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Health

The genetic complexity of prehistoric Sweden

Gene ExpressionBy Razib KhanMay 17, 2011 9:20 AM

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Thanks to the fact that northern Europe is cool and archaeological research is rather well developed in the region due to quirks of history, there are lots of findings from ancient DNA which are answering long-standing questions. In particular Scandinavia is of special interest in regards to the transition of Europeans from a hunter-gatherer lifestyle to an agricultural one. We know that hunting and gathering as dominant modes of economic production persisted relatively late in European history in this region, up to ~5,000 years before the present. From my cursory reading of the material on the spread of agriculture in northern Europe one dynamic which seems clear is that the rate of expansion was not always constant, and that at the northern fringes in particular social or ecological frontiers served to demarcate the limits to the expansion of farming groups, which often originated from the south and east. Additionally, on the maritime fringes of the North Sea and Baltic there seem to have been relatively dense agglomerations of hunter-gatherers which resisted or coexisted with farming populations for long periods of time (perhaps they were more accurately termed fisher-gatherers!). This is where Anna Linderholm's research comes into the picture. I've blogged some of her work before. Linderholm's goal seems to be to synthesize a range of results from disparate fields in understanding how two partially contemporaneous prehistoric Scandinavian cultures related to each other: the Pitted Ware Culture (PWC) and the Funnelbeaker Culture (TRB, which is an acronym for the German name for the culture). The former were hunter-gatherers who tended to rely upon marine resources, while the latter were agriculturalists who engaged in a great deal of animal husbandry. You can find her contribution to the book Human Bioarchaeology of the Transition to Agricultureonline. It's pretty accessible for an ignorant lay person, and in the chapter she outlines some really interesting detail about the relationship between the PCW, TRB, modern northern European populations, and the functional genetic characteristics of these ancient groups. The basic chronological outline seems to be that around 3000 BCE there was a period of hundreds of years of contemporaneous habitation of southern Sweden by the TRB and PCW cultures, though they were spatially segregated. TRB finds seem to be concentrated in inland regions, while PCW were found on the maritime fringe. Additionally, the island of Oland in the Baltic exhibited nearly 1,000 years of coexistence of the two cultures. After 2000 BCE these cultures eventually disappeared and gave way to a homogeneous agricultural Bronze Age society. Were the two cultures of southern Sweden during the Neolithic simply two modes of production of the same people? And were these the ancestors of modern day Swedes? And what can biological anthropology tell us about what they ate and how they ate? Let's start with mtDNA relationships:

The mtDNA results point to the fact that the TRB and PWC are two genetically distinct populations (p G 0.001) (Linderholm, 2008; Linderholm, unpublished). Furthermore, the PWC population appears to be more closely related to the modern Latvian population than the contemporaneous TRB population or any of the other modern population examined. This could imply that the PWC population may have had an eastern/central European origin, whereas the TRB population may have had a continental European origin (Linderholm, 2008; Malmstr€om et al., 2009; Linderholm unpublished). These results could also imply that in Sweden the PWC were part of a large hunter-gatherer complex that spanned vast areas of the central and eastern parts of Europe... Neither of the two populations (PWC and TRB) shows any genetic affinities to the Sami population when compared on the basis of mtDNA sequence. It therefore appears that the ancient mtDNA analyses provide genetic evidence that the TRB and the PWC cultures were Neither of the two populations (PWC and TRB) shows any genetic affinities to the Sami population when compared on the basis of mtDNA sequence....

There's a little ambiguity here about the TRB's relationship to modern Swedes, so let me jump to the discussion where this is clarified:

Over 50 individuals were successfully typed and based on these results it has been shown that the PWC and TRB cultures were genetically distinct populations. The genetic legacy of the TRB continued into the Bronze Age and further in time, suggesting that they made a genetic contribution to modern Swedish populations. In contrast, the PWC seem to have left no genetic legacy to the modern Swedish population, but their genetic signals can be detected in modern north-eastern European populations.....

Modern Sweden was settled ~10,000 years ago after the retreat of the Ice Age. The TRB culture first arrived in Sweden ~6,000 years ago. From these results one might infer then that modern Swedes derive predominantly from a migration of farmers from the north European plain ~6,000 years before the present. In these analyses the identity of the hunter-gatherers resident in region (e.g., Skane) for the 4,000 years after the Ice Age and before the arrival of farming is left unclear. The PWC itself may have been intrusive, and closely related to a broader group of northeast European hunter-gatherer societies. Do recall that this study focused on southern Sweden. We know that the Sami were indigenous to northern Sweden relatively late into the historic period. Whether they too were exogenous on this sort of time scale, we don't know yet.

