and they were selected at different times. For example, several
different genes, each working through different molecular mechanisms, helped Europeans develop
lighter skin as they moved to higher latitudes over the last 50,000 years. Interestingly, an entirely
different set of alleles with unique cellular properties helped people in northern Asia develop lighter
skin. Few of these genes have been selected to fixation, and they occur together in different
proportions, creating a lot of variability in skin color even within isolated populations. The genes for
autism may be similar to these examples in the sense that each gene may have a different natural
history. Some genes may have been meant to work cooperatively whereas other not. Different genes
for autism may have evolved independently in genetically isolated gene pools. Each gene that
predisposes toward autism may represent a different way to make an organism a little less reliant on
sociality. Like other polygenic continuous traits, the mutations responsible for autism could have been
maintained by “environmental heterogeneity,” a form of balancing selection. In other words, the genes
responsible for autism may have remained in our gene pool because as social-environmental
conditions fluctuated in the past, discrepant genetic polymorphisms, or “multiple alternate alleles”
were favored. Currently, it is not possible to scrutinize the natural history of autism genes at this level
of analysis because even though over a dozen genes have been implicated, including an oxytocin
receptor, a serotonin transporter, and putative language genes, very little agreement exists in the
literature over the specifics [3].
It is currently not possible to tell whether the frequencies of genetic variants responsible for autism are
high enough, or in the right proportions, to accommodate a model of past positive selection. The
contributions of a large number of relatively rare genes and of de novo mutations [3] tell us that
autism may not have been subject to natural selection for thousands of years. It may have a large non-
evolutionary component as well as an irregular evolutionary signature. The present article cannot fully
address this issue primarily because of the lack of knowledge about the genetics of autism. It is
unclear whether autism can be explained better by rare mutations or by rare combinations of common
genetic variants [3,6]. Future linkage and association studies though may be able to resolve whether
the responsible genes are mutated sequences encoding aberrant gene products or normal
polymorphisms acting synergistically [9]. It is also clear that much of the clinical picture of autism is not
representative of adaptation. Some cases of autism are associated with congenital anomalies and
physical stigmata that were probably not adaptive. How and why rare comorbidities such as Rett
syndrome, tuberous sclerosis, fragile X syndrome, psychomotor epilepsy, and other clinical entities
can present with autism is far from apparent [2,4]. It may be the case that several pathological
conditions present symptoms that resemble autism (or exacerbate autism) and thus clinicians and
epidemiologists have grouped them together with autism, obscuring its natural and unadulterated
visage (as described here). This is certainly the case with ADHD and schizophrenia. Idiopathic or
teratogenic neurodevelopmental insults can closely mimic these as well as other neurodevelopmental
disorders [3]. Many of these unknowns will not become clear until additional genetic, molecular, and
pathophysiological research is done.
Another important criticism of the present argument is that there may be no clearly documented
anthropological evidence for solitary foraging in modern humans. This shouldn’t be a huge blow to the
present hypothesis given that there do not seem to be any documented examples of autistic
individuals in hunter-gatherer groups either. It seems that no one has looked for these things
systematically. However, the prevalence of autism in forager groups should be equivalent to the
prevalence in the rest of the world, given that epidemiologic studies indicate that environmental
factors, such as perinatal insults, prenatal infections, teratogens, and toxins (factors that may be
particular to modern life) account for only very few cases of autism [4]. In modern society, it is
apparent many people intentionally sequester themselves or prefer solitude to different degrees, and
it is reasonable to expect that this type of behavior could have been continuous with the autism
spectrum in the ancestral past [34]. Several substantiated cases of abandoned youths, known as feral
or wild children, have survived to adulthood in the wild [52]. It is probable that far more children were
orphaned in the wilderness in ancestral times, and this may have contributed to the selective
pressures for asocial temperament. Feral children usually have many autistic features in the sense
that they are emotionally detached, prefer solitude, and eschew physical contact [52]. These features
may constitute protective, adaptive responses to solitary life, and their resemblance to the symptoms
of autism may not be coincidental.
A consistent, predictable proportion of ancestral humans (and ancestral hominins) must have been
forced to live alone during the six million years of human prehistory. From these individuals, the ones
with mutations that created disruptive abnormalities in regions of the social brain may have
experienced increased reproductive success. This should not sound like a stretch, considering that
this general pattern may have already occurred during the evolution of orangutans. After orangutans
diverged from the common ancestor that they shared with gibbons around 18 million years ago, they
moved into Southeast Asian rainforests where they were forced to give up their social lives for solitary
ones because of ecological constraints [53]. It appears that from the consistency of their behavior in
and out of the wild, their solitary tendencies were instinctual and innate and that selective pressures
acted upon their brain. For this reason, it should be very informative for autism researchers to
compare the brain areas responsible for social cognition across apes. This kind of research has not
yet been done. We may never know exactly what it was like to have autism in prehistoric times but
comparing them to known solitary species on a variety of neurological measures may provide
clarification.
Autism and Epigenetic Programming
It is my hunch that autism, like other syndromes studied by the discipline of phenotypic plasticity, is
epigenetically programmed in utero by cues indicative of social scarcity. I imagine that a number of
different cues coming from a sparse social environment might program the autistic phenotype. There
seems to be some evidence for one such environmental determinant. This section will offer tenuous
speculation as to why advanced paternal and maternal ages are risk factors for the development of
autism. Twin studies suggest that interactions between multiple genes cause "idiopathic" autism but
that epigenetic factors and environmental modifiers may contribute to variable expression of autism-
related traits. One of the most significant environmental risk factors for autism is advanced paternal
age. In fact, recent studies have shown that individuals born to older fathers (aged 45-50 years or
more) are 2 to 3 times more likely to develop autism. Studies that have attempted to correct for factors
that might confound the association between autism and advanced paternal age, such as personality
traits, have suggested that the effects of possible confounders are low and that the association is
genetic in nature [80].
A solitary strategy would work better if it was tied to an environmental cue that served as a reliable
predictor that solitary status is likely. Advanced paternal age may be such a cue. In modern hunter
gatherer groups fathers are known to play significant roles in teaching their offspring how to hunt and
become socially adept. A child that will be neglected of its father is in a unique and disadvantaged
position. But what signal could warn a developing embryo that it may be neglected of paternal care?
Surely older fathers have a higher probability than younger fathers of dying before they are able to
raise their children into full adulthood, especially in ancestral times when people lived far shorter lives.
For this reason, it may be adaptive for an individual who is born to an older father to have a
propensity for autism simply because older fathers are more likely to die before they have the
opportunity to provide their offspring with social instruction and socialization. Furthermore, it might be
harder to integrate oneself into a pack or tribe in the absence of one’s father. Because a father’s
presence is probably more important to a male than it is to a female, particularly because males
engage in hunting activities, a father may be more valuable to a male.
The Selectivity of Neuropathological Changes in Autism
The brain abnormalities in autism do not seem to constitute indiscriminate pathological abnormalities
as one might expect if autism were simply a disease. The most conspicuous brain abnormalities seem
to affect the areas of the brain that have been associated with social cognition consistently and
systematically [63]. The amygdala, the anterior cingulate cortex, the orbito and medial frontal cortex,
and the mirror neuron system have all been strongly associated with social cognition [64], and also
have been shown to be the same areas that exhibit the prominent anomalies in autism [65]. This
selectivity makes it seem that the genes that influence social areas of the brain were affected and the
others were spared (consonant with Baron-Cohen’s psychological theories of autism). According to
Gelman and Williams [66], brains have been evolutionarily organized to acquire and store particular
sorts of relevant information and disregard other, irrelevant information. In this context, individuals
along the autistic spectrum appear to have a neurological propensity to gate out social information
and thus gain conscious access to highly processed nonsocial perceptions and schemata. This
section will look at how this might be accomplished by focusing on eye contact, facial expression,
oxytocin regulation, macrocephaly, amygdala reactivity, and facial recognition in autism.
Curious phenomena that are very common in autism are averted gaze and poor eye contact [67].
Autistic individuals describe eye contact as uncomfortable and even threatening [34]. Interestingly,
eye contact is also very rare in the vast majority of solitary species, including orangutans, who actively
avoid both direct gazing and even facing [68]. Chimpanzees and gorillas share gazes and use the
eyes for communication frequently, just like most humans [69]. Chimps and gorillas use eye contact
constantly while interacting with members of their own group but may charge at an individual from
another group if it makes eye contact. Staring between unfamiliar apes is often interpreted as a threat
signal; therefore, it is best for the solitary orangutan to avoid both eye contact and direct gazing in
order to forestall an attack [69]. Orangutans actively avoid gazing and eye contact and this tendency,
very common among solitary animals, has been explicitly interpreted as adaptive for their solitary
foraging niche [68]. Instead of face-to-face direct viewing, orangutans, like individuals with autism,
glance momentarily at others sideways with the head turned away [70]. The neurological substrates
(including amygdalar sensitivity) that underlie this very specific and prominent tendency may have
evolved for the same adaptive, defensive reasons in both autism and orangutan.
Studies have shown that autistic individuals are less expressive especially with respect to facial
communication. They make fewer facial expressions and are rated as more flat or neutral in affect by
observers [71]. This absence of facial responsiveness is probably due to underlying neuronal
mechanisms and there is evidence that the facial motor nucleus is significantly reduced in size in
autism [72]. Fascinatingly, the size of the facial motor nucleus is thought to vary predictably in total
volume as a function of group size in monkeys and apes. The larger the average group size, the more
important facial expressiveness is and the larger the facial motor nucleus must be [73]. It should be
informative to analyze the anatomical organization of the autistic facial motor nucleus (VII) taking note
of the general size, the placement of motor neurons, distribution of neuron types and the general
topography of muscle representation. Experts in phylogenetic specializations of the primate facial
motor nucleus might be able to perceive social or ecological traits in the distinctive anatomical
organization of the facial motor nucleus in autism. Analyzing it relative to the orangutan facial motor
nucleus should be informative as well, as it is known that orangutans are much less facially expressive
than other apes [74]. Solitary foragers do not need to communicate with their faces as much since a
large proportion of the interaction that do occur between them and members of their species might
have taken place from a distance. This may evince that brain areas associated with facial expressions
were less important for a solitary forager and that less plasticity, learning, and potential for
interpersonal variation was needed in these domains.
Oxytocin, a neuropeptide thought to enhance social learning, social expressiveness, direct eye gaze
and the ability to remember faces [75], is reduced in autism subjects. Diminished peripheral levels of
oxytocin may play a large role in retuning social brain modules in autism [76]. It has been shown that
intravenous oxytocin produces a significant reduction in stereotypic behaviors in adult autism subjects
and increases empathy in people without autism [77]. Variation in oxytocin levels in mammals reflects
adaptations to the social environment, just as it may in the case of autism. Animals that rely on pair-
bonds and social attachment have higher levels of plasma oxytocin, especially when it is behaviorally
relevant, like during childbirth or monogamous sex [52]. Comparing prairie voles to montane voles,
two very closely related species of rodent, is interesting in this context because the two differ widely in
bonding behavior. The montane vole, relative to the prairie vole, has a much smaller number of
receptors in the brain for oxytocin and unlike the amorous prairie voles they do not form pair bonds
[55]. The montane voles have fewer receptors and thus are less responsive to oxytocin making them
more wary, suspicious and more easily frightened of other members of their species- ensuring that
they do not allow themselves to become vulnerable [55]. It is thought that the wide discrepancy in
social behavior between these two species reflects adaptation to two very different physical and social
environments [63]. Not surprisingly, interspecific, seasonal and reproductive variation in oxytocin
concentrations have been attributed ecological and adaptive significance. High levels are associated
with trust, love, pair-bonding and generosity in a variety of mammals [63]. It would be interesting to
compare the details of oxytocin action- such as receptor number and distribution- in solitary animals
with that of people with autism.
