This week’s publication of the first draft and initial analysis of the Neandertal genome sequence by a large multi-national group is of very special interest. [May 7, 2010, Science]
Our theory makes a very strong prediction about the ascendency of the behaviorally modern humans that are ancestral to all of us alive today (Chapter 11 in Death from a Distance and the Birth of a Humane Universe or DfaD). Our specific claim is that the behaviorally modern human revolution was a social revolution, not a genetic revolution. One of the symptoms of this dramatic process was that the populations of our modern ancestors began to grow and expand out of Africa, thereby driving all other non-modern humans, including the Neandertals of Eurasia, to apparent extinction. Thus, our theory implies that the behaviorally modern displacement of Neandertals was unlikely to reflect some genetic superiority of our “modern” ancestors over other human groups, like Neandertals, traditionally classified as “archaic.”
Rather, our theory requires that modern ascendency was an accident of our modern ancestors having invented a fundamentally new coercive technology (the atlatl or spear-thrower) before the Neandertals had access to any analogous new coercive technology. This single social/technological innovation allowed our modern ancestors to apply the ancient, uniquely human adaptive trick with new ecological power. This fundamental trick is kinship-independent social cooperation coercively enforced by the self-interested projection of coercive threat. We expect the scale of this cooperation to be dramatically increased in moderns in comparison to the smaller social groups characteristic of both Neandertals and the “anatomically modern” humans who were the immediate ancestors of behaviorally moderns.
This particular prediction of our theory is especially useful because the traditional views of the behaviorally modern displacement of Neandertals often propose (or implicitly assume) that modern human ascendency was the result of some significant difference in the genetic endowment of these two species. Thus, several conclusions from this week’s new Neandertal genome sequence are of particular relevance.
First, many of the distinct characteristics of the modern human genome are also present in Neandertals – consistent with their being fully “human.” [This pattern had been observed earlier for the protein coding sequence of the FOXP2 putative "language" gene.] For example, the “human accelerated regions” (HARs) are short genomic segments that appear to have evolved very rapidly in the human lineage after its divergence from the chimp lineage – in spite of these segments being strongly conserved (showing little evolutionary change) throughout the tens of millions of years of vertebrate evolution before the chimp/human divergence. HARs are, thus, good candidates for some of the information central to the genetic redesign supporting the evolution of humanness. From the new Neandertal sequence, we learn that most of the human-specific changes in these HAR regions are shared with Neandertals, as predicted by our theory.
Second, similarly, approximately 90% of the human-specific small changes in our genomes (inferred from comparison with a reconstructed proxy for the genome of our last common ancestor with chimps) are shared with Neandertals. Especially striking is the finding of only 78 or 88 (depending on measurement approach) genome changes producing amino acid substitution differences between moderns and Neandertals, with only five genes (out of ca. 23,000) containing more than one amino acid substitution. In view of the well-known fact that many amino acid substitutions are actually nearly neutral (without much adaptive effect), these observations are consistent with extensive biological/genetic similarity between us and Neandertals. Likewise, there are less than 200 substitutions in non-coding (potential regulatory) segments of mRNAs, some or most of which could also be neutral.
The larger point here is that many regions of the Neandertal genome fall within the range of variation expected among contemporary people (“modern” descendents). There is no compelling evidence from these data for a genetic revolution underlying the rise of modern humans.
Third, the authors were also able to infer which regions of the genomes of some modern humans might have been acquired from Neandertals as a result of interbreeding at or immediately after the first expansion of our modern ancestors out of Africa 40,000-60,000 years ago. Two observations are especially striking. Roughly, 1-4% of modern Eurasian and Eurasian-derived human genomes (that is, non-African genomes) are apparently inherited from Neandertals. This indicates a modest, but significant level of interbreeding between Neandertals and the first members of the behaviorally modern African disapora ancestral to all contemporary non-African moderns. Thus, Neandertals were not a different species in the biologist’s literal sense of that term. It will be of interest to determine if any Eurasian traits (lighter skin tones or protuberant noses, for example) prove to be of Neandertal origin.
Fourth, against this background of very little difference suggesting a common genetic heritage it is possible to look for rare genes or genome segments that might have experienced strong genetic selection at or after the divergence of the primary modern lineages from the Neandertals. These measurements detect 212 possible candidates for such selectively significant modern regions.
Of course, this last observation could be taken to support traditional genetic revolution hypotheses for modern ascendency. However, it is important to note that such selective sweeps continue in modern human sub-populations without creating the basis of global displacement. For example, some local human populations have recently undergone strong selection for genetic resistance to malaria or adult tolerance of lactose (milk sugar). Some or all the putative selective sweeps occurring at or since our divergence from Neandertals could easily be of this general form.
It is illuminating to consider a more recent potential analogy to the takeover of the Neandertal’s domain by our modern ancestors. The Afroeurasian displacement of Native Americans in North America produced a number of genetic signatures – replacement of Native American genetic variants and configurations with Afroeurasian ones. However, we are not required to believe that the current human population of North America reflects the product of a genetic revolution rather than a social one. Indeed, the genetic hypothesis Afroeurasian ascendency in North America is widely viewed as implausible. Our theory argues that any residual genetic after-effects of the behaviorally modern human displacement of Neandertals are equally likely to be superfluous and secondary to the social revolution that bestowed possession of the human realm on our modern ancestors.
Of course, it follows from our theory’s view of the modern/Neandertal divergence that the study of the differences between Neandertals and moderns will notinform us about the genetic/biological bases of what is means to be human. Rather, on our approach, the greatest power of this comparison will lie in identifying what Neandertals and moderns share. The evolution of these common properties is what should most capture our attention. The Neandertal sequence is equally useful to both approaches. The question is how we can best employ this rich new burst of information.
Before leaving the Neandertals, their analysis has one more important insight for us and allows us to recognize one crucial new question. First the insight. Many contemporary modern humans are the recent descendents of successful imperialists. This is obviously true of the Western Eurasian colonial diasporas (of which the authors are both members). However, it is also true, for example, of Han Chinese, Bantu Africans, non-Ainu Japanese and Aryan South Asians, among many others. The cultures and social structures perpetrating such imperialism are especially prone to produce racist (and sexist) explanations of coercive dominance (see Chapters 10 and 13, DfaD). We, their recent descendents, are extremely vulnerable to this culturally inherited preconception. As we study Neandertal and their relation to us we are well advised to be mindful of this potential bias in our underlying thought processes.
Now we can turn to the new question the Neandertal results lets us recognize. If Neandertals and moderns were fellow human species members why was there not more extensive gene flow between Neandertal Eurasia and anatomically modern Africa long before the behaviorally modern human revolution? Our theory predicts one candidate for the answer. Members of individual human social units are extremely xenophobic toward members of other units with whom they have conflicts of interest beyond their capacities for cost-effective management (Chapter 10 and Third Interlude; DfaD). These effects might have been especially intense for the small cooperative social groups sustained only by the coercion supported by elite human throwing in pre-modern humans (Chapters 5 and 7; DfaD) – blocking potential mechanisms of effective mate exchange/migration between culturally distinct lineages. The substantial expansion of the scale of social cooperation in behaviorally modern humans (Chapter 11, DfaD) might have created new avenues (“commercial”?) for such exchanges. This is an issue that will reward substantial new investigation.
Paul M. Bingham and Joanne Souza
Authors: Death from a Distance and the Birth of a Humane Universe
