Neanderthal Project

The Emergence of Modern Human Behavior Through the Inclusion of Difference

June 2nd, 2010
Neanderthal

The emergence of “modern human behavior” in our distant ancestors is an enduring mystery that continues to puzzle archaeologists and anthropologists as they comb through the fossil record, uncovering the evidence of its expression. Such behaviors emerged around 50,000 years ago during a period in which anatomically modern humans began to move out of Africa and spread into territories in western Asia and Europe which were already populated with other archaic varieties of the genus Homo, such as Homo Neanderthalensis and Homo Erectus. The behaviors which are considered “modern” are generally identified as those which spread with modern humans as they left Africa, and scientists have been primarily interested in technological, economic, social, and cognitive changes which appear to be unique to this wave of modern humans who eventually replaced the other archaic lineages throughout Europe and Asia. There are a variety of expert opinions on modern human behavior in regards to how suddenly or gradually it appeared, or whether or not it included any other archaic Homo species such as Neanderthals.

Neanderthal

Several theories have been put forward to explain the arrival of modern human behavior on the scene, and most of them describe some cognitive change which precipitated the behavioral one. Notable among such theories are the fluid linkages of mental modules, development of modes of consciousness (altered states), improvement in “working memory”, and the rise in capacity for symbolic thought. Some elements of modern behavior seem to have first arisen as far back as 100-160,000 years ago around the same time that anatomically modern humans appeared. Mitochondrial DNA evidence also indicates an expansion of anatomically modern humans out of Africa around 60,000 years ago concurrent with the change to modern human behavior. The evidence also shows a marked contrast between the behavioral adaptations of anatomically modern humans and Neanderthals as modern humans spread into Europe around 40-30,000 years ago.

Neanderthal

The archaeological record consistently defies attempts to apply progressive models of behavioral changes, however, and the failure to develop a simple “spread” model brings into question the direct link between biological changes and behavioral changes. For example, behavioral changes are less abrupt in Asia where the adoption of symbolic behavior is much more gradual. In the Levant, anatomically modern humans were present but did not display modern behavior at around 80,000 years ago, and were replaced by Neanderthals. Yet, the fossil record in Europe shows that Neanderthals had the capacity to reproduce modern human behavior in tool making and in personal ornamentation. Some researchers have suggested that the changes resulting in modern human behavior may have been precipitated by some social component, but the nature of such a social component remains ambiguous.

Neanderthal

The evidence of the arrival of modern human behavior is composed of artifacts which cover vast swaths of space and time in parallel with the expansion of modern humans into western Asia and Europe, and thus arises the simple assumption that modern humans brought the behaviors with them as they emerged from Africa. However, the record is more complex than this, and actually indicates no absolute association between anatomically modern humans and what is referred to as modern human behavior. There is no explanation for why modern human behavior arises significantly (about 100,000 years) later than the first appearance of anatomically modern humans. However, the absence of evidence for modern human behavior earlier than this is not evidence of absence of such behavior, and so the debate continues. More recently, researchers have looked to demographic changes in early human populations for clues. Such research suggests that larger and more stable groups might be more likely to adopt new inventions and reproduce them with fidelity, but this speculation does not address why such demographic changes occur at all. The emphasis on behavioral traits and the absence of a coherent theory of social and cognitive structures underlying them may be an impediment to achieving a better understanding of the rise of modern human behavior.

The following hypothesis posits that the dominant assumption that modern human behavior can be characterized or explained by a single “modern mind” against which earlier human minds might be compared is in error. Rather, a range of cognitive types within populations might offer a better explanation for the origins of modern human behavior, as social mechanisms emerged to cope with a variety of cognitive expressions. The autistic spectrum is used as an example of different cognitive expressions which, through their integration within society, present the dynamic engine behind the emergence of modern human behavior.