Diminished peripheral levels of oxytocin may cause individuals with autism to be born “expecting” or
prepared for a socially impoverished environment. Natural selection cannot act on behavior directly
but instead acts on the neural substrates that generate the psychological mechanisms that create the
behavior. Evolved psychological mechanisms, such as cognitive modules, are generally understood in
terms of specific inputs, decision rules and outputs [78]. Most of these mechanisms were naturally
selected to be sensitive to a narrow range of perceptual information. In other words, they are
biologically prepared to learn about or solve particular adaptive problems. Some mechanisms are
known to be domain-specific and many of these are assumed to exhibit variation in humans causing
some people to attend to perceptual cues that others might miss entirely. Tuning differences in
domain-specific mechanisms or modules, especially those involving oxytocin, serotonin and
adrenaline may underlie the differences in autistic cognition and, like other differences seen in nature,
may have been created by natural selection to help solitary foragers face their particular set of
recurrent or ecologically relevant threats and opportunities. They have direct access to lower level,
less processed information. People with autism have normal early vision (what a baby sees) and
normal middle vision (features extracted). Their high level vision is even ok (scene understanding).
They just never learned the social things to have a social, top down understanding.
It has been shown that one or both hippocampi are often significantly larger in autism [4]. The
hippocampus is a convergence zone that is integral to spatial ability and perhaps the large size of the
hippocampus in autism has neuroecological relevance. The study of neuroecology focuses heavily on
the relationships between ecological hardships, spatial abilities and the size of the hippocampus
[79,80]. It might make sense that a solitary forager would have to compensate for the fact that it could
not rely on the spatial abilities of its companions. Individuals with autism have also been shown to
excel at the block design subtest of the Wechsler intelligence scales, evidence of high visuospatial
abilities [6]. It may be possible that such abilities are indicative of the visuospatial rigors particular to a
solitary existence.
Brain growth is accelerated in early autism and this may have been adaptive for a youngster that was
preparing to become a solitary forager. Individuals with autism have been known for years to exhibit
increased head circumference and increased brain size (two highly correlated traits) during infancy
[81]. Macrocephaly (head circumference higher than the 97th percentile), one of the most consistently
encountered traits seen in autism, is known to affect between 14% and 34% of all autistic infants [82].
In fact, total brain size, as revealed by MRI, is on average 5-10% bigger in autism between 18 and 48
months of age [4]. Neonates who will develop autism and macrocephaly exhibit normal head
circumference at birth, but their head growth accelerates during the first year of life. This growth
continues until at least 4 years of age, slows and then decelerates prematurely [81]. This increased
rate of brain maturation and growth slows early, well before nonautistic brains start to slow, resulting in
very similar head sizes between those with autism and those without in both adolescence and
adulthood [82]. A large contributor to increased brain size in youth is myelination and it is thought that
different areas of the brain myelinate differentially according to an ecological program where primary
sensory areas begin to myelinate first well before association cortices. The dramatic increase in head
circumference from birth until 4 may be evidence that the learning arc for a solitary forager is not as
protracted as it is for other children. Hunter-gatherer children need to laboriously and perseveringly
learn a language along with social morays, a time-consuming process [84]. Researchers have
concluded that learning during human childhood is so long because social interactions are more
variable and relatively less frequent than other forms of ecological learning [85]. The fact that
individuals with autism show this increase in head and brain size between birth and age 4, may
indicate that they are programmed to learn more about their environment, faster, at an earlier age.
This may also suggest that autism represents a precocial (“out of the nest early”) strategy where
learning takes place very early so that the animal can become independent of its mother quickly, a
characteristic observed in countless nidifugous animals that do not have complex social lives.
Consistent with this conclusion are data evincing that many males with autism exhibit precocious
puberty [86], further evidence of a precocial as opposed to an altricial strategy.
Other neurological differences particular to autism may hold neuroecological significance. Individuals
with autism have been widely reported to have increased fear activity in the amygdale and high levels
of anxiety [87] which might act to up regulate vigilance, caution and a healthy fear of strangers. It
seems reasonable to assume that a solitary forager would benefit more from such caution than would
an individual who must learn to embrace companions. Some frequently reported, but only preliminarily
researched, eccentricities in autism include acute and perceptive hearing, and increased frequency
of, and interest in, smelling [88]; two features that could certainly be interpreted in a neuroethological
context. Face processing, a key factor in the development of social perception, is severely impaired in
autism [89]. The area responsible for face recognition, the fusiform face area, in particular has
demonstrated reduced activation in autism during facial discrimination tasks [90] indicating that in
autism, like in many solitary animal species, identity recognition may not be as valuable. These
neurological differences may be telling. If autism truly does represent a solitary phenotype, then the
brain abnormalities -which are highly consistent across individuals on the spectrum- should furnish
theoretically intriguing insights, not only into comparative cognition but also into human evolution.
The Relationship to Advanced Paternal Age
This section will offer a brief speculation as to why advanced paternal and maternal age are risk
factors for the development of autism. One of the most significant risk factors for autism is advanced
paternal age. In fact, recent studies have shown that individuals born to older fathers (aged 45-50
years or more) are 2 to 3 times more likely to develop autism. Studies that have attempted to correct
for factors that might confound the association between autism and advanced paternal age, such as
personality traits, have suggested that the effects of possible confounders are low and that the
association is genetic in nature (Croen et al., 2007).
In modern hunter gatherer groups fathers are known to play significant roles in teaching their
offspring how to forage and hunt. A child that will be neglected of its father is in a unique and
disadvantaged position. But what signal could warn a developing embryo that it may be neglected of
paternal care? Surely older fathers have a higher probability than younger fathers of dying before
they are able to raise their children into full adulthood. For this reason, it may be adaptive for an
individual who is born to an older father to have a propensity for autism simply because older fathers
are more likely to die before they have the opportunity to provide their offspring with social instruction
and socialization. Furthermore, it might be harder to integrate oneself into a pack or tribe in the
absence of one’s father. Because a father’s presence is probably more important to a male than it is
to a female, particularly because males engage in hunting activities, a father may be more valuable to
a male. Interestingly, autism is four times more prevalent in men than women, is accompanied by more
severe anomalies in brain structure in men and is known to have a worse prognosis in men (Frith,
1991). It also makes some sense that autism might be more valuable to males because of their lower
need for social support and parenting ability.
The phenotypic characteristics of a large number organisms ranging from plants to insects to humans
have been shown to make plastic responses to early environmental events, many of which are
thought to represent defensive or reproductive strategies (Pigliucci, 2001). Perhaps all humans, and
especially those that carry autism susceptibility genes, are sensitive to chemical markers that indicate
the age of the parents. This may be accomplished by epigenetic programming, where different
messaging pathways cause DNA acetylation or methylation changes altering gene expression. In
other words, autism could at least involve a predictive, adaptive response to anticipated social
deprivation, parental estrangement or orphanage. It is possible that autistics may represent
phenotypes that would have been able to make it on their own, from a younger age, if they were
separated from their father or mother during early childhood. In this sense, autistics may have been
more nidifugous or precocial as babies and young children. In sum, the parental age effects may
indicate that autism is associated with a genetic imprinting strategy that attempts to compensate for
the social handicaps related to having an older father, an older mother, or both.
Conclusions
This article has attempted, in an exploratory manner, to provide a characterization of autism that
reconciles known findings with evolutionary theory. In the past it was not understood how autistics
might have gotten along during prehistoric times. Yet now, a comparison with solitary species shows
that autistic individuals may have lived largely solitary lives and yet still have achieved self-sufficiency
and reproductive success. Characteristics of autism that have been interpreted as consistent with the
solitary forager hypothesis include: the high systemizing abilities, the obsessive and perseverative
tendencies, the repetitious and ritualistic tendencies, splinter skills, the deep but narrow stores of
knowledge, macrocephaly, gaze aversion, absence of eye contact, the parallels with orangutans,
increased hippocampal size, precocious puberty, the testosterone effect, reduced fusiform face area
and facial nucleus activity, reduced oxytocin concentrations and the sex ratio. Clearly, there are
exceptions to each of the lines of evidence offered here. For instance, there are innumerable
functional differences between the social mind of an orangutan and that of the average individual with
autism. The growing body of experimental literature testing the social-cognitive skills of solitary
species, may offer important insight into autistic traits though and tell us more about the similarities
and differences in these domains. It will be very difficult to determine irrefutably if what we know as
autism today was in fact an adaptive phenotype in the ancestral past. The hypothesis presented here
is underspecified and vague but may be progressive as it is thought that analyzing disease states
from an evolutionary perspective can ultimately do much to inform and influence medical theory and
ultimately even intervention strategy [12]. Furthermore, the evolutionary perspectives delineated here
could potentially provide structure for empirical investigations in animal behavior or cognitive
neuroscience.
The case that autism represents a solitary forager phenotype is a very top-heavy argument that can
and probably should be toned down in numerous ways. First, individuals with autism were probably
not totally solitary and may even have been better adapted to very small groups than to a purely
solitary existence. Second, it should be emphasized that natural selection may have only been
selecting for subclinical autistic traits and that the high-functioning corner of the spectrum was the
substrate for natural selection. Third, the most severe cases of autism may actually have been
maladaptive and due to assortative mating of two individuals that hold a high number of autism
susceptibility genes. A comprehensive solitary forager theory of autism must address these issues
and should start by partitioning variation in autism into nosological categories that represent either
pathology or nature. This article has not clearly committed to delineating which aspects of autism are
adaptive and which are not, or even committed to whether the lowest functioning forms could have
had a place in nature. It is probably too early to make these discriminations. However, it seems that if
the extreme end of the autism spectrum was selected for, that the distribution of autism prevalence
relative to severity might be bimodal or have a fattened tail yet this does not seem to be the case.
It is probable that other endophenotypes and genetic patterns in autism can be analyzed in terms of
the present hypothesis to provide convergent evidence. Eventually, genetic hypothesis testing could
provide near incontrovertible proof for the present hypothesis. Until then, comparing intron relative to
exon mutations in susceptibility genes for autism, mapping gene linkages, analyzing epigenetic
patterns, scrutinizing genome-wide microarray analyses and even looking for inter and intraspecific
trends in genographic data may all provide further support. Further comparative behavioral studies
may also be revealing. It will be instructive to look for abilities in autism that more closely resemble
foraging activities than the abilities mentioned here. Conversely, looking for anthropological evidence
for autism-like traits among successful foragers should also be revealing. It is also important to
question whether the processing deficits or cognitive findings in autism might have put a solitary
forager at a disadvantage. In order to further explicate the evolutionary roots of autism and draw new
inferences with the potential to inform medical intervention. I encourage researchers to turn to
ethology and Niko Tinbergen’s [91] “four questions” in understanding the value of behavior: 1) what is
the immediate benefit to the organism? 2) What is the immediate consequence? 3) How does it
develop in the individual across ontogeny? 4) How did it evolve in species across phylogeny?
Knowledge about the costs and benefits of solitary existence may help to guide each of these lines of
research.
A number of other, sensible evolutionary theories of autism have been put forth. Autism has been
conceptualized as a low-fitness extreme of a parentally selected fitness indicator [92]. These authors
have advanced that variation in the ability to connect socially to ones parents, an adaptive trait, may
have a maladaptive extreme form that results in autism. Another pair of researchers has envisioned
autism as a consequence of paternally imprinted genes [93]. This idea has been taken further and
autistic-like traits have been seen as a male-typical strategy geared toward parental investment, low-
mating effort and long-term resource allocation [94]. These and other theories of autism are well
supported and plausible. None of these is directly compatible with the present hypothesis but again,
neither are they directly contradictory. Like the present hypothesis they view autism as an extreme
end of a continuum constituted by adaptive traits. Oliver Sacks (1970) has formulated an interesting
view of autism that appears to be consistent with this [95]. He points out that the perspective of the
autistic individual appears aboriginal because they never succumb to our modern societal
conventions, but that this does not keep them from perceiving, thinking and behaving in original,
innovative and intelligent ways:
“The autistic by their nature are seldom open to influence. It is their ‘fate’ to be isolated and thus
original. Their ‘vision’, if it can be glimpsed, comes from within and appears aboriginal. They seem to
me, as I see more of them, to be a strange species in our midst, odd, original, wholly inwardly
directed, unlike others.”
