And truth shall come out of the mists of legend

And he will be a wild man; his hand will be against every man, and every man’s hand against him; and he shall dwell in the presence of all his brethren.

– Genesis 16:12

By now you may have seen or read two important papers which just came out in Science, 2000 Years of Parallel Societies in Stone Age Central Europe, and Ancient DNA Reveals Key Stages in the Formation of Central European Mitochondrial Genetic Diversity. The details have been extensively explored elsewhere. If you don’t have academic access I highly recommend the supplement of the second paper. It’s also very illuminating if you don’t have a good grasp of the nuts and bolts of archaeology (I do not). I can’t, for example, confirm whether the merging strategies of different archaeological cultures were appropriate or not, because I’m not totally clear in my own head about the nature of these distinct archaeological ‘cultures’ (quotations due to the fact that archaeologists infer culture from material remains, and so they may not be cultures in the sense we understand culture). But the overall finding is clear, in ancient Europe thousands of years ago there were multiple demographic replacements and amalgamations. The post-World War II thesis in archaeology that one could not infer changes in the demographic character from material remains (because the latter can diffuse purely through memetic means) seems to be false. The correspondence is surprisingly tight.

In its broad outlines this was clear before these papers emerged. There is very little I would change from my post The last days of Grendel. This confusing welter of societies in prehistoric Europe is hard for us to conceive of (or reconstruct with any plausibility) today, and as one of the authors of the broader mtDNA paper observes you could not infer this pattern of replacements based on modern patterns of variation. Phylogeography inferring the past from present distributions of variation clearly has limitations, because it is constrained by the necessity to adhere to parsimony in the absence of a dense enough data set. In the early aughts the argument was between scholars who adhered to a more dominant role for demographics in transferring the farming lifestyle (L. L. Cavalli-Sforza et al.)  from the Neolithic societies of the Middle East to Europe, and those who pushed forward the thesis of cultural diffusion (Sykes et al.). These are obviously stylized extreme positions, but they capture the essence of the dispute in regards to how cultures transform and expand. Scholars looked at present day European and Middle Eastern populations, and compared their genetic relatedness, usually with male and female lineages (Y and mtDNA).

There was a major problem with this model: the ancient DNA we have is telling us that present population genetic distributions are poorly correlated with past population genetic distributions. And, not only are the ancient populations of Europe rather well mixed and overturned, like a well tilled field, but it seems entirely likely that those of the Middle East are too. Therefore the methodology was bound to mislead from the get-go; the premise of a few major population movements was false. But there was I believe another major lacunae in our understanding: prehistoric people were not entirely atomized. Whether one believed in the central role of demographic movements or cultural transmission, both theses seemed to posit that prehistoric human populations were mobilizing and interacting mostly on a small scale. Diffusing. This seems likely to be wrong. Or at least it misses enough of the picture that it turns out to give a false impression.

To understand what I’m getting on, consider the American migration west in the 19th century. There were multiple forces at work. First, there was a real demographic pressure in many parts of the United States. New England for example was literally at capacity. It simply had no more land for subsistence agriculture which could support a larger population beyond the Malthusian limit. There were three primary responses. A transition up the “value chain” toward industry, made possible by the natural endowments of water power available in the region. Decreased total fertility rate (related to the first). And finally, a mass migration west, first to upstate New York, but then across the Great Lakes and out even to the Pacific. To a great extent these shifts can be modeled as individual (for family/firm) dynamics. People are responding rationally to changing incentives. But this misses “higher level” structural shifts.

As we all are now well aware the United State government entered into a massive program of ethnic cleansing and pacification of the native populations of the western territories, making migration a viable option. It acquired the western seaboard states through victory in war (California) or diplomatic bluster and coordinated demographic assault (Oregon and Washington). These events are linked to macroscale cultural dynamics, encapsulated in a slogan such as Manifest Destiny. Increasing the geographic scale of the model cultural and demographic changes in Europe itself also made itself felt in the United States (i.e., European migration to places such as the Midwest were important contributors to settlement of the nation, and this migration was often due to social and political dynamics in source nations). The reality of these macroscale dynamics means that demographic shifts often occurred in pulses, in a discontinuous fashion.

Credit: dbachman

Because prehistory is defined by the lack of writing from which we can draw detailed narratives, we will always be in the dark as to the specific macroscale dynamics which resulted in the cultural and genetic shifts we infer (barring the development of time machines). But, we can at least construct a correct framework get a true flavor of the context of how humans interacted in the past. As I have stated elsewhere, I believe that once autosomal and Y chromosomal results come on line (mtDNA is more copious and so easier to extract) we will see that many of the discontinuities and shifts are actually attenuated in the female lineage. What I mean here is that the picture from these papers may actually be less radical than the real shifts truly were. In India the source populations for admixture were distinct enough that it seems clear that admixture was male-mediated. West Eurasian Y chromosomal lineages are more well represented fractionally than West Eurasian autosomal ancestry, which is more well represented than West Eurasian mtDNA. The whole zone from West Asia out toward Atlantic Europe was more of a continuum, so solid inferences will have to wait on the ancient DNA.

Finally, one last big picture aspect which I think is important to note is that the genetic distances between ancient populations across small spatial scales was very large. I suspect that with the rise of agriculture, and imperial states, we have seen a massive process of genetic requilibration across vast swaths of Eurasia in particular. Though I think we mislead ourselves if we view prehistory purely as an affair of small scale bands with vague higher order structure, it is still the fact that the scale was smaller than what came later. That leads me to conclude that population genetic diversity as a function of distance in the far past was likely greater than it has been across most of recorded history. So inferences about the character of human genetic diversity derived from contemporary variation is misleading.* The large differences between Bushmen populations may be highly representative of what was the norm in the past.

* To be clear, Fst between continental populations may be the same. But Fst over small scales may have been larger.

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Source: Discover Magazine – Gene Expression