Cognitive difference

Advances in the understanding of autism in recent years has led to a new appreciation of cognitive differences in humans and a few in the field of archaeology are beginning to look again at its potential role in the development of human social structure and innovation. The role of autism has been overlooked in the study of human prehistory due to the fact that its diagnostic criteria focuses strongly on extreme and antisocial behavior associated with it, and thus individuals with autism are typically regarded as being “outside” of society. However, autism is a spectrum of cognitive differences rather than a single condition and in recent years the study of high-functioning individuals diagnosed to be on the autism spectrum, particularly those diagnosed with Asperger’s Syndrome, has led to the understanding that such individuals, while cognitively different from the majority of the population, are not significantly excluded from social interaction. Indeed, such individuals can use language effectively and are often relatively successful in life. Those who are diagnosed with Asperger’s Syndrome are unable to feel empathy, or the appropriate emotion in relation to the feelings of another. Such individuals may still be considered socially competent because they are able to predict behavior, using a rule-based method which more or less works. Although such individuals may think differently, they may not behave in extremely different ways as they are able to learn and modify the set of rules they use to predict behavior and are willing to discuss what constitutes an acceptable rule.

Individuals with Asperger’s Syndrome are typically gifted in terms of pattern recognition and in determining predictability in complex systems. This particular gift gives them the ability to provide original insight. Their unique cognitive gifts, when coupled with their tendency to shun social interaction, their hunger for learning, and a tendency towards single-mindedness to the point of obsession makes them uniquely suited to driving innovation in the arts and sciences. It may very well be the case that people with Asperger’s and other high-functioning autism may be playing a crucial role in society, and that they have been since prehistory.

Recent attempts to determine the number of people on the autism spectrum in the UK have produced a number between 0.9% and 2%, which is consistent with similar studies in other populations.

While it is clear that individuals on the autistic spectrum think differently than the rest, there are competing theories regarding its underlying neurological basis, including decreased function in mirror neurones and differences in high-level neural connections. In general terms, however, autism is most often characterized as a failure to develop a Theory of Mind, which is the ability to intuit the beliefs and intentions of other people. Despite such deficits, the individual with Asperger’s Syndrome can often negotiate through social interactions by relying on a rule-based system for determining which responses are expected.

That having been said, it is in intimate social relationships where the individual with Asperger’s is at a greater disadvantage. Such individuals are unable to empathize with the feelings of others or develop a genuine emotional rapport, thus they are unable to invest in another’s feelings and well-being. Emotional rapport is an essential ingredient in all intimate social bonds, and it is essential to development in early childhood. Reciprocal emotional bonds are also important in adulthood, forming a critical component of love, remorse, and compassion – the glue which holds society together. These complex emotional ties allow us to regularly sacrifice our time, energy, money, and even our own safety on behalf of others.

Individuals with Asperger’s Syndrome behave differently in social relationships in two important ways. In the first case, such individuals remain oblivious to the nuance of humor and subtle emotional cues which pass among people in ordinary situations. They can detect rhythm and patterns, but cannot effectively join in this mode of communication. Secondly, such people are motivated by different goals than neurotypical people, and report that they derive pleasure or satisfaction from creative breakthroughs or in bringing their ideas to life in the real world, either as inventions or technical innovations, or works of art, etc.. Those with Asperger’s Syndrome, like neurotypicals who suffer from attachment insecurities, are not a part of the network of caring in a community. Such networks are composed of people for whom compassionate action and altruistic behavior bring satisfaction. Sadly, the individual with Asperger’s Syndrome has no such compulsions. In fact, one of the criteria for diagnosing historical figures with Asperger’s Syndrome has been the failure of such figures to adequately care for the vulnerable or infirm within their own intimate social circle.

The combined talents and motivations of individuals with Asperger’s Syndrome often lead them to make significant advances in the arts and sciences. Such individuals seem to be drawn towards academia, engineering, computer science, and even politics. Indeed, there are many important historical figures representing these fields and others who have been retroactively diagnosed with Asperger’s Syndrome, leading us to reconsider the social role played by such people.

It is reasonable to speculate that those with Asperger’s Syndrome have occupied analogous social roles since prehistoric times. “Difference” in ways of thinking is often accepted and may have resulted in specialized roles in small communities. It is easy to imagine how such individuals may have excelled in producing technology or understanding natural systems, and their obsessive focus might result in exceptional hunting or tool making skill. Difficult or controlling people who are not particularly empathetic may have been used to negotiate with other groups or to ruthlessly employ force to control the behavior of others. Having a distinctly different mind may have conferred a certain spiritual status upon such people in prehistory, as well.