Studies in behavioral genetics have demonstrated that tendencies for cooperation, temperament, and
sociality have biological underpinnings. Furthermore, they show considerable variability within and
between animal species [96]. Tendencies for independence, reclusiveness, introversion and other
traits characteristic of autism certainly show a great deal of variability between species [97], but what
about within them? Perhaps populations of other social species such as chimpanzees have an equally
low but consistent prevalence of autistic individuals as well. No formal diagnostic criteria are available
for psychiatric or even social disabilities in other animals [98], but it would be interesting, although
difficult, to see if there are analogues, or possibly homologues of autism in other species. If there were
homologues of autism in species closely related to humans, it would be relatively easy to use
molecular techniques to show this given that the genes responsible could be identified. Looking at the
evolutionary signatures in the behavioral genetics of autism, might tell us more about the variability in
life-history strategies of our ancestors. Many different factors could be invoked in such a discussion
including behavioral neuroscience, cognitive primatology, the importance of group membership to
survival, fission-fusion sociality, maverick males, paleoneurology, parenting methods, phenotypic
plasticity, sexual selection, sexual strategies, social and affective neuroscience, pair bonds,
territoriality, social hierarchy, and others.
“Because humans are highly social, we like to think that complex societies represent the crowning
achievement of evolution.… Indeed, if group living were the universally superior lifestyle, we would
expect social species to outnumber solitary ones, but exactly the opposite is true. Why? Almost
certainly because in many environments, the costs of living with others are prohibitively high.”
-John Alcock [99]
Humans are an innately social and gregarious species [95]. Their ancestral environment, like that of
chimpanzees, selected them to be this way. It is probable that as with all traits selected for by
evolution, there are costs and benefits associated with social predispositions and that the large
variation in sociability in our species reflects the large variability in selective forces in the past. The
relative prevalence of autism is high enough to suggest that it was a phenotype that was naturally
selected for in the past, but also low enough to suggest that it was rarely preferable to the socially
typical alternative. Perhaps the prevalence of the autism spectrum in modern human populations tells
us something profound about the frequency of forced solitude in ancestral times, the adaptive value
of social instincts and the plasticity of human cognitive strategies. Showing that autism had ecological
viability and that it exists today because of its success in the past suggests that it should not be
considered a disease, but instead a condition. It should not be thought of as something to be
ashamed of, but as something that represents individuality, self-determination and autonomy.
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9(2): 207- 238.
Jared Edward Reser
Brain and Cognitive Science Department
University of Southern California
jared@jaredreser.com
Copyright 2006
Abstract
This article reviews etiological and comparative evidence supporting the hypothesis that some
heritable genes associated with the autism spectrum were naturally selected and represent the
adaptive benefits of being cognitively suited for solitary foraging. The systemizing theory of autism is
extended here and people on the autism spectrum are conceptualized as ecologically competent
individuals that could have been adept at learning and implementing hunting and gathering skills in
the ancestral environment. Upon independence from their mothers, young autistic individuals may
have been psychologically predisposed toward a different life-history strategy, common among
mammals and even some primates, to hunt and gather primarily on their own. Many of the behavioral
and cognitive tendencies that autistic individuals exhibit are viewed here as adaptations that would
have complemented a solitary lifestyle. For example, the obsessive, repetitive and systemizing
tendencies in autism, which can be mistakenly applied toward activities such as block stacking today,
may have been focused by hunger and thirst toward successful food procurement in the ancestral
past. Individuals on the autism spectrum share a variety of behavioral traits with solitary species. Both
solitary mammals and autistic individuals are low on measures of gregariousness, socialization, direct
gazing, eye contact, facial expression, emotional engagement, affiliative need and other social
behaviors. The evolution of the neurological tendencies in solitary species that predispose them
toward being introverted and reclusive may hold important clues for the evolution of the autism
spectrum and the natural selection of autism genes. Solitary animals are thought to eschew social
contact as part of a foraging strategy often due to scarcity and wide dispersal of food in their native
environments. Similarly, it is known that, due to frequent and prolonged dry spells, the human
ancestral environment was often nutritionally scarce as well, and this may have driven human parties
to periodically disband. Inconsistencies in group size must have led to inconsistencies in the manner
in which natural selection fashioned the social minds of humans, which in turn may well be responsible
for the large variation in social abilities seen in human populations. This article emphasizes that
individuals on the autism spectrum may have only been partially solitary, that natural selection may
have only favored subclinical autistic traits and that the most severe cases of autism may be due to
assortative mating. This solitary forager hypothesis of autism is explored in the context of
anthropology, comparative neuroscience, epidemiology, evolutionary biology, neuroethology, and
primatology.
Keywords: Theory of mind, autism, neuroecology, ethology, ecology, systemizing
1) Autism and Natural Selection
2) Autism and Evolutionary Medicine
3) Presentation of the Hypothesis
3) Autism Intelligence and Systemizing
4) Autism and Solitary Foraging
5) Autistics and Orangutans: Solitary Foragers
The Evolution of Autism
6) The Selectivity of Neuropathological Changes in Autism
7) Conclusions
Autism and Natural Selection
Autism is a condition that affects individuals from birth or infancy and is diagnosed on the basis of
three primary symptoms: social deficits, impaired communication and stereotyped and repetitive
behaviors [1]. People with autism largely withdraw from social contact and become absorbed in private
worlds of obsessive interests and repetitious activities [2]. Autism has been shown to be highly
heritable and has been associated with a number of both genetic and environmental risk factors [3]. It
is thought that the genes associated with autism create very selective abnormalities that tend to affect
brain regions associated with social cognition [4]. Unfortunately, the genetics, molecular biology and
neuroscience of autism are still, relative to many other neurological disorders, shrouded with
uncertainty due to their highly complex nature.
A portion of this complexity and uncertainty arises from the relatively large number of distinct
susceptibility genes that have been identified, many of which can be completely absent even in
pronounced autism [3]. This genetic heterogeneity may be responsible for the clinical heterogeneity,
which ranges from debilitating social deficits to minor personality traits. Medical science recognizes
certain phenotypes that are thought to lie on a continuum often referred to as the autism spectrum
disorders (ASD). Of these disorders, the DSM recognizes: autistic disorder (Kanner’s autism),
childhood disintegrative disorder and pervasive developmental disorder not otherwise specified (PDD-
NOS, or atypical autism). [1]. Autistic disorder itself has also been broken into 4 subgroups: Asperger
syndrome the highest functioning form [5,6], and then high, medium and low functioning autism [7,8].
Geneticists report that although the clinical distinctions do not map neatly onto specific genes or
patterns of genes, lower functioning individuals may have a higher total number of susceptibility
alleles [3]. For this reason this article will not make evolutionary distinctions between individual autistic
disorders but will focus on the autism spectrum and the range of related genes as a whole.
A number of very different theories about the causes of autism have attempted to explain the
scattered facts. It has been difficult for theorists to create a grand synthesis and autism has been
reconceptualized many times. At one point, it was thought that autistic children were purely a result of
poor parenting, an idea that was once widely embraced but is now utterly rejected [2]. Autism is now
known to be a biological phenomenon in which a genetic diathesis or susceptibility may interact with
early environmental circumstances to determine the severity of outcome [9]. However, why this genetic
susceptibility to autism is so prevalent and how the genes persisted despite the perceived negative
effects is unclear. It is the author’s view that conceptualizing autism in terms of evolutionary biology will
offer insight into the underlying factors, and help to make sense out of its seemingly incongruous
characteristics.
The risk of developing autism approaches .5 percent in the general population, making it a highly
prevalent “disorder” from an evolutionary perspective [3]. In the past two decades alone, the
incidence of autism has increased from 2-5 to 15-60 per 10,000 children, a dramatic increase, which
is attributed to heightened attention by the medical community as well as the broadening of diagnostic
criteria [10,11]. Disorders, like autism, that are so prevalent that they exceed common mutation rates
are thought to have persisted because the genes responsible for them conferred some advantage in
the ancestral environment [12]. To fully understand the evolutionary provenance of such a syndrome
it is important to attain a rough date of its origins in natural history.
For years autism was thought to be a disorder that affected Caucasians exclusively. If this had in fact
been the case, an evolutionary explanation would have to explain why autism originated and spread in
Europe over the last 20 to 40 thousand years. It is now known, however, that autism affects all human
populations. In fact, autism presents with approximately the same prevalence rate, worldwide, in all
studied races and ethnic groups [13,14] and has no association with immigrant status or
socioeconomic status [14]. That autism has the same prevalence in all ethnic groups strongly
suggests that it existed in a fully developed form well before the first humans left Africa. Autism and
the autism spectrum appear to be very ancient, yet how were the responsible genes selected and
maintained over tens or perhaps hundreds of millennia?
This article will delineate the “solitary forager” hypothesis of autism which proposes that some genes
contributing to the autism were selected and maintained because they facilitated solitary life, an
infrequent but unavoidably persistent condition that must have threatened survival during human
evolution. Individuals on the autism spectrum are described here as having had the potential to be
self-sufficient and capable foragers in hunting and gathering times that would have been predisposed
toward taking up a relatively solitary lifestyle. Certain psychological characteristics of autism are taken
here as a suite of cognitive adaptations that would have facilitated a lone existence. Like other solitary
mammals with similar cognitive adaptations, they were probably not completely and obligately solitary;
rather, they may have done much of their foraging solitarily instead of in groups. The article will use
the perspectives from evolutionary medicine and evolutionary psychopathology to expound upon this
hypothesis in a conjectural and exploratory manner. Moreover, corroborating evidence is sought from
evolutionary medicine, the systemizing theory of autism, anthropology, primatology and comparative
neuroscience.
Autism and Evolutionary Medicine
Many medical conditions that are pathological in the present are known to have been adaptive in the
ancestral environment, and the science of evolutionary medicine is concerned with identifying and
understanding these conditions [15]. It is thought that evolutionary perspectives on disease will
elucidate pathophysiology and ultimately inform treatment strategy [12]. The accomplishments of
evolutionary medicine in explaining disease in terms of adaptation to environment have been
extensive and have provided insights into the origins of atherosclerosis, cardiovascular disease, cystic
fibrosis, diabetes mellitus, obesity, sickle cell anemia and many others [16]. Evolutionary medicine has
also attempted to explain the origins of certain mental disorders. Several well-received theories have
been formulated based on these attempts.
The growing fields of evolutionary psychopathology [17,18], and evolutionary psychiatry [19] are
concerned with the application of evolutionary rationale to the understanding of psychological and
psychiatric disorders. Researchers in evolutionary psychology have concluded that there were
multiple, alternative cognitive strategies used to deal with the problems and obstacles that recurred in
our evolutionary past [20]. Furthermore, they have emphasized that individual differences in
developmental patterns may not always represent the effects of idiosyncratic life experiences but in
fact represent biological, naturally selected responses to pressing environmental concerns [21]. Many
articles in the last two decades have espoused this view and reconceptualized various forms of
psychopathology (e.g., anxiety, depression, obsessive compulsive disorder, psychopathy and post
traumatic stress disorder (PTSD)) as adaptive, cognitive syndromes that have ecological utility
[22,23,24,25,26].
