The Holocene lattice

Citation: Towards a new history and geography of human genes informed by ancient DNA, Joseph Pickrell, David Reich, doi: 10.1101/003517

440px-PazuzuDemonAssyria1stMilleniumBCEThere were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.

– Genesis 6:4

and David Reich have put up a preprint at BioRxiv, Towards a new history and geography of human genes informed by ancient DNA. Since it’s a preprint at BioRxiv you can 1) read it for free 2) comment on it. It is a magesterial review of “where we are,” though close readers of this weblog may not find much that is new in their survey of the empirical results which are coming out of human population genomics and ancient DNA analysis. In regards to this let me highlight two sentences. First, it is now clear that long-range migration, admixture and population replacement have been the rule rather than the exception in human history. Second, the serial founder effect model is no longer a reasonable null hypothesis for modeling the ancient spread of anatomically modern humans around the globe. For the second I’m thinking in particular of Sohini Ramanchandran’s 2005 paper, Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa, though the model is older than that obviously, as is made clear in the acknowledgments. For the massive ground that the paper covers when it comes to the latest findings it is highly concise, and I commend it to anyone wishing to dive into this exploding literature. Pickrell & Reich show how the analysis of dense marker data sets with more powerful techniques has allowed for the teasing apart of the interlaced layers of the historical genetic palimpsest. But, the complement to this has been the development of the field of paleogenomics, which allows for the explicit analysis of ancient genomes. Another section of the preprint touches upon the technological changes which are allowing for more and more DNA analysis of ancient samples. In particular they point out that rather than focusing on sequencing very rare pristine remains the near future may be in looking at known SNPs on a larger number of samples, because the technical challenges for such typing are far lower.

Credit: Maulucioni
Credit: Maulucioni, Haplogroup R

The preprint is focused on the genomic aspects of this research because the authors are statistical geneticists, but it does not hesitate in offering up a host of historical and archaeological hypothesis which might be tested in the next few years. Also, they do not take a definitive position on the role of long distance migration and punctuated admixture events, as opposed to more continuous gene flow (though the methods which analyze contemporary populations seem to be better at detecting the former). So I will hazard a general model. It seems that root of what is driving these demographic changes are cultural changes. And cultural changes over the past ~30,000 years have been very fast and punctuated, and have accelerated. To given an example, the cultural chasm between a Egyptian in 500 AD as opposed to one in 500 BC would be far greater than that between two that lived in 500 BC and 1500 BC. Whether the word “revolution” is necessary for cultural adaptations such as the acquisition of agriculture, it seems clear that these were shifts in lifestyle which radically changed the local human demographics, as some populations entered into a phase of rapid population expansion in a condition of land surplus (e.g., farmers can extract many more calories per unit of land than hunter-gatherers, so the first farmers invariably encounter massive land surplus and  operate at the higher boundary of productivity). Basically Peter Bellwood’s model in captures many of the broad features of what occurred in the Holocene to produce ubiquitous admixture we see in the map at the top of this post (the methods pick up the strongest signals, and so usually underestimate admixture). Small group of individuals acquired a cultural adaptations which resulted in a winner-take-all scenarios of demographic expansions until a new equilibrium was attained repeatedly over the past 20,000, and especially 10,000, years. These the primary layers in the palimpsests that geneticists are teasing apart. Additionally, I will add the proviso that I suspect these long distance leapfrogs often became strongly male-biased in the genetic signal. It would be totally unsurprising to me if haplogroup R has its origins in the North Eurasian population which has left a legacy in Native Americans and Europeans.

Archaeologists and historians are going to be reluctant to shift from a dominant position which is skeptical of migrationism. Part of this is due to an ideological bias which emerged after World War 2. It is also simply the fact that the statistical methods employed by the newest batch of researchers are abstruse and difficult for outsiders to decrypt (though I find the methods in Ancient Admixture in Human History comprehensible after a close reading, so it’s not impossible). But archaeologists and historians are essential in constructing plausible models which can explain the genetic patterns we see around us. The motive engine for these changes are cultural phenomena, and cultural researchers are the ones who can shed the most light on the possibilities.

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