Friday, October 28, 2011

The origin of our species

The  origin of our species might seem a question of purely scientific interest, but it is actually also an intensely political one. Stringer gives us a dialectical understanding of our evolution, argues Elaine Graham-Leigh. 

Written by Elaine Graham-Leigh 
27 October 2011

The origin of homo sapiens might seem a question of purely scientific interest, but it is actually also an intensely political one. From the Just So stories of evolutionary psychology to various ideas about innate human behaviour, there are no shortage of arguments seeking to justify aspects of modern society with reference to the prehistoric past. This is not restricted to the history of our own species, homo sapiens, as our earlier evolutionary history can also be held to be extremely relevant for understanding the present.

An example of this was the development of the theory, in the 1950s and 1960s, about the supposedly violent behaviour of an ancient hominid species called Australopithecus africanus, which was then usually thought to be an ancestor of all later hominid species (the group of species to which homo sapiens belongs).

A palaeontologist, Raymond Dart, who discovered various africanus fossils in southern Africa, believed that he had found evidence of hunting, murder and cannibalism, and that it was this which had enabled africanus to give rise to the later hominid species. He called it ‘the predatory transition from ape to man’, and this was developed in the popularisation of his theory in the 1960s into the conclusion that violence was not only innate to modern humans, but that it was in some way what made us human.  The usefulness of this theory to right-wing arguments against the peace movement was, of course, entirely coincidental.

The science underpinning this argument for innate human violence was fairly swiftly demolished, with poor old africanus appearing now to have been more prey than predator. However, this is not the only instance of perceived hominid history being used to justify a right-wing view of society. A particular point of contention has been the question of where homo sapiens first evolved from its precursor hominid species, and what that means for differences (or not) between modern human populations; the biological reality of race.

The older theory for the development of homo sapiens is what is known as the multiregional model. This is based on theories first developed in the 1930s, reworked by US scientist Carleton Coon in the 1960s, and picked up in the 1970s and 1980s by a number of multiregionalists. Broadly, this theory argues that homo sapiens populations in different parts of the world evolved separately in those areas. The multiregionalists accepted that earlier hominid species had evolved in Africa, but argued that once the hominid species called homo erectus had dispersed from Africa about one million years ago, the erectus populations in each area had given rise to their own modern humans. For Europe, the argument was that homo erectus had arrived from Africa and developed into the Neanderthals, who had then in turn developed into modern Europeans, but also that modern humans in other parts of the world would have had a different post-erectus evolutionary history.

This separate evolution meant that perceived racial differences – in skin colour, stature, facial features and so on – were real differences expressing the fact that a million years separated, for example, a sub-Saharan African and a WASP. In addition, the idea that there were real different racial groups of modern humans, which had evolved separately, enabled the multiregionalists to suggest that they might have evolved at different rates. All were still just about human, but now some seemed to be more human than others. Carelton Coon even singled out Australian aboriginies as an example of a group which was less evolved, just in case anyone could not work out for themselves that it would not be white Europeans who were thought to have lagged behind in evolutionary terms.

Some aboriginal groups have tried to turn this on its head, arguing that their right to the land is strengthened if they are effectively homo erectus, but the general racist effect of this theory should be evident. For the multiregionalists, what was key was that the unavoidable African part of their evolutionary history was as peripheral, and as long ago, as they could make it. They could not argue that the evolution of hominids up to homo erectus happened outside Africa, because the weight of evidence was against them, but they could locate their own homo sapiens origins safely in Europe, via the Neanderthals. They were unable to believe that anything as impressive as white men could have anything recently to do with Africa. As Coon argued, ‘If Africa was the cradle of mankind, it was only an indifferent kindergarten.’