Researchers in evolutionary psychopathology have made impressive strides in explaining the
evolutionary basis of psychological disorders. In fact, several psychological and psychiatric conditions
are now thought to be well understood in terms of the adaptive roles that they played in the ancestral
environment. These disorders include: anxiety, thought to represent a careful, cautious strategy [27];
depression, a socially submissive strategy [28]; psychopathy, a socially selfish and opportunistic
strategy [29] and PTSD, a threat avoidant strategy. Significant comparative evidence supports these
four theories. The symptoms of anxiety, depression, psychopathy and PTSD have been shown to exist
in other animals, serving adaptive purposes. Imagine the plight of a guppy born without the capacity
for predator anxiety or a social monkey born without the instinct to act subordinate to larger, dominant
monkeys. Despite the cultural stigma and psychological pain associated with these four disorders
today, genetic susceptibility to them would have conferred specific adaptive benefits in prehistoric
times. Certainly, a person who meets the criteria for an anxiety disorder takes this “cautious strategy”
to an extreme; however, researchers speculate that such people may have achieved high
reproductive success during highly volatile, violent or unpredictable times [17]. Similarly, self-
subordination, threat avoidance and selfishness all represent important behavioral proclivities that
have strong neurological foundations in a large variety of species. Importantly, social and asocial
tendencies also have neurological bases and vary widely between species. Innate predispositions
toward sociality are also thought to vary within species [29], and it is thought that the severity of
autism, like the severity of these other disorders, can be shown to exist on a continuum. As a final
example of the success and applicability of evolutionary psychopathology to mental disorder, let us
consider the popular reconceptualization of attention-deficit / hyperactivity disorder (ADHD).
Thom Hartmann [30] coined the phrase “hunter in a farmer’s world” to explain the predicament of
individuals with ADHD. He totally rethought the condition, explaining that in the environment of
evolutionary adaptedness, children were not naturally selected to listen for hours on end, to pay
careful attention to lessons, or to concentrate for long periods on assignments. Shapiro and others
[31] have taken this theory further. They explained that the predilection for individuals with ADHD to
be impulsive, hyperactive and scattered could have given them an advantage in prehistoric times. It is
likely that the ability to focus on a single concept for a prolonged period was not necessarily beneficial
in hunting and gathering times. High levels of activity and scanning, though, may have kept humans in
tune with a rapidly changing environment and allowed them to identify and act on both immediate
threats and immediate opportunities. Additionally, Panksepp [32] has concluded that even some
severe cases of ADHD may reflect normal variation in these adaptive tendencies. Likewise, this article
will take the perspective of evolutionary psychopathology and make the argument that autism
represents ‘a lone forager in a social world,’ and, as with ADHD, perhaps a large proportion of the
autism spectrum, maybe even pronounced cases of autism, may reflect normal variation. The autism
spectrum will be conceptualized as representing naturally occurring variation, at one extreme end of
the sociality continuum.
Presentation of the Hypothesis
Today, the cognitive disabilities associated with autism are clear and well documented; however,
modern social, occupational, and mating practices may conceal the evolutionary or adaptive benefits.
From an anthropological perspective, the society we live in is very different from the environment of
human evolutionary adaptedness. Because of the large group sizes seen in modern day cities and the
very social nature of modern employment, social abilities such as congeniality, extraversion and
savoir-faire are highly regarded. Even minimal social dysfunction or awkwardness can be
professionally and socially problematic. In fact, the DSM considers social ability as a critical
component of psychological health [1]. Social ability may have been less critical though in the
prehistoric past, especially during certain contexts. During the six million years since our branch
diverged from that of chimpanzees, our ancestors lived in relatively small groups that probably
fluctuated greatly in size [12]. Under these conditions, the abilities to relate effortlessly to a stranger,
to display affection in a demonstrative manner, or to charm a foraging companion were probably
exposed to inconsistent selective pressures. Inconsistencies in the level of social ability that our
environment demanded may well be responsible for the large variation in social abilities seen in
human populations as well as the extremes of social “dysfunction.”
The adaptive value of social ability may have been inconsistent in the ancestral past for many
reasons. Depending on the period of human evolution, the geographical location, the ecological
setting and the cultural habits of the group in question, ancestral foragers may have foraged
intimately with others throughout the day, may have spent weeks foraging alone, or anything between
these two extremes. Humans are a highly social species and this tells us that, like chimpanzees, we
must have spent much of our formative years congregating together. However, just as our social
abilities are a testament to our social past, our asocial abilities may indicate the opposite. In other
words, during prehistoric times, a certain proportion of breeding individuals may have lived under
solitary conditions or under conditions where group size was very small and this may have led to the
numerous autism genes found in the modern-day gene pool.
Certain eccentricities or impediments peculiar to autism may have precluded past researchers from
considering that autistic individuals may have survived well during prehistoric times. As in neurotypical
individuals, behavior in autism can vary markedly from individual to individual. Some individuals with
autism are awkward and unwieldy while others are graceful and nimble. Some are gentle and kind
while others can become angry and violent. Some are severely mentally handicapped whereas others
are cognitively gifted. These extremes, often barely tempered by social constraints and morays, might
make these individuals appear dangerously unnatural. Given this, and given that a proportion of
individuals with autism cannot be mainstreamed comfortably in elementary school, it may appear that
autism would have been maladaptive. Hypothetically though, it would be much more difficult to place a
young orangutan in a human elementary school classroom than it would be to place the average
autistic child and yet orangutans are capable of reaching full ecological independence before age 10.
Although, some individuals with autism have grave social deficits, many may have been able to
survive and prosper as effectively as other solitary mammals in their natural environment. A great deal
of new research supports this line of reasoning by illustrating that autistic individuals may have great
trouble with social cognition but that their other cognitive abilities are largely intact [33]. The next
section will consider some of the findings in the literature on autistic intelligence, supporting the notion
that autistic individuals can be very competent in areas that do not require social cognition.
Autistic Intelligence and Systemizing
Simon Baron-Cohen and others have changed the way that many researchers think about autism by
pointing out that a social deficit alone may account for most of the symptoms. Previous theories of
autism have characterized autistics as cognitively confused, disorganized or incoherent. Baron-
Cohen, on the other hand, has characterized autistics as having profound social disabilities but
otherwise being coherent and able minded. His theories of autism emphasize that the main deficit is in
the social domain, specifically, in the inability to empathize with and model the minds of others [34]. A
large number of subsequent studies have substantiated that “theory of mind,” or the ability to
empathize with others, is impaired in autism [33]. Baron-Cohen and others have argued persuasively
that impaired theory of mind in autism can be independent of learning ability and general intelligence.
Furthermore, he has documented that some autistic individuals can have superior technical
understanding of physical systems (sometimes referred to as folk physics as opposed to folk
psychology) compared to their age-matched peers and he has termed this ability “systemizing.”
Baron-Cohen has demonstrated that even though autistics are poor at empathizing, they are good at
systemizing. Systemizing is the ability to observe a physical system and make inferences or
conclusions about how it works and what causes it to work the way it does. He explains that
systemizing works well for phenomena that are lawful and deterministic but that systemizing is of
almost no use when it comes to predicting moment-by-moment changes in a person’s behavior.
Empathizing and forming theories about the other person’s mind is required for the latter task.
According to Baron-Cohen, abilities that are associated with empathizing include responding
empathetically to distress, intuiting another’s emotional state, being sensitive to facial expressions,
and demonstrating ability with language. Systemizing, on the other hand, is associated with ability in
domains such as map reading, mental rotation, physics, mathematics and motoric systems [34].
People who are good at empathizing are thought to lean toward jobs in the areas of social work,
psychology, and nursing. High systemizing is thought to lead to jobs in engineering, construction, and
science.
Unlike autism, empathizing seems to be unimpaired in individuals with Down syndrome, William’s
syndrome, and some other forms of mental retardation, despite their decreased ability to systemize
and their lower general intelligence [35,36]. This dissociation between intelligence and empathy
indicates a real genetic and cognitive difference between the two types of disorders. Many children
with autism have IQs well below the average for their age; however, the intelligence tests that correct
for social disability have shown that many autistics may in fact not be intellectually impaired at all [37].
Contrary to speculations, researchers have found little evidence of a deficit of executive function in
autistic children younger than 4 years of age. This suggests that the mild executive function deficits
seen after age 4 are not primary to the disorder but may arise later because of the absence of social
learning [38]. In fact, several “less learned, more innate,” executive functions, including inhibition and
visual working memory, appear to be spared in autism [39].
The present author has theorized as to the evolutionary origins of several syndromic forms of
congenital mental retardation, pointing out that they may represent predictive adaptive responses to
environmental cues predictive of nutritional scarcity [23]. Many forms of mental retardation feature a
suite of traits associated with low metabolic rate and have risk factors, such as low birth weight,
associated with the “thrifty genotype” phenomenon. Autism does not seem to be closely associated
with such risk factors or low metabolic rate. Autism does not seem to be a thrifty foraging strategy and
similarly, most forms of mental retardation do not seem to be solitary foraging strategies. Social and
empathic abilities are usually very high relative to IQ in most forms of mental retardation. In fact,
William’s syndrome appears to be the ecological opposite of autism, featuring high empathy,
surprising strengths in language ability and facial processing and a profound impairment in spatial
cognition. An comparable analysis of William’s syndrome would be interesting.
Baron-Cohen, and many who have followed him, has performed a great deal of research showing that
systemizing is part of the genetic profile of autism and that the autistic spectrum is continuous with
normal human variation. He has demonstrated that the parents and relatives of people with autism are
good at systemizing but not very good at empathizing. He has compiled data showing that fathers and
grandfathers of males with autism are twice as likely to be engineers compared to males in the
general population [8]. He has also shown that students in the natural sciences have a higher number
of relatives with autism than do students in the humanities [8]. Baron-Cohen and others have also
shown that in Asperger’s syndrome and in high functioning autism, individuals can perform at normal
or often superior levels in tasks requiring the systemization of information. People with Asperger’s
performed better on his Systemizing Quotient (SQ) [40] and tests of intuitive physics [41] and can
reach extremely high levels of achievement in systemizing domains, such as mathematics, physics,
and computer science [42]. This has been called the “autism advantage” in popular autism advocacy.
It is thought that these high functioning autistic individuals may be neurologically well suited for certain
jobs that are detail oriented, repetitive, and involve the identification of technical errors [37]. A popular
conceptualization of this purports the idea that quirky computer specialists working in Silicon Valley,
for example, desire jobs allowing them to work alone on computing problems, tend to marry others
who are high on systemizing but low on empathy, and tend to have a propensity to have children that,
like them, are on the autism spectrum. Baron-Cohen pointed out that, “If systemizing talent is genetic,
such genes appear to co-segregate with genes for autism [8].”
Baron-Cohen has extended his theories into the “extreme male brain theory” of autism [33]. He
pointed out that women, when compared to men, are more verbal, more socially adept, and have
more sophisticated capacities for empathy and theory of mind. He asserted that the differences
between non-autistics and autistics could be thought of as analogous to the differences between
women and men. This extreme male brain theory is fascinating, well supported, and may produce
testable hypotheses. Unfortunately, the theory can be difficult to relate back to natural selection and,
in its present state, may not appropriately address ultimate or evolutionary concerns. Baron-Cohen’s
analogies could be expanded if one concludes that autism represents the cognitive style of an
individual who is well suited for living alone. Comparing non-autistic and autistic individuals then may
be more like comparing a dog (a pack animal) to a cat (a largely solitary animal) rather than
comparing a woman to a man.
Baron-Cohen and others have been successful in convincing many researchers that autistic people
are intelligent in a different way and very good at systemizing and understanding non-social natural
processes. Baron-Cohen has even posited that evolution may have maintained the genes for autism
precisely because of their systemizing ability, suggesting also that many cases of autism may be due
to the assortative mating of two high-systemizing parents [8]. The present paper hopes to extend
these arguments by pointing out that these natural systemizing abilities could have predisposed
people on the autism spectrum to be able-minded, able-bodied, self-sufficient, and solitary foragers in
the prehistoric past.
