Monday, March 26, 2012

Ancient Autosomal DNA Confirms Uniparental Conclusions

There are many samples of ancient mitochondrial DNA, fewer samples of ancient Y-DNA, and fewer still samples of ancient whole autosomal genomes. But, a new examination of four ancient autosomal DNA samples from Scandinavian individuals ca. 3000 BCE, three farmers and one a hunter-gatherer, confirms prior gentic evidence.

Analysis of these sequences in the context of the present day European gene pool suggests that the spread of agriculture involved the northward migrations of farmers. Thus, these data provide the most direct and compelling support for the demic diffusion model of agriculture (as opposed to cultural diffusion) described to date.

Razib, at Gene Expression, draws his conclusions more cautiously. He cautions against all or nothing conclusions, suggests that farmer colonization may have tracked fertile crop growing areas rather than broader geographic distances, and refers to violent conflict at martime boundaries. He concludes:

It seems likely that the expansion of agriculture was more spatially patchy, and exhibited more starts and stops, then the samples we have allow us to infer with any confidence. The arrival of startlingly distinctive populations genetically and culturally across which likely rapidly traversed territory in a point-to-point fashion, and their subsequent extinction or assimilation into local substrate, is less surprising when we keep in mind the discontinuous nature of much of cultural change.

Still, estimates that put the contribution of immigrating farmers to the gene pool at something on the order of 80% relative to European indigeneous hunter-gatherers, with a fair amount of regional variation, seems to be in the ball park of the indications from the evidence. The case, for example, for elite domination by immigrant farmers, who culturally transferred a farming package to a subordinate predominantly indigeneous population, at least in Europe, is weakening in the face of ancient DNA data.

Similarly, DNA studies of modern populations suggest that while modern Bantu language speaking populations in Africa sometimes assimilated a substantial share of indigeneous hunter-gatherer populations, that there was also almost alway a meaningful demic contribution of Bantu immigrants as well.

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