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Health

No one knows who Europeans are descended from (yet)

Gene ExpressionBy Razib KhanSeptember 5, 2009 4:57 AM

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After reading Genetic Discontinuity Between Local Hunter-Gatherers and Central Europe's First Farmers I'm left scratching my head a bit. Cut-out black & white models really benefit from lack of data, and now that there's some serious data I think perhaps that we need to think about starting from a clean slate in many ways. The title of the article is rather justified in the local contexts:

on the mitochondrial DNA (female lineage) there is an enormous difference between farming and non-farming populations.

Here are the locations of these samples:

farmhun.jpg

The genetic disance seems to have been 5-6 times greater than between modern European populations. Additionally, modern Europeans are very different from both these prehistoric samples (though somewhat more distance from the hunter-gatherers than farmers). One of the major reasons for this difference is the very high frequency of haplogroup U among the hunter-gatherers, in particular the subclade U5. This lineage is still around, in fact it was labelled "Ursula" in Bryan Sykes' The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry, the earliest European lineage. It would seem though that this lineage was far more frequent after the Ice Age than Sykes' model would have predicted. In The Real Eve: : Modern Man's Journey Out of Africa the author asserts that most of the genetic variation we see around us was already fixed by the Last Glacial Maximum, and these data would strong suggest that that extreme model is falsified. Today the frequency of U5 exhibits a south-north cline in increased frequency in Europe, as noted in the text of the paper:

Europeans today have moderate frequencies of U5 types, ranging from about 1-5% along the Mediterranean coastline to 5-7% in most core European areas, and rising to 10-20% in northeastern European Uralic-speakers, with a maximum of over 40% in the Scandinavian Saami.

Does this then mean that the Uralic peoples of northern Europe, in particular the Saami, are the exemplars of Ice Age European man? Perhaps. But it is also true that populations change over time, so even if the Saami have more in common in terms of their ancestry with the pre-Neolithic peoples of Europe, they may have changed a great deal in terms of their traits over the past 10,000 years. Finally, it may be that a particular non-agricultural lifestyle may select for particular mtDNA lineages! Or, perhaps the U haplogroup confers advantages in colder climates, selective pressures which were naturally relaxed after the last Ice Age. These sorts of muddled confounds have to be entertained, especially in light of the fact that the farmers who were contemporaneous with the European hunter-gatherers seem different from modern Europeans. That leaves us with the reality that a parsimonious model with a few simple historical-demographic parameters may not explain the variation of genes in modern Europe, or in much of the world. But why should we expect such simplicity in the past when we don't have it today? Consider this map of China, which greatly simplifies, but illustrates that as the Han spread south of the Yangtze they settled the bottom-lands first and left the less arable regions for native peoples. Farming does not spread on a wave of advance as the crow flies, but rather percolates like water, seeking out rich soils and favorable climates. This would especially be the case as one pushes in new lands where one's crop toolkit may not be optimized in the first place. The archaeology in Europe suggests this occurred as farmers pushed north of the Alps and up the Danubian river systems, pockets of agriculture slowly expanded outward into less favored lands. This sort of spatial topology does not lend itself to succinct verbal treatments, or perhaps even elegant mathematical ones. Cite:Science DOI: 10.1126/science.1176869

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