David Reich’s talk at SMBE 2014 has come and gone, and it seems like from the reports on Twitter that it was a synthesis of the results in their bioRxiv preprint from last fall, Ancient human genomes suggest three ancestral populations for present-day Europeans, and the ancient DNA samples from Samara in Russia. The major takeaway being that genetically modern Europeans are by and large an admixture between three very distinct population groups, which fused together only during the Holocene (last ~10,000 years). A stylized variant of the model is represented in the figure I’ve taken from the bioRxiv preprint.
But a question that’s nagged me is how realistically to take the proposition that some of these nodes are genuinely distinct populations separated by barriers to gene flow, as opposed to being part of a broader continuum of genetic variation? For example, populations separated by water barriers such as those of Sahul almost certainly exhibited enough attenuated gene flow so that drift could work to shift their allele frequencies away from the populations of Sundaland. On the other hand, it seems reasonable to me that genetic variation on the broad plain from western Europe to the Urals in northern Europe may have been mostly clinal, with each population exchanging genes with the next, from the Atlantic to the fringes of Siberia. Some have argued that the Paleo-Siberian population which has been termed “Ancient North Eurasian” (ANE) is only part of a cline across Eurasia which extends out toward the European hunter-gatherers (to be clear, I’m skeptical of this because the genetic distance seems too great, but who knows how rapidly genetic distance increased as a function of distance in the Pleistocene?). On the other hand one might posit regions of extremely low population density during the Pleistocene due to inclement conditions in many regions so that various ancestral groups may have been isolated enough to drift apart due to more conventional genetic isolation (for example, it seems to me that the ancestors of groups such as the Han Chinese have been isolated from western Eurasians for ~40,000 years, unless you count relatively recent fusions such as the Uygurs and the peoples of Turkestan more broadly).
And yet some discussions I’ve had recently (on Twitter) have made me clarify my thoughts and admit that for some purposes it really doesn’t matter whether ANE was part of a genetic continuum or not in relation to European hunter-gatherers. The reason is that I believe that the human past was characterized by many powerful demographic sweeps which we are beginning to comprehend due to the power of ancient DNA. If the expansions occur from specific narrow geographic zones, and overwhelm a huge area adjacent, then whether the genetic variation is characterized by clines or not is irrelevant, as it will look like a discontinuous replacement in regions far from the core point of origination.
This brings me to a major update in my own personal views on these sorts of dynamics. I recently read Richersen, Boyd, and Heinrich’s Gene-culture coevolution in the age of genomics. It’s a good overview of the intersection of the fields of cultural evolution and genomics, but too often it struck me that the authors were keen on ascertaining how genomics could illuminate problems in cultural evolution, without considering the converse. That is, what can our understanding of cultural evolutionary process tell us as to what patterns of genomic variation we should see around us? Modern human genomics has a surfeit of data, and population genetic theoretical machinery of yore is being drafted to hammer away at the massive rich empirical seams, but in the domain of paleodemography a model of culture is probably more informative in allowing us to gain an expectation of the distribution of dynamics. More concretely cultural and economic factors are clearly critical in understanding why a few nations of western Europe* entered into massive settlement of the New World after 1492, and others did not. Obviously we’ll never have historical records from 50,000 years in the past, but a better understanding of the processes of cultural evolution might allow us to judge whether rapid archaeological transitions signal demographic shifts, or not. And these then might serve as an interpretative framework for genomic results.
* I specify western Europe because the genetic distances here are small, and the major settler nations, the British and Iberians, are not particularly clustered together.