Autism is highly heritable, and some research suggests that preferential mating between like-minded people with Asperger’s Syndrome may explain the maintenance of autism in the general population, and it appears that such unions may be generating more extreme forms of autism in their offspring. This contingency is supported by evidence of high rates of autism amongst engineers and their families, and in particular those who work in information technology. Furthermore, there are geographical hot spots of severe autism at Cambridge, MIT, and Stamford. There are other factors which help to maintain autism, however, such as the potential for success and achievement for people with “a dash of autism” which is sometimes translated into an attractive social status. Autism, then, is sometimes advantageous to those who possess the genetic basis for it and is being maintained in populations through diverse means.

It is clear that autism and autistic individuals may have played a significant role in evolutionary history, from both a biological and cultural perspective. Autism and other cognitively based differences in mind may have a more crucial role in modern society and in its historical precursors than we have so far allowed, if only because our modern perspective of cognitive difference as a “defect” precludes our ability to conceive of it. Certainly, the evidence is strong enough to warrant a reappraisal of the history of humanity through this lens.

Autism in Prehistory

Acknowledgment of the talents and social situation of those on the autism spectrum may help us to gain insights into the novel developments in human culture around the Middle-Upper Paleolithic period.

For example, the cave art found in south-west Europe which is dated to the Upper Paleolithic has been found to share distinct similarities to artwork produced by so-called “autistic savants”. An unusual attention to detail and a literal representation of the world are analogous to the artwork created by such savants. The archaeological record contains other artifacts from the same period in prehistory which indicate a more symbolic way of thinking about the world, including burial practices and personal ornamentation, however, and furthermore there are earlier examples of cave art, such as that found in the Blombos Cave, which demonstrate a more symbolic thought process. It seems rather unlikely that all of the artworks found during this period were the product of autistic genius, however, considering the high frequency and similarity of cave art and portable art against the expected rareness of such autistic individuals in those populations.

There is no need to resort to an either/or scenario when considering whether or not autistic individuals were responsible for the artwork found in the Upper Paleolithic era. Again, it is clear in modern times that there are many who fall on the functional end of the autism spectrum, such as those with Asperger’s Syndrome, and it is also quite clear that such individuals are successful not only at generating unique artwork and technological innovations but also in transmitting those ideas to others. Consider such historical luminaries as Einstein, Darwin, Newton, Mozart, and Van Gogh, for instance. In today’s world, many of us have adopted the regular use of such innovations as the Internet and cell phones, though we undoubtedly owe a debt of gratitude to individuals on the autistic spectrum for the breakthroughs leading to these advances. Beyond embracing the manifest products of autistic genius, it may also be the case that analytical thought and the rules of logic, for example, were codified by autistic minds. The artwork of the Upper Paleolithic then, may be in part the direct work of autistic savants, but may be more broadly interpreted as the evidence for the transmission of ideas engendered by them.

Besides the stunning artwork from the Upper Paleolithic, there are other finds, such as microblade industries at Howiesoon’s Poort, which indicate standardized innovation and attention to detail and which could be construed as the product of an autistic mind. The technological transitions during the Upper Paleolithic offer an example of a dissemination of technologies and ways of thinking which are analogous to the dissemination of ideas from original thinkers in modern times into industry and broader social integration. The analogy is strong because of the apparent adherence to strict rules and evidence of rigid analytical thinking. Throughout this period, tools and tool making steadily became more efficient and the innovation in projectile weapons bear the imprimatur of engineered solutions. These improvements in technology undoubtedly made men better hunters and thereby reduced stress based on scarcity of resources. Simultaneously, time-factoring notation systems were being developed which, combined with the other innovations of the period, represented a revolution in human thinking which epitomized a willingness to break from the norm or to conceive of the world in new ways. This revolution could easily be imagined as the integration of the specialized, analytical, and independent autistic minds within social and communicative populations.

Here it must be emphasized that it is not our conjecture that the rise in autistic thinking is solely responsible for the changes in human society and technology. Rather, it may be more accurate to note that something in human society changed which allowed the incorporation of different-mindedness into the social group rather than excluding it, and that such inclusion led to the adoption of new ways of thinking and behaving.