Autism and Foraging
In modern society, due to cultural, historic, and sociological circumstances, many individuals with
autism do not learn to become self-sufficient. Due to their social deficits, many people with autism do
not follow their peers on a path toward social and occupational mobility. Today, children must go
through a prolonged period of education and socialization to reach a point where they can hold a job
and live autonomously. Autonomy and even food procurement depend on social abilities as well as
learning that took place in a particularly social setting, the school classroom. Schooling can be very
difficult for individuals with autism because, as the systemizing theory of autism has demonstrated,
individuals with autism have a proclivity to learn things on their own rather than from others.
Unfortunately, the interests chosen and knowledge acquired by individuals with autism often do not
coincide with financial, professional, or social opportunities. Even when carefully guided by loving
parents, it is difficult for many individuals with autism to teach themselves how to become self-
sufficient in today’s world. However, this may have been much more natural in the ancestral
environment, particularly considering that merely 10,000 years ago, neither food procurement nor the
acquisition of a place to sleep were necessarily contingent on social ability as they are today. The
discipline of behavioral ecology, which may be applicable here, is based on the premise that cognitive
traits and the neural substrates responsible for them are shaped by natural selection and hence are
fine-tuned to respond to a particular environment. The autism spectrum may be explicable in terms of
behavioral ecology in the sense that it represents the adaptive value of being fine-tuned to thrive in a
solitary environment.
Similar to mothers in contemporary foraging societies, mothers in the ancestral past would have
required their children to learn how to forage for food from an early age. Virtually all mammals,
whether of social or asocial species, use their systemizing abilities to learn food procurement
techniques from their mothers. The offspring of asocial mammalian species may not be motivated to
empathize or socialize much with their mother but are nonetheless driven by hunger to attend to her
examples and internalize her tactics. This strategy works well for hundreds of species of asocial
mammals (even ones with very small brains and limited systemizing abilities) and presumably would
have worked well for individuals with autism. Of course, it would be an immoral experiment to place an
autistic toddler and its mother in the wilderness for years in order to see if he or she learns and
thrives. However, would such an experiment be needed if it could be shown that autistic behaviors are
normal relative to those of other solitary animals taken out of the wild since birth (say a pet reptile, a
domestic cat, or a captive orangutan)? Are there reasons to think that an individual with autism, living
in prehistoric times, would not be strongly motivated by thirst, appetite, discomfort, sexual urges, and
other innate instincts to become nutritionally independent and to increase its reproductive success?
The behavior of autistic individuals is often seen as bewilderingly inappropriate in a social context,
because they so often become interested in or obsessed with socially meaningless activities. In a
natural environment though, it is likely that hunger would have motivated them to redirect their
obsessive tendencies toward food procurement. Today, their hunger for food does not drive them to
refine food procurement techniques because their parents feed them every time they are hungry.
Modern humans are responsible for social and academic learning and are rarely given the chance to
be positively reinforced by successful food acquisition. This temporal or causal pairing between
learning and satiety, integral for wild animals, has been artificially taken away from modern children.
Because the compelling and coercing natural instinct of hunger does not actuate or motivate modern
individuals with autism, their efforts and skills are misplaced onto irrelevant stimuli.
The powerful and mobilizing asocial fascinations and preoccupations seen in modern day autism
could have aided their prehistoric counterparts in self-preservation. Humans habituate to things that
they are not interested in and systemize things that they find rewarding, motivating, or intrinsically
interesting. In the ancestral past, activities leading up to the sating of hunger would have been highly
reinforced, and thus food procurement and food processing strategies would have been the primary
variables of the reinforcement schedule for individuals with autism. Perhaps, when children with autism
ignore their parent’s examples of social behavior today, it is because these examples seem boring
and meaningless whereas in the ancestral past they would have been inspired by their parent’s
examples of hunting and gathering activities. Today, because they are not able to forage or to watch
their parents forage and because they can obtain food free of effort, their interests are redirected
toward salient, nonsocial activities, like stacking blocks, flipping light switches, lining toys up in rows,
playing with running water, chasing vacuum cleaners, and collecting bottle tops. This kind of
misapplication of an innate tendency is known in behavioral ecology as “displacement behavior.”
The interests of autistic individuals often lead to the development of islets of aptitude or competence.
Despite obvious deficits, many show that they have refined or even mastered certain skills, often
referred to as splinter skills. These may include eccentric proficiencies, such as penchants for
drawing, music, rote-memory, or puzzle solving. In extreme cases, these skills might include what are
often called savant abilities including things like counting, hyperlexia, pattern finding, and calendar or
mathematical calculation. Some of these skills and abilities in autism were probably acquired because
the individual focused on a particular type of problem solving, they learned to systemize a particular
system, and then they retained and elaborated on the ability. In the past, these elaborate abilities
would probably have mapped onto the acquisition of foraging techniques, which, would have been
honed to proficiency through rote repetition and practice.
Many anthropologists studying hunter-gatherer groups exclaim that the foragers that they are
observing have cultivated amazing naturalistic abilities and are able to sense and perceive things to
which they themselves are virtually blind. These abilities, like tracking, ranging, stalking, food
processing, mapping of terrain, and knowledge of flora and fauna may be areas through which the
potential for autistic or savant-like abilities were channeled in the ancestral past. The same could be
said for the deep stores of specialized or technical knowledge exhibited by people on the autism
spectrum. This knowledge could have been dedicated to understanding edible and inedible species,
the habits of prey items, self-protection, maximization of food-collection efficiency, fashioning tools,
and procuring shelter.
Aside from their asocial preoccupations, individuals with autism also exhibit stereotyped and repetitive
actions. In fact, many mammals engage in perseverative or repetitious behaviors especially when
placed into unnatural or confining environments [43]. Repetitive, stereotypical behavior (including self-
injurious behavior) is very rare in the wild but very common in captive animals. More intelligent animals
seem to have increased capacity for self-injurious behaviors. It has been estimated that 10 to 15
percent of rhesus monkeys living alone in a cage develop self-biting, head banging and self-slapping
[43]. This may indicate that the living conditions that many young individuals with autism experience
are artificial, and possibly inhumane as they are not as stimulating or motivating as the wild
environment that they are born expecting. Surely if you aren’t focused on social interaction, you are
destined to focus on other systems. Recent clinical studies have shown that the intensive practice of
social skills and social subskills, from a very early age, can ameliorate many autistic symptoms. This
demonstrates that individuals with autism, like everyone else, have a wide array of potential talents
available to them from an early age but that because of neurological reasons, social developmental
windows close earlier for them.
The propensity toward strict adherence to routine seen in autism also close analogues in the “master
routines” and “subroutines” of many non-social species (e.g., waking, cautious emergence,
defecation, grooming, basking, foraging, returning, bedding, sleeping). Although such neurologically
mediated rituals can be repetitive and inflexible at times in many vertebrate species, they are
important regulators of adaptive behavior. Perhaps the persistent, stereotyped behaviors
characteristic of autism had ecological utility in the sense that they allowed structure, order, and self-
regulation. Why is there the tendency toward tedium and invariability then? Most of the variety in the
life of a human hunter-gatherer is probably found in its social interactions. There would probably be
much less variety in the life of a lone hunter-gatherer. A lone forager would be forced to engage in
lonely, repetitive, and stereotypic activities, such as scanning repeatedly for threats, picking and
processing fruit, searching for and extracting vegetables, and locating and capturing prey items. An
obsessive desire for sameness, repetition, and ritual makes little sense in the context of a social
setting but seems applicable in a lone setting. In fact, it might be bad to expect variability and to thrive
off unpredictability in a solitary scenario because of the monotony of living alone.
A mind that is highly geared toward using social cognition and forming emotional relationships would
have been disadvantageous in an individual who was forced by circumstance to live in a solitary
scenario. This may help explain why many individuals with autism have an undue preference for their
own company, pay attention to the non-social aspects of people, and treat others as if they were
inanimate objects. Concentrating on empathy or theory of mind would probably have been impractical
and counterproductive. Martin Brune [29] has speculated that theory of mind and social cognition are
probably vulnerable to dysfunction. He pointed out that most people at one time or another have
strong desires to ascribe intentions to non-animals. A solitary forager without autism may be likely to
attribute agency or ascribe intentions to inanimate objects, amounting to displaced and confused
behavior.
In her book “Animals Make Us Human” autistic professor Temple Grandin (who herself engaged in
repetitive, restricted and self-injurious behavior at a young age) points out that even though domestic
dogs and cats both live comfortably with humans, only cats can leave a human family to live solitarily
in the wild. “If you put the family poodle out in the countryside, his chances of surviving are low unless
he finds another family to live with. But abandoned cats do fine.” Here she underscores the fact that
some animals are obligately social whereas others can transition between social and solitary lifestyles.
Her life story is attestation to the fact that individuals with severe autism can make this transition while
maintaining the “cognitive coherency” to contribute profoundly to both industry and academia.
Like asocial animals, some individuals with autism avoid close bodily contact and fail to establish
emotional relationships. It seems that these tendencies might have facilitated solitary life whereas the
inclinations to seek out physical contact and emotional relationships could have made solitary life
miserable or unbearable. Classic works of fiction portraying humans abandoned or marooned from
others emphasize the human need for companionship. The protagonists in such works frequently
have unremitting obsessions with imaginary friends, behavior that would be extraneous and probably
maladaptive for a solitary human. People on the autism spectrum would be less likely to yearn for
companionship and more likely to focus on survival. Solitary species have very limited abilities for
social cognition. This is probably because gregarious predispositions, companionable inclinations,
and social instincts in general are maladaptive in a solitary context.
Autism and Orangutans: Solitary Foragers
Most animals, many mammals, and several species of primates seek out food on their own. It is clear
that such a solitary foraging strategy can even be effective for apes. In fact, orangutans, the species
that is genetically third closest to humans as revealed by molecular studies, live solitarily in the wild.
Orangutans eat, sleep, hunt, and forage on their own [44]. They are often described as cautious and
introverted, and it has been estimated that Bornean orangutans spend at least 95% of their time
alone [45]. Orangutans have low interaction and association rates, and only infrequently meet up with
conspecifics, often only to mate [46]. They have been reported to congregate in small groups
temporarily, but only to eat from a particularly fruit-laden tree. Several specialists emphasize that
orangutans have limited social aptitude and tend to prefer solitude [45]. Van Schaik [45] has
concluded that, unlike all other species of apes, well-defined communities do not appear to exist in
any orangutan population studied so far.
In orangutans, inculcation in infancy and juvenility consists of learning about food procurement from
the mother. In stark contrast, chimpanzees are indoctrinated into learning about both food
procurement and socialization [44]. Orangutan mothers teach their babies to recognize edible food
species, implement foraging techniques, find shelter from the elements, protect themselves from
predators, and develop effective ranging capabilities [48]. Compared to chimpanzees, young
orangutans do not learn much from their mothers about communication or socialization; however, they
do learn everything that they need to survive and reproduce.
Unlike their orangutan counterparts, chimpanzee adolescents reliably stay with (or at least frequently
visit) their mothers for months or years after they have been weaned [49]. Their strong affective ties,
emotional attachments, and gregarious nature keep mother and child in contact into adulthood or, at
times, throughout life [50]. Orangutans on the other hand, exhibit very different behavior. Adolescent
orangutans reliably leave their mother and go off on their own to forage very soon after weaning. The
bond between the mother and child is based largely on the provision of milk [48]. One of the most
frequent complaints of parents with autistic children is that they find it very difficult to build an
emotional bond with the child [6]. One might imagine that in an ancestral setting the bond between a
mother and her autistic child could have been as delicate (and yet at times, as ecologically
appropriate) as the bond seen in orangutans. Despite this relative delicacy, the vast majority of
babies and children with autism do develop true psychological attachments to their caregivers. This
attachment would probably have facilitated the acquisition of foraging techniques as it does in
orangutans.