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Next Linderholm reviews previous results on the prevalence of lactose tolerance among these two societies. The results are pretty straightforward: the PWC generally lack the ability to digest lactose. This makes sense, why would they need this mutation if they weren't engaged in animal husbandry? There's an allele in northern Europeans which is strongly correlated with lactose tolerance. It operates in a dominant fashion, so the fact that the frequency of the allele in modern Swedes is 75% means that lactose tolerance is nearly at 95%. But the question left unanswered is the state of the situation in the TRB. It seems that this is work that is proceeding now (from what I can tell there was less success amplifying the TRB remains for whatever reason), but the author politely hinted strongly: "Analysis has also been performed on individuals representing the TRB population; the outcome of these analyses has not yet been totally verified. In the dataset we can detect a large rise in T-allele frequency in Sweden amongst the farming community (unpublished data)." This is the reasonable expectation, as the TRB kept cattle. It seems plausible that the rise of farming on the northern European frontier was in large part a function of the innovation of dairying, as in these regions the primary productivity of Middle Eastern cereal crops was far less. Rather, less palatable crops needed to be cultivated, and it may be that it was more efficient to use this as feed for cattle which then produced milk (e.g., oats). Finally, they assayed the CCR5-D32 variant in their PWC and TRB samples. This is the "HIV resistance" allele found in many Europeans, whose origin has been hypothesized to be 1400 CE or 3000 BCE, depending on which coalescence model you trust. Interestingly Linderholm found that both of these ancient groups carried the resistant allele, likely pushing back the origin of the variant. So what does this tell us in the end? We need to keep in mind other facts, and not lose the big picture. The populations of Norden, whether Finnic or Scandinavian, are genetically similar. But clearly there is a distinction as one moves from Sweden to Finland. The relationship between the Baltic populations and the Nordic ones is somewhat confused. What I can say from ADMIXTURE runs performed by genome bloggers is that it seems that Scandinavian, as opposed to Finnic, populations seem to have an ancestral component at low, but consistent, levels which is modal in the Middle East. The lack of this distant Middle Eastern affinity among the Finns, and to a lesser extent the Lithuanians, is to me telling. I suspect what you're seeing is an ancient admixture event, greatly diluted by the time one approaches the far north, which was carried from the southwest by the ancestors of modern Scandinavian populations. The Finns were not impacted by this migration, and so do not have any imprint of that distant Middle Eastern ancestral component. Second, the relationship of the PWC, hunter-gatherers, and TRB, farmers, needs to be teased apart. It does not seem this was a classic replacement of hunter-gatherers by farmers. Both these groups were intrusive to southern Sweden in relation to the Mesolithic hunter-gatherers who originally repopulated the peninsula. It could be that there was a symbiosis between the PCW and TRB, but it is notable that the genetic distinction between these groups is very strong. Additionally, the authors note that there is no connection between the PCW and modern Swedes, though there is one between the TRB and modern Swedes (in the late Neolithic and early Bronze Age the TRB were replaced by the "Battle-Axe" culture, which may or may not have been an organic outgrowth of the TRB). It doesn't take rocket science to make an inference of "who won." All that said, I wonder if one could make the case for the PWC being highly efficient superior hunter-gatherers, the "last stand" so to speak of a non-marginal manifestation of this lifestyle in Europe. Their concentration in coastal fringes suggests that they were focusing on regions where their per unit labor had the greatest returns. It may be that for a period they were at demographic parity with the TRB because the farmers had simply not perfect their cultural toolkit. An analysis of isotope ratios in the bones of the remains points to the possibility that the TRB took a very long time to acclimate to the north: "The results show a gradual transition from a hunter-gatherer diet to a diet based on agriculture took place by the TRB people (which is thought to be associated with the introduction of farming on the island) only by the end of the Neolithic and hence much later than had been previously thought." If the TRB were facultatively engaging in behavior more typical of foragers then it seems understandable that they didn't push aside the PWC as easily as they might have, because they lacked the traditional farmer demographic advantage. There's a lot more to come I assume. Though there was some analysis of nuclear ancient DNA the focus here was on mtDNA. That's because mtDNA is copious. But as the techniques get better I assume museums might be more willing to part with parts of their specimens. Overall these results make me more cautious of maximalist clean models of simple replacements. Here's a speculative scenario which I think might have been: - TRB emerges out of a hybridization of native north European plain marine foragers and farmers from central Europe, who are themselves a hybridization of southern migrants and indigenes. - Once TRB crystallizes with an agricultural toolkit more appropriate to the far north it quickly sweeps back the older farming frontier. The old-stock hunter-gatherers of the north melt away. But the TRB then are challenged by the "climax" culture of the northern hunter-gatherers, the very efficient lifestyle of which the PWC were partaking of, focusing on marine resources which were easy to obtain in large quantities. The rise of these hunter-gatherers may partly have been competition, but their lifestyle may also have been dependent upon trade with the dense agricultural societies to their south and west.

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