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Friday, April 27, 2012

Prehistoric Scandinavians genetically most similar to modern Poles


Scientists from Uppsala University have managed to extract genome-wide markers from the early Neolithic remains of three hunter-gatherers and one farmer from southern Sweden. They only pulled a few thousand SNPs from each sample, but that was enough to successfully compare the ancient remains to modern Europeans. The results of their study, published in Science Magazine today, reveal that Poles top the allele sharing list with the the hunter-gatherers. Interestingly, Poles also show higher allele-sharing with the farmer than Swedes do, but not as high as Cypriots and Greeks. The figure below illustrates this clearly.




Now, if we look at the STRUCTURE analysis from the study, it suggests that the farmer was also very similar to Sardinians, albeit with more North European admixture.


So I think it's pretty clear we're dealing here with an individual of mostly deep East Mediterranean origin, whose ancestors made their way from West Asia to Western Europe, probably via maritime routes, and settled islands like Sardinia in the process. They possibly moved into what is now Sweden via Western and Central Europe, but then again, they might have gone straight from the Mediterranean to Scandinavia by boat.

But why is it that Poles show higher similarity to these Neolithic Scandinavians than Swedes do? Firstly, it's important to realize that the differences aren't that great. Note, for instance, that Swedes are the second most similar population to the hunter-gatherers after Poles. However, clearly, the data suggests that there had to be other population movements into Scandinavia after the late Neolithic. These also likely affected Poland, but to a lesser degree.

No one yet knows what these were exactly, but if I had to guess, I'd say the Bell Beaker folk of the Copper Age represented one of the major waves (see figure below from "Europe during the third millennium BC and Bell Beaker culture phenomenon: peopling history though dental non-metric traits study" by Jocelyne Desideri). Also, another factor might be that the hunter-gatherers tested by Skoglund et al. belonged to the Pitted Ware culture, which arrived in Scandinavia from the Eastern Baltic.



Anyway, I'm absolutely delighted with the results from this study. The reason is that they correlate very closely with the experiments I've been running with ADMIXTURE, aimed at untangling the story of the peopling of Europe. Note, for instance, the close correlation between the STRUCTURE plot above, and the results from my Hunter-Gatherer vs. Farmer analysis (see here). All you have to do is add up the blue and purple components from the STRUCTURE graph, and you'll basically get my "Baltic hunter-gatherer" cluster. Also, the orange component is very similar to my "Mediterranean farmer" cluster.

If Skoglund et al. had access to more prehistoric samples, then it's likely these would create their own clusters. That's because the four Neolithic individuals they tested, especially the hunter-gatherers, seem to fall outside the range of modern European genetic variation, like on some of the PCAs below. The appearance of ancient clusters wouldn't invalidate the current results, because such clusters would no doubt show a close relationship to those created by modern samples. However, I’m pretty sure they'd give us a better idea of how much hunter-gatherer ancestry survives in modern Europeans, because they wouldn't be affected by such factors as genetic drift since the Neolithic. So that’s something to look forward to in the future.



Reference...

Skoglund et al., Origins and Genetic Legacy of Neolithic Farmers and Hunter-Gatherers in Europe, Science 27 April 2012: Vol. 336 no. 6080 pp. 466-469 DOI: 10.1126/science.1216304


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