The False Dawn of modern humans in Eurasia

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A little over a month ago I asserted that “The Cro-Magnons Have No Descendants in Europe Today”. By “Cro-Magnon” I really meant the modern humans who spread the Aurignacian culture through the continent ~40,000 years ago (that is explicitly stated in the post). This was the first human culture across the continent associated with populations which had recently expanded out of Africa, and are termed “anatomically modern.” A few weeks ago in The New York Times Carl Zimmer on recent ancient DNA results from the past 10,000 year clarifying ancient and modern population relationships had a line, “The first [“Western European hunter-gatherers, WHG, -Razib] were hunter-gatherers who arrived some 45,000 years ago in Europe. Then came farmers who arrived from the Near East about 8,000 years ago.” I responded on Twitter to Carl that it seemed very unlikely that the ancestry of European Mesolithic hunter-gatherers dated to the initial settlement of the continent 45,000 years ago. On Twitter I offered up that a proportion of ancestry on the order of 1% might be possible to Chris Stringer.

Why did I say this? Because ancient DNA over the past few years has indicated that across much of Eurasia population turnover has been very common on a 10,000 year time scale (see Joe Pickrell and David Reich’s review for details). With that in mind, why would one expect that European hunter-gatherers would exhibit population continuity across the more than 30 thousand years between the initial replacement of Neanderthals ~40 thousand years ago, to the first arrival of farmers? Not to mention the non-trivial fact that the Last Glacial Maximum was situated right in the middle of this period. The fact is that many of the ancient DNA results suggest that the populations which defined the range expansion of humans across Eurasia were characterized by very small breeding populations. Not only would these be subject to genetic risk of extinction via mutational meltdown, but small populations are more vulnerable to stochastic environmental events which result in range contractions. This also applies to “archaic” hominins; Neanderthals and the Denisovan human seem to be either inbred or genetically homogeneous. The Neanderthal samples from the Altai all the way to Western Europe are much more closely related than is plausible for a lineage whose regional geographic populations exhibit time depths of hundreds of thousands of years (at minimum Neanderthals flourished for ~100 thousand years, but to my knowledge the extant individuals are not that different at all).

Another reason I made that statement is that people have been leaving telltale clues for a while. Nothing explicit, but sometimes you can infer things from radio silence. In any case, here’s the Nature paper on the Romanian ancient sample dating from ~40,000 years before the present, An early modern human from Romania with a recent Neanderthal ancestor. From the last paragraph of the discussion:

The Oase 1 genome shows that mixture between modern humans and Neanderthals was not limited to the first ancestors of present-day people to leave Africa, or to people in the Near East; it occurred later as well and probably in Europe. The fact that the Oase 1 individual had a Neanderthal ancestor removed by only four to six generations allows this Neanderthal admixture to be dated to less than 200 years before the time he lived. However, the absence of a clear relationship of the Oase 1 individual to later modern humans in Europe suggests that he may have been a member of an initial early modern human population that interbred with Neanderthals but did not contribute much to later European populations….

Modern Europeans can be thought of as compounds. The first element are a set of populations which descend from, or are genetically very close in nature to, Mesolithic hunter-gatherers who likely descend from groups extant during the late Pleistocene on the fringes of the continent. The second element seems to be a population which is an outgroup to all other non-Africans. That is, this group diverged from the ancestors, jointly, of European Mesolithic hunter-gatherers, the people which gave rise of the Mal’ta boy, as well as Oceanians, East Asians and Andaman Islanders. Like the “Ancestral South Indians” my impression is that this group does not exist in “pure” form today, but rather must be inferred. As this 40 thousand year old individual from Pe┼čtera cu Oase, Romania, is no closer to Europeans than to East Asians, it seems implausible that it was ancestral in any substantial fraction to modern Europeans. The third element has affinities to central Eurasian groups.

51yuNuckdiL._SY344_BO1,204,203,200_Influenced in part by Clive Finlayson’s The Humans Who Went Extinct, I think it seems likely that the earliest group in western Eurasia to contribute substantial ancestry to modern Europeans are the Gravettians. Though even they flourished prior to the Last Glacial Maximum, so this is not assured in my mind. But it seems plausible to me that the population cluster which David Reich’s lab term “Ancient North Eurasian” (ANE) may have diverged from that ancestral to Mesolithic Western European hunter-gatherers as the Gravettian culture spread and subdivided.

The main fly in the ointment of this narrative is the Kostenki 14 remains, which date to a few thousand years later than Oase 1, and is from its north and east (the Don river valley). Kostenki 14 seems to exhibit all the hallmarks of carrying the signatures of ancestry which define Northern Europeans. That is, the hunter-gatherer heritage, the mysterious Eurasian element defined by the Mal’ta boy and found in Native Americans and brought to Europe by groups from the east ~4,000 years ago. So where does that leave us? My understanding is that people don’t have good clarity on Kostenki 14 right now. But the possibility that most ancient human populations left no descendants would be an easy explanation. It was an earlier admixture of the basic elements which came together again during the Holocene.

Addendum: A minor aspect is to note how Y and mtDNA may give different inferences. I suspect that that is a function of the fact that we don’t know about the ancient distribution of the Y and mtDNA well enough.

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