Razib Khan One-stop-shopping for all of my content

September 22, 2018

The great human migrations (coming in waves)

Filed under: Migrations,Population genomics — Razib Khan @ 8:53 pm

The figure above is from a new paper, Estimating mobility using sparse data: Application to human genetic variation, which uses genomic data from late Pleistocene to the Iron Age in western Eurasia, and then infers migration rate considering both spatial distribution and the variable of time (remember that samples apart in time should also be genetically different, just as those apart in space often are).

The empirical results are shown above, but they validated their method first by running some simulations. Interestingly they modeled the migration as a Gaussian random walk. Which is fine. But I wonder how true this is for a lot of the Eurasian migrations of the last 10,000 years. Perhaps the the distribution of distances from the place of birth would turn out be multi-modal, with a minority of individuals tending to make “long jumps”?

With that out of the way, it’s fascinating that migration peaks around the Neolithic transition, the Bronze Age, and then the Iron Age. If you read a book like 1177 BC, you know that there was a major regression in the 13th century BC across the Near East, and for several centuries the region was in a “Dark Age.” In The Human Web William H. McNeill argues that one of the reasons for the length and depth of this Dark Age is that the network of complex societies exhibited less density and so less redundancy to failure.

The authors conclude:

We find that mobility among European Holocene farmers was significantly higher than among European hunter–gatherers both pre- and postdating the Last Glacial Maximum. We also infer that this Holocene rise in mobility occurred in at least three distinct stages: the first centering on the well-known population expansion at the beginning of the Neolithic, and the second and third centering on the beginning of the Bronze Age and the late Iron Age, respectively. These findings suggest a strong link between technological change and human mobility in Holocene Western Eurasia and demonstrate the utility of this framework for exploring changes in mobility through space and time.

Earlier they say:

We find strong support for a rise in mobility during the Neolithic transition in western Eurasia, likely corresponding to a well-established demic expansion of farmers, originating in the Middle East and resulting in the spread of farming technologies throughout most of Western Eurasia

One of the main findings of Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past is that oftentimes change is not gradual. Consider the transition to the Corded-Ware society in Northern Europe.

The “demic diffusion” model is an easy one because it relies on the mass-action of individuals and family-groups as they expand in space through high fertility rates. And yet one thing that I think it misses is the socio-political context of that demic diffusion. For prehistoric periods we don’t have writing, and so no socio-political context. This is why in War Before Civilization the author focused on ethnographies of historical societies which came into contact with literate cultures which recorded their organization and folkways. The short summation is that these societies were often very aggressive and well organized for war. Additionally, hunter-gatherers themselves were keen on expanding farmers, and it seems clear they too could mobilize for violence.

The upshot is we need to think of the rise and expansion of strong states and expansionist polities as the context for an increase in the rate of migration. The reality of low migration rates in Pleistocene Europe was pretty evident even before this formal analysis. The pairwise genetic difference due to drift, and therefore low migration rates, for some nearby populations in the Pleistocene and early Holocene indicates that small-scale societies tend to be quite insulated from each other. In contrast, the Iron Age has witnessed a great deal of admixture, as large states and polities, as well as meta-ethnic identities, have broken down genetic barriers.

A regression around 1000 BC correlates neatly with reduced migration, This was almost certainly due to the fact that without larger states much of West Eurasian society, such as in Greece, had disintegrated into smaller tribal units.

Future historians and geneticists will notice that in the period between 1500 and 2000 the distribution of the Y chromosome lineage R1b1a1a2 expanded far beyond Western Europe. They will also understand the political context for this expansion of the lineage…

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