Many years ago I was perplexed by particular patterns in some genes which have been subject to very strong selection. In particular, the locus SLC24A5 has been subject to a powerful sweep over the last 10,000 years across Western Eurasia, to near total fixation in Europe, but still at high frequencies as far south as India. Yet the derived variant is relatively uncommon in East Asia. Groups which carry the West Eurasian variant, such as the Uyghurs, almost certainly obtained it through admixture processes over the last 10,000 years (in the case of the Uyghurs and various northeast Eurasian ethnicities such as the Mongols, this admixture from West Eurasians is mostly in historical time over the past 2,000 years).

The common sense explanation is that vast regions of interior Eurasia were not highly populated for tens of thousands of years. Even after the Ice Age retreated the Eurasian interior would have been particular inhospitable. Though maps of human migration show where humans have lived at some frequency all across the world, they do not usually show any sign of the density. If densities were low enough in the inter-montane zones of Inner Asia, then for all practical purposes the idea of isolation-by-distance gene flow may not have held for the two antipodes of Eurasia for much of the Pleistocene and early Holocene. So have things changed? I believe so. And it comes down to agriculture, which enabled much higher population densities in areas which were previously simply not feasible areas for hunter-gatherers.

A new paper in Science outlines this for Tibet, Agriculture facilitated permanent human occupation of the Tibetan Plateau after 3600 B.P.. I’ll quote the relevant sections of the paper:

On the basis of the above evidence, the prehistoric human occupation of the NETP can be subdivided into three phases. During the first phase (pre–5200 cal yr B.P.), hunter-gatherers made occasional forays to altitudes reaching above 4300 masl, presumably tracking game. During the second phase (5200 to 3600 cal yr B.P.), a longstanding tradition of millet farming that had become widely established along the middle and lower reaches of the Yellow River extended upstream into the NETP. Millet farming had spread across the Loess Plateau after 5900 cal yr B.P. (17) and subsequently spread across these lower reaches of the NETP from 5200 cal yr B.P. Toward the end of the second phase (4000 and 3600 cal yr B.P.), two significant additions are observed in the crop repertoire (text S4 and fig. S6). The North Chinese crops of broomcorn and foxtail millet were joined or displaced on some sites by the principal cereals of the Fertile Crescent, barley and wheat. There has been much interest in the chronology and consequences of the meeting of east and west staple crops in prehistory (18–20). Here, its notable consequence was to facilitate the sustained settlement of the Tibetan Plateau’s higher altitudes. The importation of wheat and barley enabled human communities to adapt to the harsher conditions of higher altitudes in the Tibetan Plateau, a possibility raised in previous studies (15, 21). The key addition was barley. During phase three, from around 3600 cal yr B.P., sites can be divided into those that lie above or below 2500 masl. In the lower-altitude group, the longstanding crops, broomcorn and foxtail millet, are joined by barley as a third component in an otherwise traditional dietary repertoire. In the higher-altitude group, however, the frost-sensitive millet is absent, and the cold-tolerant barley has moved to a primary position (Fig. 2D). Alongside the presence of wheat (also relatively cold-tolerant) and sheep, the diet at these high altitudes has clearly been transformed, but in a manner that enabled sustained settlement at unprecedented altitudes.

There’s been a lot of interesting work on the genetics of Tibetans recently, from altitude adaptation from archaics, to the inference that a great deal of Tibetan ancestry is actually shared with the Han and other lowland groups in the past three to four thousand years. These results make more sense if you realize that the arrival of more advanced agricultural techniques reshaped the possibilities of habitation for humans at higher densities. In fact, it is almost certainly no coincidence that it is during the period of agriculture that the great fusions between the disparate “branches” of the human family tree came back together; higher population densities across huge areas mean that de facto gene flow no go zones disappeared.