The alternative theory, developed by Chris Stringer, among others, throughout his career, is the Out of Africa Theory, or theory of Recent African Origins (RAO), so renamed because there appears to have been not one African exodus but several. As the name implies, this theory contradicts the multiregional model by arguing that homo sapiens evolved relatively recently in Africa and spread out from there to become the globally-dispersed species we are today. This theory turns the idea of the biological reality of race on its head. If we are all so recently related, it follows that differences in appearance between populations from different areas must be literally skin deep. Indeed, work on the DNA of different populations shows that because everyone whose ancestors were part of the out of Africa dispersal are descended from a fairly limited gene pool, there is more variation between different modern population groups within Africa than there is between groups from anywhere else in the world.

The RAO theory has become the most generally accepted version of human origins, and presents a conveniently clear scientific refutation of some unpleasant racist theories. However, theories of human origins are changing all the time as new remains are found, or new techniques developed for dating finds or tracking DNA variations. This new book from Chris Stringer brings us up to date with recent years of developments, including the startling discovery of a new hominid species, (although not a human ancestor), the tiny ‘Hobbit’, homo floriensis. It is a fascinating read, but from the point of view of the political arguments over human origins theories, a rather challenging one.

It is always a bad idea to rest the repudiation of right-wing interpretations of ‘facts’ about the world on the basis of our own ‘facts’ alone, rather than on disputing the ideology behind reactionary arguments. Scientific facts are always subject to new discoveries and can be challenged. The problem with a broad-brush rendering of the RAO as an answer to racist science, is that it can imply that it is only the fact of homo sapiens’ recent evolution in Africa which means that racist arguments about the superiority of some groups over others are not correct. In other words, it can appear that if new evidence emerged in support of regional differences in homo sapiens’ evolution, that would pull the rug out from underneath our repudiation of scientific justifications for racism. The challenge presented by Stringer’s new book is that the nuanced version of the RAO which it presents has had to accommodate some regional differences. The simple story of African origins told by summaries of the RAO turns out not to be quite right.

Part of the multiregional hypothesis was that homo sapiens in Europe was descended from European Neanderthal populations. By contrast, the RAO held that Neanderthals died out without leaving descendants, being replaced by the more successful homo sapiens, either by competition or genocidal violence, depending on how far the individual commentator was in favour of the ‘nature red in tooth and claw’ version of human prehistory. However, advances in studies of Neanderthal DNA now indicate that while homo sapiens clearly did not evolve from Neanderthals in Europe or anywhere else (the evolutionary progenitor of homo sapiens appears to have been homo heidelbergensis, from which both homo sapiens and Neanderthals evolved), modern Europeans and Asians have DNA which is closer to Neanderthal DNA than is modern Africans’. This indicates that modern humans outside Africa may have had some Neanderthal ancestors. The Neanderthal DNA is about 2%, a reasonably significant input, which requires explanation in any model of human origins.

Stringer maintains that it is unlikely that this interbreeding happened in Europe, as European homo sapiens do not appear to have inherited their cold adaptions of light skin, hair and eyes from the similarly cold-adapted European Neanderthal populations. The most likely hypothesis, Stringer argues persuasively, is that homo sapiens leaving Africa encountered people descended from an earlier exodus of homo heidelbergensis into the Middle East. This is not the only evidence of interbreeding between homo sapiens and other hominid populations. Modern Melanesians, for example, have as much as 8% DNA which appears to have originated in the Denisovians, a hominid species also descended from homo heidelbergensis and closely related to Neanderthals. This is so high a retention of archaic DNA that Stringer speculates that it may be providing some selective advantage, such as resistance to endemic diseases.

There is clearly a danger that these discoveries could be used to resuscitate the otherwise moribund multiregionalist theory. It is probably not a coincidence that the multiple authors of a recent paper published in Science on Neanderthal and Denisovian DNA, ‘The shaping of modern human immune systems by multiregional admixture with archaic humans’ [italics mine].  Stringer however makes it clear that the RAO theory is amended but not contradicted by these discoveries of contact between homo sapiens and other hominid species.

