About ten years ago a standard model of the understanding of the peopling of the world by modern humans was that ~50,000 years ago a massive demographic swell out of eastern Africa overwhelmed, to elimination, all other human populations. With a few exceptions, such as the New World, these modern Africans quickly settled down, and the extant distributions of genes, generally mtDNA and Y lineages, reflected the long equilibration between then and now (the recent changes in the New World being an exception to that). Human genetic variation then could be understood as having been shaped by a rapid pulse expansion, and then a subsequent stabilization where genetic variation was maintained by geographic barriers across founding populations, and diminished by gene flow governed by isolation-by-distance. To a great extent this is the story you’ll find in Stephen Oppenheimer’s Out of Eden: The Peopling of the World.
To a large extent that story was wrong. Ancient DNA in particular, though not exclusively, has reshaped our understanding of the past (see Toward a new history and geography of human genes informed by ancient DNA). The ubiquity of population discontinuity and admixture suggest that it was naive to assume that modern genetic variation in any way reflects the founding stock which initially arrived in the first wave in many regions of the world. Additionally, the existence of archaic admixture in most modern lineages attests to the fact that the chasm between “them” and “us” was not perhaps quite as great as some might have claimed.
The ubiquity of population replacement is the reason I recent predicted that the first Aurignacian genome would show no relation to modern Europeans. (I was correct for what it’s worth) That is, modern humans in Europe have no special relationship to the first modern humans that settled Europe 45,000 years ago. The work on ancient DNA does suggest that modern Europeans have hunter-gatherer ancestry…but how deep does this go? I hazarded that perhaps the Gravettians are the earliest candidates for being the direct ancestors of the “Western Hunter-Gatherers” (WHG), who contribute a substantial portion of their genes to modern Europeans through Mesolithic hunter-gatherer populations. But, I wouldn’t be surprised if the genomic character of European Mesolithic hunter-gatherers was determined after the Last Glacial Maximum, ~20,000 years ago.
A new paper in Current Biology, seems to tip toward the latter conclusion. Pleistocene Mitochondrial Genomes Suggest a Single Major Dispersal of Non-Africans and a Late Glacial Population Turnover in Europe. In particular, using a confluence of “best of breed” phylogenoomic methods and archaeological dating the authors contend that there was a major turnover in the mtDNA heritage of Europeans ~14,500 years ago, during the Bølling-Allerød interstadial, a relatively mild and warm period of the Pleistocene before the sharp and harsh regression of the Younger Dryas. The big result is that some very old (pre-LGM, Gravettian) belong to mtDNA haplogroup M. This is one of the two major groups common outside Africa, but it is absent in Europe today (the Roma harbor M because of their South Asian heritage).
The lineage that to a great extent has been canonical as that of European hunter-gatherers, U5, seems to have increased in frequency only late in the Pleistocene, during the above warm period. Because of the nature of random genetic drift we do expect lineage to go extinct over time. These are mtDNA, direct maternal lineages, so only one locus in the genome (though mtDNA is copious, so tends to be low hanging fruit for any new extraction technique). The combination of low long term effective population sizes and meta-population dynamics on the Eurasian fringe might mean that these are not unexpected results. But as suggested in the paper there is also a great possibility that the disruption of the interstadial resulted in some advantage to a particular subset of Pleistocene Europeans, who expanded rapidly, replacing their competitors. Many of the hunter-gatherers of the Mesolithic have relatively low genetic diversity in comparison to modern populations, suggestive of the small population sizes on the European frontier.
The expansion of U5 at the expense of other lineages though around ~14,500 years ago does seem to not be attributable purely to chance according to the models tested within the paper. Then what? One hypothesis is that the climate change resulted in extinction of many populations ill equipped to adapt to climate change, and these were later replaced by newcomers. Another, not exclusive, model is that there was conflict between different groups, and the climate change opened up opportunities for one subculture. There is an allusion to megafaunal extinction in the paper around this period…perhaps we should think of humans as just another megafauna for the super-predator cultures?
One way to look at geological process is that it is uniformitarian, not catastrophic. But it strikes me that with human demography catastrophic pulses are quite common on a geological scale. Why? Likely because cultural evolution is not quite so gradual and continuous, but that innovative revolutions and rapid sweeps of inter-group competition “thin the field,” so to speak.
The main caution I would add is that though we know a lot more than we did, we still no little. What was the ancient population structure in prehistoric Europe? Really we don’t know much, as the sampling is thin at best. That is changing.