The Social Integration of Difference

The question arises as to what kind of cultural mechanisms emerged in prehistoric times which allowed for the integration of differences in minds in society. One might look first to language itself as a possible candidate. People have described Einstein as “eccentric”, for example, whereas other talented individuals on the autism spectrum have been labeled “disordered” or even “retarded”. Language codifies meaning in a culture shared by different minds. High-functioning autistics may not be able to understand dynamic empathetic social interactions, but they are in many cases very capable of communicating their ideas and views through the use of language. Language is not a perfect medium, as the use of idiom or reference to a foreign emotional state to an individual with Asperger’s may be misunderstood. However, such misunderstandings can themselves be discussed and explained; thus language solves even its own problems.

Besides language, material language and material symbolism also codify meanings. Codes of dress, ornamentation, and the use of ritual objects work in a way similar to language. Material symbols such as wedding rings codify interpersonal and emotional relationships so that the social meaning of the bond between two individuals will not be misunderstood by autistic minds. Subtle shades of meaning may be lost on autistic minds, and ambiguities might be exploited by others. Nevertheless, language and symbols do work to make it possible for different minds to collaborate and communicate.

Other kinds of relationships between people have been codified, such as the universal tit-for-tat of commerce. The autistic sense of “fairness” is expressed very eloquently in the monetary system, though the system itself may be gamed. Reciprocation and alliances are ritually codified in contracts, war documents, and other symbols.

No matter how advantageous it may have been in prehistory to incorporate different minds into the group, the autistic traits of single mindedness, lack of concern with social norms, egocentricity, and obsessive concerns with detail would have also created social tensions. Societies would have had to develop mechanisms for coping with such tensions, and story telling, dance, and music would have promoted group cohesion between minds through boundary loss. In hunter-gatherer groups, shamans with a strong empathetic or socially oriented personality may have arisen to perform a crucial role in nurturing social cohesion. In fact the evidence for music, collaborative ritual (including burial), and shamanic practices are associated with “modern human behavior”.

There are other spheres of human social life which provide a level of lawfulness and predictability through clear rules, little change, and a high rule base to social interaction ratio. In such arenas, those on the autism spectrum would have enjoyed particular success. Such social environments would remain sheltered from the mercurial and empathetically confusing spheres of social interaction, in very much the same way they are today in such places as courts of law and universities. In addition, other social groups may live in ethnographically segregated populations which stay to themselves. The study of the social environment in Mesolithic Europe indicates that in the emotional and social arena burial rites are so intensely changeable that they defy any attempt at overall generalization, and in such places one also finds that innovation in material culture remains stagnant and unchanged. This may support the existence of spatially separated spheres of social life and the possibilities for certain individuals to isolate themselves from intense emotional or unpredictable social interaction.

Social Controls

Clearly, the non-empathetic and ego-centric mind may surpass the boundaries of tolerance and accommodation and become a more serious threat to group autonomy. Such different and often difficult individuals are always found in ethnographic studies, and the social responses to such types were varied. In small scale societies, tolerance of very different behavior is typically broad but never unlimited. Such studies indicate a variety of types of responses, as well. In hunter-gatherer groups, moralistic stances typically motivate the expulsion or assassination of dominant individuals. Failure to abide by certain rules may also lead to exclusions from different kinds of social interaction rather than outright expulsion. The development of social controls in modern human behavior is well, documented and is found even within the burial record of Mesolithic Europe.

Society of Difference

Societies composed of different cognitive styles held together through social mechanisms are very different from societies of people sharing similar cognitive styles, and it is argued here that the rise of modern human behavior is the expression of the social mechanics of the former kind of society, which is composed of emotional and social minds as well as anti-social and autistic minds of varying degrees of difference.

