In Science Ann Gibbons has a very long reported piece, Close Encounters of the Prehistoric Kind. It’s well worth reading, but behind a pay wall. If you don’t have access though, I want to spotlight one particular section:
The discovery of interbreeding in the nuclear genome surprised the team members. Neandertals did coexist with modern humans in Europe from 30,000 to 45,000 years ago, and perhaps in the Middle East as early as 80,000 years ago (see map, p. 681). But there was no sign of admixture in the complete Neandertal mitochondrial (mtDNA) genome or in earlier studies of other gene lineages…And many researchers had decided that there was no interbreeding that led to viable offspring. “We started with a very strong bias against mixture,” says co-author David Reich of Harvard Medical School in Boston. Indeed, when Pääbo first learned that the Neandertal DNA tended to be more similar to European DNA than to African DNA, he thought, “Ah, it’s probably just a statistical fluke.” When the link persisted, he thought it was a bias in the data. So the researchers used different methods in different labs to confirm the result. “I feel confident now because three different ways of analyzing the data all come to this conclusion of admixture,” says Pääbo.
The finding of interbreeding refutes the narrowest form of a long-standing model that predicts that all living humans can trace their ancestry back to a small African population that expanded and completely replaced archaic human species without any interbreeding. “It’s not a pure Out-of-Africa replacement model—2% interbreeding is not trivial,” says paleoanthropologist Chris Stringer of the Natural History Museum in London, one of the chief architects of a similar model. But it’s not wholesale mixing, either: “This isn’t like trading wives from cave to cave; the amount of admixture is tiny,” says molecular anthropologist Todd Disotell of New York University in New York City. “It’s replacement with leakage.”
The power of data to overwhelm human prejudice is sometimes very awesome. And the bias which Reich and Pääbo exhibited was not unfounded; Pääbo was involved in the sequencing of the Neandertal mtDNA, and found no evidence of admixture there. These data were strong, and I believe they should now shift our assessment of probabilities in relation to earlier papers which claimed some admixture between the population which derives from the Out-of-Africa expansion, and the Others.
In the second section it is notable that Chris Stringer has discarded replacement as not viable. He uses the term “not trivial,” which means that it’s a significant finding of note which one can’t simply ignore when generating inferences from a set of facts which one takes as axiomatic. Disotell’s attempt to minimize the finding is more a matter of rhetoric. He does not dismiss the admixture, he simply consigns it to the undefined category “tiny.” To some extent this reminds me of the neutralist vs. selectionist arguments of the 1970s, and more recently of the Out-of-Africa vs. Multi-regionalism disputes in human evolutionary origins. An argument over the meaning of words is a matter of law, an argument grounded in empirical data and quantitative estimates is an argument about science. No one holds to the extreme caricatures of any four of the models at this point; we’ve established that all these paradigms are unchaste, now we’re just haggling over price. We know that humans and the Others did the deed, we’re now mapping out the what, where, and how often.
But this is not the closing of the gate of itjihad. Dienekes presents an alternative model which may explain the data:
There is an alternative explanation. It involves the emergence of Homo sapiens and Homo neanderthalensis from a common ancestor and the subsequent admixture of Homo sapiens with populations that have branched out before this divergence. This would account for increased similarity between Eurasians and Neandertals, but without the problem of explaining how “Neandertal” ancestry is so similar in Europeans and East Asians.
What about Africans? Why do they stand further away from Neandertals? The answer is simple: low-level of admixture with archaic humans in Africa itself. It is fairly clear to me that the sapiens line whose earliest examples are in East/South Africa must have been an offshoot of an older African set of populations. We are lucky that Neandertals lived in a climate conducive to bone (or even DNA) preservation, while the African populations inhabiting the tropics left no traces of their existence.
In conclusion: I am not at all convinced that the authors have uncovered evidence of Neanderthal admixture in Eurasians; the alternative explanation is that modern humans and Neandertals were related, modern humans spread from East Africa/West Asia and as they entered deeper into Africa, they interacted with archaic human populations there.
Intermixing does not surprise paleoanthropologists who have long argued on the basis of fossils that archaic humans, such as the Neandertals in Eurasia and Homo erectus in East Asia, mated with early moderns and can be counted among our ancestors—the so-called multiregional evolution theory of modern human origins. The detection of Neandertal DNA in present-day people thus comes as welcome news to these scientists. “It is important evidence for multiregional evolution,” comments Milford H. Wolpoff of the University of Michigan, the leading proponent of the theory.
The new finding shows that “gene flow across taxonomic boundaries happens,” observes geneticist Michael F. Hammer of the University of Arizona. Hammer is among the minority of geneticists who have espoused the idea of gene flow between archaic and modern populations. His own studies of the DNA of people living today have uncovered, for example, a stretch of DNA that seems to have come from encounters between moderns and H. erectus.
I assume Wolpoff is exultant. I do not personally think that this finding necessarily is going to result in a renaissance in Multi-regionalism, but Wolpoff has been the subject of a rising tide of skepticism and dismissal these past few decades. But rather than a more robust discussion between a revived Multi-regionaism and Out-of-Africa, I think these findings, and those that are likely to follow, will force us to move past simplistic typologies and accept that human evolutionary history works itself out through the principles of population genetics, and so can only roughly be modeled in words. The devil is in the parameters.