Aside from orangutans, all apes, including gibbons, gorillas, chimpanzees, and bonobos are social
species, much like humans [51]. The ability to make inferences about the mental states of others
probably first emerged in a clear form during early primate evolution due to heavy selection from the
social environment [52]. Apes, relative to monkeys though, have elaborated greatly upon this ability
presumably because their social environment was even more important. To some degree, it seems
that orangutans regressed in their social abilities relative to other apes because, as their group size
decreased, pressure from the social environment must have decreased along with it.
Primatologists generally agree that orangutans live solitary lifestyles primarily because the islands of
Borneo and Sumatra, where they are found, have poor food density. Relative to the lush African
regions that chimpanzees and gorillas inhabit, these islands cannot support large groups of social
apes [44]. If orangutans lived in larger social groups, they would have to travel extremely far (up to 20
miles) each day simply to get enough food to sustain themselves [53]. Because orangutans are large
animals, and because the trees and bushes in their non-seasonal habitat go through various phases
of producing buds, shoots, and fruit, the orangutans must forage through many plants daily to ferret
out the right dietary foodstuffs [44]. Because they must traverse so much land everyday in order to
obtain a sufficient number of calories, orangutans must define large territories for themselves. It is
thought that in Sumatra, each orangutan needs nearly a square kilometer and that in Borneo each
requires an even larger area of spatial isolation [54]. Fascinatingly, Borneo is thought to be less food-
dense, and Bornean orangutans are thought to be less socially inclined when compared to Sumatran
orangutans [45]. Food density of the habitat is actually known to be an important determinant of group
size in a number of animals [55]. In fact, when food becomes scarce, even chimpanzees split up to
search for food alone [56].
Harsh and unpredictable climates marked much of the period of human evolution [57]. The
Pleistocene and Pliocene ice ages caused frequent aridity of the African plains, which would have
dried up many sources of food for our foraging ancestors [58]. These conditions would have rendered
our habitat nutritionally scarce [59], perhaps forcing groups of our ancestors to disband. Humans may
have been forced to do this habitually in the past despite the fact that we are an inherently social
species. In sum, environmental forces prominent during our evolution caused nutritional scarcity that,
at least in some geographic regions, may have created the same conditions that selected orangutans
to become solitary. Another condition that influences selection for sociality is the distribution of food
resources. It is known that when food is approximately evenly dispersed in an environment, solitary
foraging yields higher energy returns but when food is clumped, group foraging becomes more
efficient [55]. One may not even need to invoke scarcity or food distribution patterns to conclude that
many pre-social species, including humans, may have experienced historical disruptions (either
punctuated or gradualistic) in their social continuity that left lasting traces in the genome.
Adaptations particular to orangutans, which are thought to reflect their life-history strategies, may
have implications for autism. Orangutans show large sexual dimorphism [47] and much of this is
thought to be due to high testosterone levels in the males. Autistics are also thought to exhibit
elevated testosterone levels, which may be associated with increased fetal exposure to testosterone
[60]. It might make sense that a solitary forager strategy is better suited to males given that female
animals of many species have an increased need for social support and parenting proficiency. Male
orangutans actually spend a greater proportion of their time alone compared to females (most of the
time spent between two males is thought to be attributable to random, unintended encounters) [45].
Interestingly, autism is four times more prevalent in men compared to women, is accompanied by more
severe anomalies in brain structure in men, and is known to have a worse prognosis in men [6].
Higher testosterone levels in autistic males may have increased their sexual aggressiveness as well
as their sexual attractiveness. Mating practices of individuals on the autism spectrum may have been
similar to those of the orangutan. Females with autism or on the autistic spectrum probably had little
difficulty in procuring receptive sex partners. Males, on the other hand, could have procured partners
using the same tactics used by male orangutans, such as propositioning, perseverative soliciting,
provisioning/food sharing, and others. Like the association with testosterone, orangutans have other
biological predispositions that might elucidate enigmas in autism. Relative to chimpanzees,
orangutans have tendencies toward late weaning, prolonged lactational amenorrhea, and long inter-
birth intervals [48]. It might be interesting and instructive to look for tendencies like these, as well as
other peculiar life-history phenomena, in autism births. Moreover, specific tendencies seen in autism,
such as atypical eating, food consumption rituals, food faddism, and oral fixations may prove to have
ecological significance.
It would be amiss to neglect to mention two important things. First, there is some contention over
whether orangutans have been selected to be solitary or simply have relocated in the last 10,000
years to an environment that requires solitary foraging. If future comparisons demonstrate that
orangutans are relatively socially adapted, the present argument would be forced to abandon the
evolutionary (yet not necessarily the behavioral) comparison with orangutans. Regardless,
evolutionary comparisons with known solitary primates and other solitary animals that lack biological
instincts for gregariousness, affection, and strong social relationships should be informative. It is also
important to affirm that no offense is intended toward autistic individuals in this comparison with
orangutans. Only traits related to sociality are compared here, and individuals with autism are not in
any way equated with apes in general. In fact, individuals with autism are only compared to
orangutans here to the same extent that neurotypical individuals are compared to chimpanzees.
Orangutans were selected as a basis of comparison for this article simply because they are the
species of solitary foragers that is most closely related to humans.
The Evolution of Autism
The evolution of social tendencies in animals is a poorly understood topic of research. It is clear that
there are costs and benefits to living in groups and that the pressures from the social environment
can change abruptly [52]. Surely, in the past, individuals on the autism spectrum were sometimes born
into the arms of supportive and cohesive social groups, much like today. If autism does represent a
solitary forager strategy then this circumstance could have constituted a mismatch that may have
been negatively selected against. However, most adaptations have risks associated with them and
instead of representing a panacea; they reflect tradeoffs that, over evolutionary time, produce on
average more benefits than costs. This mismatch may not even have decreased fitness. Given the
propensities of modern day people with autism, such individuals might have chosen to journey off
alone nomadically or hermitically. Alternatively, they might have chosen to forage alone but return to
their group infrequently in order to interact and travel with group members. This may have depended
on “how autistic” the individual was. Even individuals on the far side of the autism spectrum may not
have even been truly solitary during prehistory. Social chess abilities or Machiavellian intelligence are
not well developed in people on the autism spectrum. Their speech and mannerisms can be
monotonous and mechanical, yet this may not have kept them from offering valuable skills and
forming functional relationships with other humans. In many cases, autistic individuals exhibit
pragmatic competence that could have made them valuable attributes to their foraging companions.
Today, a large proportion of individuals with Asperger’s have been assimilated into elite social and
professional spheres despite the fact that they are not adroit social conformers [34]. It is reasonable
to imagine that a proficient hunter with autism could be not only tolerated but highly valued by a small
band of wild men who were each eccentric themselves, not having been exposed to our modern,
relatively conformist, post-industrial society. This would be akin to the situation today where millions of
mainstreamed autistic children around the world have become accepted on the playground, despite
their differences. It is well accepted that despite valuing their time alone, many individuals with autism
enjoy friendships and have a documented predilection for valuing quality of friends over quantity [6].
Similarly, when in captivity, or when gathering around a large, fertile fruit tree, orangutans can be
observed to have social competencies, even if they are not as facile or effusive as chimpanzees.
As a rash speculation about the origins of autism, the present author would like to point out that
autism could be a remnant of genetic introgression that took place before humans were the lone
species in our genus. Perhaps the genes for autism evolved not in our direct ancestral line but in an
asocial subspecies or hominin which later merged genetically with our line of descent through gene
flow. Demes, or subpopulations of relatively solitary humans, could have been assimilated into our
gene pool after a fair amount of interbreeding following the migration. Such autistic subpopulations
could have arisen in ecologically or climatologically different geographic regions of Africa at anytime in
the Plio-Pliestocene.
The evolution of human populations with short stature may shed light on this issue. The pygmy
phenotype, characterized by small body size, appears to have evolved independently in South
American, African, and Southeast Asian populations in the last 200,000 years. The phenotype is
mostly restricted to rainforest environments and is thought to represent a response to the selective
pressures associated with living in a tropical rainforest [61]. This powerful climatological association
has motivated adaptationist hypotheses as to how small size would increase reproductive success in a
warm, wet environment with dense foliage. It now seems clear to physical anthropologists that short
stature may have increased the capacity to cope with one or more of the following: caloric limitations,
high temperature, high humidity, extensive forest undergrowth, increased mortality, or vitamin
deficiencies [61]. Multiple genetic disruptions of the growth hormone and insulin-like growth factor I
pathway are likely to be involved etiologically but no specific DNA mutations have been uncovered
[62]. It is clear though that when pygmies intermarry with non-pygmies, the result is a spectrum;
children tend to be intermediate in stature to those of the two parental populations [62]. Pygmy
populations show us a few things that are interesting in the present context: 1. isolated pockets of
humans can remain reproductively insulated for long enough to evolve discrepant ecological
strategies; 2. such populations can quickly (less than 40,000 years in the South American and Asian
pygmies [62]) develop features that vary markedly from the norm; 3. these traits can involve multiple
genes at different loci; and 4. interbreeding can result in either continuous or polymorphic variation in
subsequent generations. It is interesting to note that, as these indigenous people become assimilated
into other gene pools, the genes for short stature will persist and may affect phenotypic variability in
sporadic and unpredictable ways for a long time to come.
The genes that make up a polygenic trait usually all have different natural histories. They appeared
as mutations at different periods, and they were selected at different times. For example, several
different genes, each working through different molecular mechanisms, helped Europeans develop
lighter skin as they moved to higher latitudes over the last 50,000 years. Interestingly, an entirely
different set of alleles with unique cellular properties helped people in northern Asia develop lighter
skin. Few of these genes have been selected to fixation, and they occur together in different
proportions, creating a lot of variability in skin color even within isolated populations. The genes for
autism may be similar to these examples in the sense that each gene may have a different natural
history. Some genes may have been meant to work cooperatively whereas other not. Different genes
for autism may have evolved independently in genetically isolated gene pools. Each gene that
predisposes toward autism may represent a different way to make an organism a little less reliant on
sociality. Like other polygenic continuous traits, the mutations responsible for autism could have been
maintained by “environmental heterogeneity,” a form of balancing selection. In other words, the genes
responsible for autism may have remained in our gene pool because as social-environmental
conditions fluctuated in the past, discrepant genetic polymorphisms, or “multiple alternate alleles”
were favored. Currently, it is not possible to scrutinize the natural history of autism genes at this level
of analysis because even though over a dozen genes have been implicated, including an oxytocin
receptor, a serotonin transporter, and putative language genes, very little agreement exists in the
literature over the specifics [3].
It is currently not possible to tell whether the frequencies of genetic variants responsible for autism are
high enough, or in the right proportions, to accommodate a model of past positive selection. The
contributions of a large number of relatively rare genes and of de novo mutations [3] tell us that
autism may not have been subject to natural selection for thousands of years. It may have a large non-
evolutionary component as well as an irregular evolutionary signature. The present article cannot fully
address this issue primarily because of the lack of knowledge about the genetics of autism. It is
unclear whether autism can be explained better by rare mutations or by rare combinations of common
genetic variants [3,6]. Future linkage and association studies though may be able to resolve whether
the responsible genes are mutated sequences encoding aberrant gene products or normal
polymorphisms acting synergistically [9]. It is also clear that much of the clinical picture of autism is not
representative of adaptation. Some cases of autism are associated with congenital anomalies and
physical stigmata that were probably not adaptive. How and why rare comorbidities such as Rett
syndrome, tuberous sclerosis, fragile X syndrome, psychomotor epilepsy, and other clinical entities
can present with autism is far from apparent [2,4]. It may be the case that several pathological
conditions present symptoms that resemble autism (or exacerbate autism) and thus clinicians and
epidemiologists have grouped them together with autism, obscuring its natural and unadulterated
visage (as described here). This is certainly the case with ADHD and schizophrenia. Idiopathic or
teratogenic neurodevelopmental insults can closely mimic these as well as other neurodevelopmental
disorders [3]. Many of these unknowns will not become clear until additional genetic, molecular, and
pathophysiological research is done.