The incorporation of autistic minds into society would present its challenges, but would also lead to certain advantages. It is easy to imagine a gradual accumulation in society of ritual and mechanisms to cope with the challenges and the talents of individuals with different minds. Such talents might be exploited for the development of technological and economic changes, the consequences of which would bear directly on social dynamics. Autistic individuals with a narrow focus of interest and a high capacity for technical thinking and pattern recognition would easily filter into specialized roles in the technological and natural realms as their gifts would make them inventors of technology, keen observers of pattern in weather and in animal behavior, as well as star gazers and calendar makers. The rigid and analytical thinking of such people could lead to the breakthroughs in human behaviors which reduced resource stress and increased longevity, ultimately leading to population increase. Indeed, such thinking could be taught to others who themselves were not autistic yet were perfectly capable of following a standardized method of observation and behavior. The more communicative and social members of society might themselves play a crucial role in propagating innovations outward. Social tensions might also arise between different minds and leadership roles in situations of crisis, and such clashes of minds or personalities might have played a large motivating role in geographic expansions.

Changes in the willingness or desire of members of a society to embrace and/or support those who do not make an emotional contribution to the group might be made possible by converging intentionality or shared common goals across different minds within the group. Though such motivations may never be retrieved from fossils and artifacts, it may nonetheless be inferred that they existed, and would explain the parallel development in technological advances and social innovations, the evidence of which is in abundance in the archaeological record.

Different Minds in the Emergence of Modern Human Behavior

At this time it is important to make clear that though the archaeological evidence may be viewed through the lens of autism, let it not be concluded that autistic gifts alone are responsible for the revolution in human culture leading to modern human behavior, but rather that it was such gifts and gifted individuals within the broader social context which included and supported them that must be credited.

The identifiable components of modern human behavior fit neatly into the picture drawn in an hypothesis for the development of social mechanisms for including people with different minds into social groups. The symbolic use of shells and other ornamentation, the use of pigments in rituals such as burials, and communal musical styles all contributed to a shared identity among different peoples proximate to one another geographically. In such artifacts we see the means for forging and maintaining alliances and social bonds with a widening sense of group identity.

Not only the inclusion of the talents of autistic individuals, but also the willingness to adopt and spread the unique insights and technological advances was involved in promoting modern human behavior and in turn inviting its consequences on social conditions. Novel technology would lead to novel uses of materials, which in turn would lead to further specialization in industries from livestock to the production of projectile points and weapons. More elaborate and structured uses of fire and materials in and around living spaces would create their own novel solutions, and the ritual use of symbols would need to expand, as it did in the Paleolithic, into structured notation systems and time-factoring technologies, such as calendars.

Besides in-group specialization, the archaeological record shows that social groups were behaving differently in different geographical regions at the same time, and thus between-group specializations likely emerged as well, and indeed the technology to forcibly resolve conflicts between groups was also steadily improving during this period. Innovation in war-making technologies and resource exploitation also played its part in the tensions created through population expansion into new territories and in rising population densities in existing territories.

The time line of events in the human revolution is consistent with the expansion of anatomically modern humans outward from Africa, and in fact the earliest evidence for integrative social behaviors comes from South Africa. The evidence found there shows the sporadic use of symbolic culture in the Middle Paleolithic. Adopting social mechanisms for coping with differences in minds might have been a hit and miss proposition as the pressure rose to codify and formalize such mechanisms into a shared culture, and natural selection would continue its relentless process through the successful adaptation of social mechanisms and structures for inclusion and tolerance of different minds within the social group.

The spread of anatomically modern humans and the sequence of modern human behavior as shown by the fossil record in the Levant is also consistent with a gradual and progressive model. Evidence for such ritual behavior as burial is found there, though sporadically, and is associated with anatomically modern humans between 130,000 and 80,000 years ago. At around 80,000 years ago, they were replaced by Neanderthals in this region. This may have been due to their sheer physical strength and fearless, confrontational style of hunting. Anatomically modern humans return to replace the Neanderthal population in the Levant some time around 43,000 to 30,000 years ago, at which point we see a rather abrupt and widespread emergence of elaborate social rituals and symbolic communication. Perhaps social mechanisms may reach a “point of no return”, where complexity and consistency are fostered by intuitive and emotional support, and where autistic thinking has a clear role in producing and maintaining efficiencies in technology and subsistence.

The expression of modern human behavior concurrent with the expansion of modern humans from the Middle East outwards into Europe and western Asia demonstrates a complete suite of behaviors, including clear social mechanisms and the capacity for long distance communication and exchange. Structured dwellings and symbolism in art are found alongside tools made of bone and antler, and small, standardized precision cutting tools masterfully chipped from stone to produce edges sharp as razor blades.