In the first place, Africa retains its importance in human origins, rather than being pushed aside as in multiregionalist theory. By the last chapter, it is clear that what we are moving towards is a multiregional theory of human origins within Africa, with different archaic populations potentially encountering and breeding with each other over a long period of time, interspersed with incidences of dispersal out of Africa. While the theories discussed earlier in the book of how homo sapiens could have bred with Neanderthals and Denisovians remain valid, the point here is that different homo sapiens populations could have acquired their differing traces of archaic DNA before they ever left Africa. Regional differences between early homo sapiens populations could have been due to the accidents of which groups ended up in which areas, rather than to the archaic populations they encountered there. It is important also to understand that ancient regional differences would not be the same as perceived more recent ones.

Just as important as this reassertion of the importance of Africa for human origins is Stringer’s consideration of what modernity means in human evolution. It is easy when thinking about the evolution of homo sapiens to reduce it to a question of genetics, but this would be a mistake. Stringer points out that in fact genetic divisions between species are often not as clear cut as a lay understanding would have it. The definition of a species in fact is closer to being groups which in the wild would not breed with each other, rather than groups which could not successfully breed with each other even if persuaded to do so by scientists in a lab. In other words, the definition of a species is behavioural as much as it is genetic, and for Stringer the evidence for the emergence of modern human behaviours is as important as that for the development of modern human bodies.

The evidence for modern behaviours like tool use, use of symbolic art and burying the dead, which set hominids apart from other animal species, is patchy, with incidences appearing early in human pre-history but separated from each other in time and space. Stringer suggests that this suggestion of patchy cultural development is not an accident of evidence survival but reflects a reality about how modern human behaviour developed. His point is that it needs a certain group size to maintain cultural and technological knowledge over generations, and a certain degree of contact with other groups to share further developments.

If small groups become isolated they can lose important knowledge, as for example seems to have happened to the first settlers in Tasmania. When they arrived there, about 40,000 years ago, the island was connected to the Australian mainland, and the settlers had a variety of tools, clothing and shelters. However, about 14,000 years ago, climatic changes turned Tasmania into an island, and the now-isolated inhabitants gradually seem to have lost much of their cultural and technical knowledge, even eventually the knowledge of how to make fire. Stringer suggests that this may be an illustration of how culture was developed and lost again many times among different groups in human prehistory. The key breakthrough for the development of fully modern human behaviour was not ability to develop this sort of culture, but the ability to sustain a large enough population to keep it.

It was this, rather than genetic differences, which ultimately made the difference between homo sapiens and Neanderthals. There is considerable evidence for Neanderthal culture, including burying the dead, tool-making and decorative painting, and it seems unlikely that these were simply imitations of homo sapiens’ culture, as has been suggested. The problem for Neanderthals may well have been that, given the ice-age climate which they had to deal with, they were never able to develop sufficiently large and connected groups to maintain and extend these cultural developments. The survival of us, rather than Neanderthals, may be more to do with the contingent effects of climate than any innate superiority of homo sapiens’ evolution. As Stringer says, lacking the last Ice Age, it might have been Neanderthals rather than homo sapiens which survived: ‘Would they have eventually looked back at their own success, at the benefits of the Eurasian environment, and the problems of surviving in Africa for their failed relatives, the ones with the weird foreheads?’ (p.238).

It is reasonable to be concerned that any suggestion that some people today have DNA from non-homo sapiens hominid species will be interpreted as meaning that some people are less human than others. However, the lesson of this valuable book is that these archaic hominid species, if the contingent events which drove them extinct had worked out differently, would have been different but actually no less human than us. Engels argued in the Dialectics of Nature that the essential element in the transition from ape to man was labour; humans became human because their bipedality freed their hands to make tools and therefore work to transform their environment.  Stringer’s conclusions seem to lead us back to this insight. That some of us may have Neanderthal or Denisovian DNA is fascinating for human history, but tells us nothing about modern regional differences. What made us human was behaviour, not our genes.

The loss of the basic RAO theory has taken away the one-sentence simple response to racists who like to imply that black people are less evolved than they are. In its place, Stringer has given us a dialectical understanding of our evolution and what it means to be a modern human, which is ultimately a much more powerful refutation of the ongoing perversion of the study of human origins by the right.

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