Another important criticism of the present argument is that there may be no clearly documented
anthropological evidence for solitary foraging in modern humans. This shouldn’t be a huge blow to the
present hypothesis given that there do not seem to be any documented examples of autistic
individuals in hunter-gatherer groups either. It seems that no one has looked for these things
systematically. However, the prevalence of autism in forager groups should be equivalent to the
prevalence in the rest of the world, given that epidemiologic studies indicate that environmental
factors, such as perinatal insults, prenatal infections, teratogens, and toxins (factors that may be
particular to modern life) account for only very few cases of autism [4]. In modern society, it is
apparent many people intentionally sequester themselves or prefer solitude to different degrees, and
it is reasonable to expect that this type of behavior could have been continuous with the autism
spectrum in the ancestral past [34]. Several substantiated cases of abandoned youths, known as feral
or wild children, have survived to adulthood in the wild [52]. It is probable that far more children were
orphaned in the wilderness in ancestral times, and this may have contributed to the selective
pressures for asocial temperament. Feral children usually have many autistic features in the sense
that they are emotionally detached, prefer solitude, and eschew physical contact [52]. These features
may constitute protective, adaptive responses to solitary life, and their resemblance to the symptoms
of autism may not be coincidental.
A consistent, predictable proportion of ancestral humans (and ancestral hominins) must have been
forced to live alone during the six million years of human prehistory. From these individuals, the ones
with mutations that created disruptive abnormalities in regions of the social brain may have
experienced increased reproductive success. This should not sound like a stretch, considering that
this general pattern may have already occurred during the evolution of orangutans. After orangutans
diverged from the common ancestor that they shared with gibbons around 18 million years ago, they
moved into Southeast Asian rainforests where they were forced to give up their social lives for solitary
ones because of ecological constraints [53]. It appears that from the consistency of their behavior in
and out of the wild, their solitary tendencies were instinctual and innate and that selective pressures
acted upon their brain. For this reason, it should be very informative for autism researchers to
compare the brain areas responsible for social cognition across apes. This kind of research has not
yet been done. We may never know exactly what it was like to have autism in prehistoric times but
comparing them to known solitary species on a variety of neurological measures may provide
clarification.
Autism and Epigenetic Programming
It is my hunch that autism, like other syndromes studied by the discipline of phenotypic plasticity, is
epigenetically programmed in utero by cues indicative of social scarcity. I imagine that a number of
different cues coming from a sparse social environment might program the autistic phenotype. There
seems to be some evidence for one such environmental determinant. This section will offer tenuous
speculation as to why advanced paternal and maternal ages are risk factors for the development of
autism. Twin studies suggest that interactions between multiple genes cause "idiopathic" autism but
that epigenetic factors and environmental modifiers may contribute to variable expression of autism-
related traits. One of the most significant environmental risk factors for autism is advanced paternal
age. In fact, recent studies have shown that individuals born to older fathers (aged 45-50 years or
more) are 2 to 3 times more likely to develop autism. Studies that have attempted to correct for factors
that might confound the association between autism and advanced paternal age, such as personality
traits, have suggested that the effects of possible confounders are low and that the association is
genetic in nature [80].
A solitary strategy would work better if it was tied to an environmental cue that served as a reliable
predictor that solitary status is likely. Advanced paternal age may be such a cue. In modern hunter
gatherer groups fathers are known to play significant roles in teaching their offspring how to hunt and
become socially adept. A child that will be neglected of its father is in a unique and disadvantaged
position. But what signal could warn a developing embryo that it may be neglected of paternal care?
Surely older fathers have a higher probability than younger fathers of dying before they are able to
raise their children into full adulthood, especially in ancestral times when people lived far shorter lives.
For this reason, it may be adaptive for an individual who is born to an older father to have a
propensity for autism simply because older fathers are more likely to die before they have the
opportunity to provide their offspring with social instruction and socialization. Furthermore, it might be
harder to integrate oneself into a pack or tribe in the absence of one’s father. Because a father’s
presence is probably more important to a male than it is to a female, particularly because males
engage in hunting activities, a father may be more valuable to a male.
The Selectivity of Neuropathological Changes in Autism
The brain abnormalities in autism do not seem to constitute indiscriminate pathological abnormalities
as one might expect if autism were simply a disease. The most conspicuous brain abnormalities seem
to affect the areas of the brain that have been associated with social cognition consistently and
systematically [63]. The amygdala, the anterior cingulate cortex, the orbito and medial frontal cortex,
and the mirror neuron system have all been strongly associated with social cognition [64], and also
have been shown to be the same areas that exhibit the prominent anomalies in autism [65]. This
selectivity makes it seem that the genes that influence social areas of the brain were affected and the
others were spared (consonant with Baron-Cohen’s psychological theories of autism). According to
Gelman and Williams [66], brains have been evolutionarily organized to acquire and store particular
sorts of relevant information and disregard other, irrelevant information. In this context, individuals
along the autistic spectrum appear to have a neurological propensity to gate out social information
and thus gain conscious access to highly processed nonsocial perceptions and schemata. This
section will look at how this might be accomplished by focusing on eye contact, facial expression,
oxytocin regulation, macrocephaly, amygdala reactivity, and facial recognition in autism.
Curious phenomena that are very common in autism are averted gaze and poor eye contact [67].
Autistic individuals describe eye contact as uncomfortable and even threatening [34]. Interestingly,
eye contact is also very rare in the vast majority of solitary species, including orangutans, who actively
avoid both direct gazing and even facing [68]. Chimpanzees and gorillas share gazes and use the
eyes for communication frequently, just like most humans [69]. Chimps and gorillas use eye contact
constantly while interacting with members of their own group but may charge at an individual from
another group if it makes eye contact. Staring between unfamiliar apes is often interpreted as a threat
signal; therefore, it is best for the solitary orangutan to avoid both eye contact and direct gazing in
order to forestall an attack [69]. Orangutans actively avoid gazing and eye contact and this tendency,
very common among solitary animals, has been explicitly interpreted as adaptive for their solitary
foraging niche [68]. Instead of face-to-face direct viewing, orangutans, like individuals with autism,
glance momentarily at others sideways with the head turned away [70]. The neurological substrates
(including amygdalar sensitivity) that underlie this very specific and prominent tendency may have
evolved for the same adaptive, defensive reasons in both autism and orangutan.
Studies have shown that autistic individuals are less expressive especially with respect to facial
communication. They make fewer facial expressions and are rated as more flat or neutral in affect by
observers [71]. This absence of facial responsiveness is probably due to underlying neuronal
mechanisms and there is evidence that the facial motor nucleus is significantly reduced in size in
autism [72]. Fascinatingly, the size of the facial motor nucleus is thought to vary predictably in total
volume as a function of group size in monkeys and apes. The larger the average group size, the more
important facial expressiveness is and the larger the facial motor nucleus must be [73]. It should be
informative to analyze the anatomical organization of the autistic facial motor nucleus (VII) taking note
of the general size, the placement of motor neurons, distribution of neuron types and the general
topography of muscle representation. Experts in phylogenetic specializations of the primate facial
motor nucleus might be able to perceive social or ecological traits in the distinctive anatomical
organization of the facial motor nucleus in autism. Analyzing it relative to the orangutan facial motor
nucleus should be informative as well, as it is known that orangutans are much less facially expressive
than other apes [74]. Solitary foragers do not need to communicate with their faces as much since a
large proportion of the interaction that do occur between them and members of their species might
have taken place from a distance. This may evince that brain areas associated with facial expressions
were less important for a solitary forager and that less plasticity, learning, and potential for
interpersonal variation was needed in these domains.
Oxytocin, a neuropeptide thought to enhance social learning, social expressiveness, direct eye gaze
and the ability to remember faces [75], is reduced in autism subjects. Diminished peripheral levels of
oxytocin may play a large role in retuning social brain modules in autism [76]. It has been shown that
intravenous oxytocin produces a significant reduction in stereotypic behaviors in adult autism subjects
and increases empathy in people without autism [77]. Variation in oxytocin levels in mammals reflects
adaptations to the social environment, just as it may in the case of autism. Animals that rely on pair-
bonds and social attachment have higher levels of plasma oxytocin, especially when it is behaviorally
relevant, like during childbirth or monogamous sex [52]. Comparing prairie voles to montane voles,
two very closely related species of rodent, is interesting in this context because the two differ widely in
bonding behavior. The montane vole, relative to the prairie vole, has a much smaller number of
receptors in the brain for oxytocin and unlike the amorous prairie voles they do not form pair bonds
[55]. The montane voles have fewer receptors and thus are less responsive to oxytocin making them
more wary, suspicious and more easily frightened of other members of their species- ensuring that
they do not allow themselves to become vulnerable [55]. It is thought that the wide discrepancy in
social behavior between these two species reflects adaptation to two very different physical and social
environments [63]. Not surprisingly, interspecific, seasonal and reproductive variation in oxytocin
concentrations have been attributed ecological and adaptive significance. High levels are associated
with trust, love, pair-bonding and generosity in a variety of mammals [63]. It would be interesting to
compare the details of oxytocin action- such as receptor number and distribution- in solitary animals
with that of people with autism.
Diminished peripheral levels of oxytocin may cause individuals with autism to be born “expecting” or
prepared for a socially impoverished environment. Natural selection cannot act on behavior directly
but instead acts on the neural substrates that generate the psychological mechanisms that create the
behavior. Evolved psychological mechanisms, such as cognitive modules, are generally understood in
terms of specific inputs, decision rules and outputs [78]. Most of these mechanisms were naturally
selected to be sensitive to a narrow range of perceptual information. In other words, they are
biologically prepared to learn about or solve particular adaptive problems. Some mechanisms are
known to be domain-specific and many of these are assumed to exhibit variation in humans causing
some people to attend to perceptual cues that others might miss entirely. Tuning differences in
domain-specific mechanisms or modules, especially those involving oxytocin, serotonin and
adrenaline may underlie the differences in autistic cognition and, like other differences seen in nature,
may have been created by natural selection to help solitary foragers face their particular set of
recurrent or ecologically relevant threats and opportunities. They have direct access to lower level,
less processed information. People with autism have normal early vision (what a baby sees) and
normal middle vision (features extracted). Their high level vision is even ok (scene understanding).
They just never learned the social things to have a social, top down understanding.
It has been shown that one or both hippocampi are often significantly larger in autism [4]. The
hippocampus is a convergence zone that is integral to spatial ability and perhaps the large size of the
hippocampus in autism has neuroecological relevance. The study of neuroecology focuses heavily on
the relationships between ecological hardships, spatial abilities and the size of the hippocampus
[79,80]. It might make sense that a solitary forager would have to compensate for the fact that it could
not rely on the spatial abilities of its companions. Individuals with autism have also been shown to
excel at the block design subtest of the Wechsler intelligence scales, evidence of high visuospatial
abilities [6]. It may be possible that such abilities are indicative of the visuospatial rigors particular to a
solitary existence.
Brain growth is accelerated in early autism and this may have been adaptive for a youngster that was
preparing to become a solitary forager. Individuals with autism have been known for years to exhibit
increased head circumference and increased brain size (two highly correlated traits) during infancy
[81]. Macrocephaly (head circumference higher than the 97th percentile), one of the most consistently
encountered traits seen in autism, is known to affect between 14% and 34% of all autistic infants [82].