During the early Paleolithic, Neanderthals and anatomically modern humans were not dissimilar in their use of symbols and tools. The sudden emergence of modern human behavior as a working set of social mechanisms arose in the Levant as humans replaced the Neanderthal population there. Prior to this time, it is clear that Neanderthal’s use of symbols was similar yet different from the use of symbols by their “cousins”, the anatomically modern humans. It is fascinating to consider that Neanderthal use of ornamentation or tool making may have been inspired by their interaction with anatomically modern humans during the thousands of years where both species occupied the same geographical territories. This might indicate a “hijacking” of mechanisms or symbol sets used for within species communication for the purpose of intra-species communication. It is also possible that some kinds of language or symbolic reference was not of a character that could be shared across the species-specific cognitive divide.

Without question, the application of “different minds theory” to the archaeological record will raise more questions while potentially answering others. For example, the emergence of symbolic thought in Asia was a more gradual process which seemed to happen along a somewhat different trajectory. There are other explanations for the rise of modern human behavior, many of which are not mutually exclusive.

Further Issues

“Different minds theory” will require more in-depth study. Did cognitive differences exist among anatomically modern humans prior to the “cultural revolution” marked by the emergence of modern human behavior? If so, then this model suggests that individuals with cognitive differences would have been excluded from the social group. An alternative explanation would be that the emergence of autistic cognitive styles is relatively new to the biology of anatomically modern humans and such talents might have been gradually exploited by the group into which such individuals were born.

Some researchers insist that autism is a spectrum of differences expressed widely and diversely across populations, and yet others maintain that autism is a very clearly defined disorder. The author of this research is more inclined to accept the former proposition, but in either case social mechanisms for inclusion and support for those individuals with autistic minds would produce similar results in the archaeological record. More research may shed more light on this issue.

There are other differences between minds besides autistic versus non-autistic thinking. For example, schizophrenia follows a parallel pattern of selection and maintenance in human populations. Intense expressions of schizophrenia are debilitating and disadvantageous, however at a lower intensity the genetic basis for schizophrenia might confer certain selective advantages in terms of creativity. Like autism, the degree of difference within individual minds and the social environment in which they exist are variable factors which determine whether such individuals and their gifts will be incorporated into the larger society.

Other biologically-based differences might also be explored. Some researchers have suggested the relative importance of a few individuals with extraordinary social skills and “higher order intentionality” in human evolution. Such individuals might occupy crucial roles in facilitating social cohesion and the integration of different minds.

Difference as a social construct may have helped to develop modern human behavior, apart from biologically based differences. The concept of separating social roles into distinct spheres, such as gender-based division of labor, may have been influenced by the social mechanisms for inclusion of different minds. It is argued that the social/cultural solutions for inclusion engendered the creative and analytical innovations resulting in the emergence of modern human behavior, and it may be further conjectured that the extent of the consequences of such innovations has not been fully determined.

Other kinds of differences in minds are involved in the creation of personality such as traumatic or early childhood experiences and such differences, though not based in biology, might also lead to distinct or specialized social roles. Social construction and biology are complementary forces which act upon one another mutually, and the process of integrating different minds into one social identity is itself a distinctive hallmark of modern human behavior. Further ethnographic research into social construction and biologically based cognitive difference will yield more insights into this.

Conclusions

While the development of social mechanisms for the inclusion of different minds is argued to be the key to explaining modern human behavior, any discussion of difference in modern times is problematic and sensitive. Human beings are grounded in their own identities through their social roles and relationships not only with one another within groups, but also in their group relation to others outside of the social group. Inquiry into seldom-questioned or underlying motivations for pervasive notions of identity can itself seem threatening for some, and thus arises some measure of unpredictability for the consequences of fear.

There is a cartoon by Carleton Coon which depicts a Neanderthal riding on the New York City subway system in a suit and tie, and it illustrates an important point which has been emphasized here already. The image suggests that despite his obvious physical differences, he might be able to integrate into ordinary society if only he would observe the social rules and structures that we all take for granted. His ability to lead a successful existence among human beings would come down to whether or not he could conform just enough to his social environment in order to engender the emotional and social support of the humans around him. The code of acceptable difference, apart from the different minds which contained it, may have been responsible for the human revolution.

