The palearctic conveyor belt

Palearctic ecozone
Palearctic ecozone
Citation: Nature (2014) doi:10.1038/nature12960

After reading the supplements of the La Brana hunter-gatherer paper I have a few extra thoughts in a very general sense. One result you see on PCA plots with Europeans is that the La Brana sample (along with the Swedish hunter-gatherers) are shifted toward modern Northern Europeans, and to some extent even Finns. Obviously this is noteworthy because they don’t cluster with modern Spaniards (even Basques). But when you plot them on a world-wide distribution you see something else: the La Brana individual is the most shifted toward Asians of any of the European samples (this includes British, Iberians, Utah whites, Finns, and Tuscans). This isn’t due to cryptic East Asian ancestry. Rather, it’s that Paleo-Siberian ancestry which can be found in many West Eurasians and Native Americans. This is  in contrast with the Luxembourg hunter-gatherer which is dated ~1,000 years earlier than the La Brana individuals. I don’t know if the dates are reliable enough, but it seems plausible enough that there may have been events of demographic change on the scale of 1,000 years in ancient Europe.

Citation: Nature (2014) doi:10.1038/nature12960

To get a better sense they used D-statistics to estimate possible admixture/relatedness between Mal’ta and various populations. A negative value indicates more drift shared between H2 and H1 (admixture, gene flow, common ancestry, etc.). Sardinians are a European floor for “Ancestral North Eurasian” (ANE), which the Mal’ta individual represents. La Brana is statistically significantly more related to Mal’ta than all tested populations except for Orcadians and Russians. It is more shifted, but not to a statistically significant degree in those cases. The main qualification I want to add here is that Lazaridis et al. have reported that there is a “Basal Eurasian” population which has admixed across Western Eurasia, and increases the distance between Mal’ta and the populations with which it has mixed. The farmers which introduced agriculture to Europe seem to have brought this element, and it is found at a high fraction in the Caucasus, explaining the value for the Adygei. In fact the Adygei have more not less ANE than Northern Europeans. Since the La Brana individuals likely lack Basal Eurasian that could be affecting the D-statistics, inflating the implied ANE in the La Brana.

With all that qualification, it does seem that these La Brana individuals have genetic affinities with populations as far afield as Central Siberia, and the New World (via these Siberians). Additionally, like the other ancient European hunter-gatherers it exhibits signs of reduced genetic diversity compared to modern populations. What are we to think of this? In the broad view this is really not that surprising. 400,000 year old Iberian hominins seem to have had affinities to populations which were later found in Siberia. The Neandertals, from the Altai to Spain, seem to be surprisingly similarly and genetically homogeneous. Eurasian wolves have also gone through a population bottleneck during the Pleistocene. What this suggests to me is that the Palearctic ecozone has been characterized by a high degree of population mobility, and, extinction. Ancient DNA is sampled from northern locations due to likelihood of preservation, but these regions are also on the settlement frontier, and it wouldn’t be surprising to me that the populations are going to be characterized by low effective population sizes because they’re expanding rapidly from small founding groups.

Finally, this has some implications for our model of population assimilation and replacement. It seems that modern Europeans are a synthesis of disparate strands which have co-mingled over the past 10,000 years. One element, the majority element in the north and east, are hunter-gatherers which descend from the early West Eurasian settlers of the northwest. These were the Ice Age inhabitants of Europe. Likely they had long had connections across the latitudes of the Palearctic zone. One thing that we are seeing with the pigmentation genes is that these Western European hunter-gatherers were very different from modern Europeans. If about ~50% of the ancestry of Western Europe derives from these populations, then we’re confronted with the possibility that several of these loci have experienced nearly complete selective sweeps after an admixture event. This is not impossible, but, another option is presented us when we consider that the far north has long been a conveyor belt of peoples. Eastern cousins of the Western hunter-gatherers, could have brought whole-genome affinities into modern Europeans similar to these ancient individuals. But these may already have changed in their modal phenotype. The paper’s supplements reports that the Y chromosome of the La Brana seem to be ancient branch of haplogroup C, which is dominant in Eastern Eurasia. A possible connection to the Mal’ta people? Perhaps. But it is important to note that very low frequencies of this haplogroup still exist in Southern Europe. So the hunter-gatherers are likely not gone in toto. But combined with the mtDNA evidence of massive changes, this may point to later Bronze Age demographic shifts, being masked by the wide scope of genetic homogeneity in Eurasia.

Obviously the picture is still only partially formed. But it’s nice to have the past being painted so vividly.

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