In fact, total brain size, as revealed by MRI, is on average 5-10% bigger in autism between 18 and 48
months of age [4]. Neonates who will develop autism and macrocephaly exhibit normal head
circumference at birth, but their head growth accelerates during the first year of life. This growth
continues until at least 4 years of age, slows and then decelerates prematurely [81]. This increased
rate of brain maturation and growth slows early, well before nonautistic brains start to slow, resulting in
very similar head sizes between those with autism and those without in both adolescence and
adulthood [82]. A large contributor to increased brain size in youth is myelination and it is thought that
different areas of the brain myelinate differentially according to an ecological program where primary
sensory areas begin to myelinate first well before association cortices. The dramatic increase in head
circumference from birth until 4 may be evidence that the learning arc for a solitary forager is not as
protracted as it is for other children. Hunter-gatherer children need to laboriously and perseveringly
learn a language along with social morays, a time-consuming process [84]. Researchers have
concluded that learning during human childhood is so long because social interactions are more
variable and relatively less frequent than other forms of ecological learning [85]. The fact that
individuals with autism show this increase in head and brain size between birth and age 4, may
indicate that they are programmed to learn more about their environment, faster, at an earlier age.
This may also suggest that autism represents a precocial (“out of the nest early”) strategy where
learning takes place very early so that the animal can become independent of its mother quickly, a
characteristic observed in countless nidifugous animals that do not have complex social lives.
Consistent with this conclusion are data evincing that many males with autism exhibit precocious
puberty [86], further evidence of a precocial as opposed to an altricial strategy.
Other neurological differences particular to autism may hold neuroecological significance. Individuals
with autism have been widely reported to have increased fear activity in the amygdale and high levels
of anxiety [87] which might act to up regulate vigilance, caution and a healthy fear of strangers. It
seems reasonable to assume that a solitary forager would benefit more from such caution than would
an individual who must learn to embrace companions. Some frequently reported, but only preliminarily
researched, eccentricities in autism include acute and perceptive hearing, and increased frequency
of, and interest in, smelling [88]; two features that could certainly be interpreted in a neuroethological
context. Face processing, a key factor in the development of social perception, is severely impaired in
autism [89]. The area responsible for face recognition, the fusiform face area, in particular has
demonstrated reduced activation in autism during facial discrimination tasks [90] indicating that in
autism, like in many solitary animal species, identity recognition may not be as valuable. These
neurological differences may be telling. If autism truly does represent a solitary phenotype, then the
brain abnormalities -which are highly consistent across individuals on the spectrum- should furnish
theoretically intriguing insights, not only into comparative cognition but also into human evolution.
The Relationship to Advanced Paternal Age
This section will offer a brief speculation as to why advanced paternal and maternal age are risk
factors for the development of autism. One of the most significant risk factors for autism is advanced
paternal age. In fact, recent studies have shown that individuals born to older fathers (aged 45-50
years or more) are 2 to 3 times more likely to develop autism. Studies that have attempted to correct
for factors that might confound the association between autism and advanced paternal age, such as
personality traits, have suggested that the effects of possible confounders are low and that the
association is genetic in nature (Croen et al., 2007).
In modern hunter gatherer groups fathers are known to play significant roles in teaching their
offspring how to forage and hunt. A child that will be neglected of its father is in a unique and
disadvantaged position. But what signal could warn a developing embryo that it may be neglected of
paternal care? Surely older fathers have a higher probability than younger fathers of dying before
they are able to raise their children into full adulthood. For this reason, it may be adaptive for an
individual who is born to an older father to have a propensity for autism simply because older fathers
are more likely to die before they have the opportunity to provide their offspring with social instruction
and socialization. Furthermore, it might be harder to integrate oneself into a pack or tribe in the
absence of one’s father. Because a father’s presence is probably more important to a male than it is
to a female, particularly because males engage in hunting activities, a father may be more valuable to
a male. Interestingly, autism is four times more prevalent in men than women, is accompanied by more
severe anomalies in brain structure in men and is known to have a worse prognosis in men (Frith,
1991). It also makes some sense that autism might be more valuable to males because of their lower
need for social support and parenting ability.
The phenotypic characteristics of a large number organisms ranging from plants to insects to humans
have been shown to make plastic responses to early environmental events, many of which are
thought to represent defensive or reproductive strategies (Pigliucci, 2001). Perhaps all humans, and
especially those that carry autism susceptibility genes, are sensitive to chemical markers that indicate
the age of the parents. This may be accomplished by epigenetic programming, where different
messaging pathways cause DNA acetylation or methylation changes altering gene expression. In
other words, autism could at least involve a predictive, adaptive response to anticipated social
deprivation, parental estrangement or orphanage. It is possible that autistics may represent
phenotypes that would have been able to make it on their own, from a younger age, if they were
separated from their father or mother during early childhood. In this sense, autistics may have been
more nidifugous or precocial as babies and young children. In sum, the parental age effects may
indicate that autism is associated with a genetic imprinting strategy that attempts to compensate for
the social handicaps related to having an older father, an older mother, or both.
Conclusions
This article has attempted, in an exploratory manner, to provide a characterization of autism that
reconciles known findings with evolutionary theory. In the past it was not understood how autistics
might have gotten along during prehistoric times. Yet now, a comparison with solitary species shows
that autistic individuals may have lived largely solitary lives and yet still have achieved self-sufficiency
and reproductive success. Characteristics of autism that have been interpreted as consistent with the
solitary forager hypothesis include: the high systemizing abilities, the obsessive and perseverative
tendencies, the repetitious and ritualistic tendencies, splinter skills, the deep but narrow stores of
knowledge, macrocephaly, gaze aversion, absence of eye contact, the parallels with orangutans,
increased hippocampal size, precocious puberty, the testosterone effect, reduced fusiform face area
and facial nucleus activity, reduced oxytocin concentrations and the sex ratio. Clearly, there are
exceptions to each of the lines of evidence offered here. For instance, there are innumerable
functional differences between the social mind of an orangutan and that of the average individual with
autism. The growing body of experimental literature testing the social-cognitive skills of solitary
species, may offer important insight into autistic traits though and tell us more about the similarities
and differences in these domains. It will be very difficult to determine irrefutably if what we know as
autism today was in fact an adaptive phenotype in the ancestral past. The hypothesis presented here
is underspecified and vague but may be progressive as it is thought that analyzing disease states
from an evolutionary perspective can ultimately do much to inform and influence medical theory and
ultimately even intervention strategy [12]. Furthermore, the evolutionary perspectives delineated here
could potentially provide structure for empirical investigations in animal behavior or cognitive
neuroscience.
The case that autism represents a solitary forager phenotype is a very top-heavy argument that can
and probably should be toned down in numerous ways. First, individuals with autism were probably
not totally solitary and may even have been better adapted to very small groups than to a purely
solitary existence. Second, it should be emphasized that natural selection may have only been
selecting for subclinical autistic traits and that the high-functioning corner of the spectrum was the
substrate for natural selection. Third, the most severe cases of autism may actually have been
maladaptive and due to assortative mating of two individuals that hold a high number of autism
susceptibility genes. A comprehensive solitary forager theory of autism must address these issues
and should start by partitioning variation in autism into nosological categories that represent either
pathology or nature. This article has not clearly committed to delineating which aspects of autism are
adaptive and which are not, or even committed to whether the lowest functioning forms could have
had a place in nature. It is probably too early to make these discriminations. However, it seems that if
the extreme end of the autism spectrum was selected for, that the distribution of autism prevalence
relative to severity might be bimodal or have a fattened tail yet this does not seem to be the case.
It is probable that other endophenotypes and genetic patterns in autism can be analyzed in terms of
the present hypothesis to provide convergent evidence. Eventually, genetic hypothesis testing could
provide near incontrovertible proof for the present hypothesis. Until then, comparing intron relative to
exon mutations in susceptibility genes for autism, mapping gene linkages, analyzing epigenetic
patterns, scrutinizing genome-wide microarray analyses and even looking for inter and intraspecific
trends in genographic data may all provide further support. Further comparative behavioral studies
may also be revealing. It will be instructive to look for abilities in autism that more closely resemble
foraging activities than the abilities mentioned here. Conversely, looking for anthropological evidence
for autism-like traits among successful foragers should also be revealing. It is also important to
question whether the processing deficits or cognitive findings in autism might have put a solitary
forager at a disadvantage. In order to further explicate the evolutionary roots of autism and draw new
inferences with the potential to inform medical intervention. I encourage researchers to turn to
ethology and Niko Tinbergen’s [91] “four questions” in understanding the value of behavior: 1) what is
the immediate benefit to the organism? 2) What is the immediate consequence? 3) How does it
develop in the individual across ontogeny? 4) How did it evolve in species across phylogeny?
Knowledge about the costs and benefits of solitary existence may help to guide each of these lines of
research.
A number of other, sensible evolutionary theories of autism have been put forth. Autism has been
conceptualized as a low-fitness extreme of a parentally selected fitness indicator [92]. These authors
have advanced that variation in the ability to connect socially to ones parents, an adaptive trait, may
have a maladaptive extreme form that results in autism. Another pair of researchers has envisioned
autism as a consequence of paternally imprinted genes [93]. This idea has been taken further and
autistic-like traits have been seen as a male-typical strategy geared toward parental investment, low-
mating effort and long-term resource allocation [94]. These and other theories of autism are well
supported and plausible. None of these is directly compatible with the present hypothesis but again,
neither are they directly contradictory. Like the present hypothesis they view autism as an extreme
end of a continuum constituted by adaptive traits. Oliver Sacks (1970) has formulated an interesting
view of autism that appears to be consistent with this [95]. He points out that the perspective of the
autistic individual appears aboriginal because they never succumb to our modern societal
conventions, but that this does not keep them from perceiving, thinking and behaving in original,
innovative and intelligent ways:
“The autistic by their nature are seldom open to influence. It is their ‘fate’ to be isolated and thus
original. Their ‘vision’, if it can be glimpsed, comes from within and appears aboriginal. They seem to
me, as I see more of them, to be a strange species in our midst, odd, original, wholly inwardly
directed, unlike others.”
Studies in behavioral genetics have demonstrated that tendencies for cooperation, temperament, and
sociality have biological underpinnings. Furthermore, they show considerable variability within and
between animal species [96]. Tendencies for independence, reclusiveness, introversion and other
traits characteristic of autism certainly show a great deal of variability between species [97], but what
about within them? Perhaps populations of other social species such as chimpanzees have an equally
low but consistent prevalence of autistic individuals as well. No formal diagnostic criteria are available
for psychiatric or even social disabilities in other animals [98], but it would be interesting, although
difficult, to see if there are analogues, or possibly homologues of autism in other species. If there were
homologues of autism in species closely related to humans, it would be relatively easy to use
molecular techniques to show this given that the genes responsible could be identified. Looking at the
evolutionary signatures in the behavioral genetics of autism, might tell us more about the variability in
life-history strategies of our ancestors. Many different factors could be invoked in such a discussion
including behavioral neuroscience, cognitive primatology, the importance of group membership to
survival, fission-fusion sociality, maverick males, paleoneurology, parenting methods, phenotypic
plasticity, sexual selection, sexual strategies, social and affective neuroscience, pair bonds,
territoriality, social hierarchy, and others.
“Because humans are highly social, we like to think that complex societies represent the crowning
achievement of evolution.… Indeed, if group living were the universally superior lifestyle, we would
expect social species to outnumber solitary ones, but exactly the opposite is true. Why? Almost
certainly because in many environments, the costs of living with others are prohibitively high.”
-John Alcock [99]
Humans are an innately social and gregarious species [95]. Their ancestral environment, like that of
chimpanzees, selected them to be this way. It is probable that as with all traits selected for by
evolution, there are costs and benefits associated with social predispositions and that the large
variation in sociability in our species reflects the large variability in selective forces in the past. The
relative prevalence of autism is high enough to suggest that it was a phenotype that was naturally
selected for in the past, but also low enough to suggest that it was rarely preferable to the socially
typical alternative. Perhaps the prevalence of the autism spectrum in modern human populations tells
us something profound about the frequency of forced solitude in ancestral times, the adaptive value
of social instincts and the plasticity of human cognitive strategies. Showing that autism had ecological
viability and that it exists today because of its success in the past suggests that it should not be
considered a disease, but instead a condition. It should not be thought of as something to be
ashamed of, but as something that represents individuality, self-determination and autonomy.
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