Spikins, P. (2009) Autism, the integrations of ‘difference’ and the origins of modern human behaviour. Cambridge Archaeological Journal, 19 (2). pp. 179-201. <http://dx.doi.org/10.1017/S0959774309000262>

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Neanderthal genes may have conferred an evolutionary advantage to the modern humans who acquired them.

May 11th, 2010

Researchers at the Howard Hughes Medical Institute and the University of Chicago think that interbreeding between and human and Neanderthals may have resulted in enhanced brain function.

According to Bruce T. Lahn, a Howard Hughes Medical Institute researcher at the University of Chicago and the senior author of the research, ancient humans may have interbred with other Homo lineages, introducing gene variants, known as alleles, which may have contributed to the reproductive fitness of our own species.

“By no means do these findings constitute definitive proof that a Neanderthal was the source of the original copy of the D allele. However, our evidence shows that it is one of the best candidates.”

~Bruce T. Lahn

The studies were conducted by a team of University of Chicago researchers, including Lahn, Patrick D. Evans, Nitzan Mekel-Bobrov, Eric J. Vallender, and Richard R. Hudson.

The team conducted a statistical analysis of the DNA sequence of the gene microcephalin, which is related to brain size in humans.

The microcephalin gene is subdivided into two distinct classes of alleles, D alleles and non-D alleles. Although currently present in roughly 70 percent of the human population, Lahn’s research suggests that the D allele in modern humans came from a single progenitor around 37,000 years ago. The explosive spread of the D allele within the human population further indicates that the D allele conferred upon its possessors some advantage in terms of reproductive success.

The researchers’ belief that the D allele made its way into the human gene pool through a single prognitor around 37,000 years ago is based upon the sequence differences in the D allele found in modern humans. With each successive generation, gene mutations are introduced at a steady rate, thus the greater the number of generations, the greater the sequence difference between different copies of the same gene.

In comparing sequences of D alleles and non-D alleles, the researchers have determined that the two classes developed in separate lineages for around 1.1 million years, with the non-D alleles having evolved in the Homo Sapiens lineage, and the D alleles having evolved in a now extinct Homo lineage. At some point – around 37,000 years ago – the D allele made its way into the Homo Sapiens lineage, and the most likely explanation for this is interbreeding between the distinct branches on the Homo family tree.

The sequence differences between the D allele and the non-D allele is the evidence suggesting the estimated 1.1 million years separating the the Homo Sapiens lineage from the archaic Homo species from which it must have acquired the D allele. The now extinct Neanderthal lineage is the most likely candidate, according the Lahn and his associates, beacuse the species separation duration of 1.1 million years is consistent with existing estimates of the amount of evolutionary time separating Homo Sapiens and Neanderthal. In addition, the calculated time of introgression of the D allele into the human genome – about 37,000 years – marks a time in history when humans and Neanderthals coexisted in Europe and Eurasia.

“A third line of evidence, albeit weaker, is that the D alleles are much more prevalent in Eurasia and lower in sub-Saharan Africa, which is consistent with an origin in the former area. And we know that Neanderthals evolved outside of Africa,” said Lahn.

The role played by the D allele in human evolution remains unknown, though Lahn suggested that it might make brain function slightly more efficient. “For example, someone inheriting the D allele may have only a slightly more efficient brain on average. While that enhancement might confer only a subtle evolutionary advantage on that person, when that effect is propagated over a thousand generations of natural selection, the result will be to drive the D alleles to a very high prevalence.”

The two prominent competing theories in modern times regarding the origin of the human lineage are sometimes referred to as the “Out of Africa” theory, which holds that fully modern humans moved out of Africa and replaced archaic homo lineages, and the “Multiregional” theory, which posits that modern humans are the result of gene flow between the original Homo Sapiens in Africa and the other Homo lineages populating Eurasia.

Source:

Could Interbreeding Between Humans and Neanderthals Have Led to an Enhanced Human Brain? N.p.: HHMI, 2006. N. pag.
University of Chicago. Web. 12 May 2010. <http://www.hhmi.org/news/lahn20061006.html>.

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