One of the things you notice when you look at genome-wide data are peculiar populations that seem to be shifted on PCA and other metrics in relation to exotic genetic affinities. For example, Sardinians, Japanese, and Taiwanese aborigines exhibit this pattern. When looking at Han Chinese data, many of the southern samples seem a bit further shifted away from West Eurasians than all the northern Chinese. That is, almost all northern Chinese seem to have low levels of West Eurasian affinity. Some of the southern Chinese do not.
When you look at West Eurasian data, you see evidence of East Eurasian gene flow into parts of Eastern Europe. Among Lithuanians, it seems to be there. It’s old and well-mixed, so it doesn’t jump out at you. But it’s there. Even more striking is that many of the Muslim populations in the Near East seem to have some proportion of East Asian ancestry because of the Turkic expansions.
We know the reason for this ancestry in West Asia. The rise of the Turks in the Islamic world is historically attested (thank you al-Mu’tasim!). Similarly, the arrival of Tatars and Magyars in Eastern Europe is also recorded. In China, various Turkic and West Asian populations arrived after the fall of the Han dynasty in the northern half of the country. I’ve documented on this weblog strong evidence of Indian ancestry across Southeast Asia.
As more ancient DNA comes to light I think one phenomenon that will become more clear is that the cultural tookit of humans over the last 10,000 years has allowed for more continuous, constant, and frequent, long-distance gene flow. Pairwise Fst values crashed with the rise of agriculture and larger-scale polities. But the adoption of the horse and the emergence of agro-pastoralism also served as a reciprocal conveyer belt of genes across the two antipodes of Eurasia.
West Eurasians and East Eurasians still remain genetically distinct. But evidence from Japanese and Sardinians gives a clear indication that within the last few thousand years have substantial reciprocal gene flows.*
* I am aware that in some of the work in David Reich’s lab there is evidence of East Eurasian gene flow into Mesolithic hunter-gatherers in Europe.