The Yamnaya origins of the Tocharians?

Eurogenes points me to a new paper in Current Biology, Ancient Genomes Reveal Yamnaya-Related Ancestry and a Potential Source of Indo-European Speakers in Iron Age Tianshan. The conclusion:

Combining both the genetic and archaeological evidence, we here provide the first direct evidence of an early stage of population admixture around 2,100 BP in Xinjiang in Western China. Our study supports the “Steppe hypothesis” over the “Bactrian Oasis hypothesis” for the peopling of the Xinjiang region. The high amount of Yamnaya or Afanasievo-related ancestry in the Iron Age Xinjiang individuals indirectly supports the introduction of Indo-European languages into the region that survived in the form of Tocharian until the late first millennium CE. We note that we need more individuals from different sites and time periods to shed more light on the genetic history of the Tarim basin and the whole Xinjiang region.

The ethnolinguistic pattern of the Tarim Basin in early historic times was complicated. In addition to very distinct “Tocharian” languages, some of the cities were dominated by Iranian peoples. The modern-day Uyghurs are almost certainly descended from some of these populations. Uyghur men carry both R1b and R1a.

These results from ~2,000 years ago of ten individuals (five of them well dated) suggest that the northern Tianshan was populated by peoples that were direct descendants of the post-Yamnaya Afanasievo, who mixed to varying degrees with trans-Siberian populations (the mixture seems to have had a wide range of fractions). Two of the males carried R1b, the haplogroup dominant among the Yamnaya, but not latter cultures of the steppe (Andronovo). These individuals lacked European farmer ancestry, again indicative of their isolation from dynamics on the western forest-steppe that resulted in genetic differences between the successors of the Yamnaya and the Yamnaya proper.

The argument here is somewhat by elimination. Historical records indicate that some of the cities of the Tarim, particular those of the southern fringe of the basin, were Iranian speaking. Additionally, Iranian cultures are associated with haplogroup R1a, and the Sintashta-Andronovo cultures all had European farmer ancestry. In contrast, R1b is rare outside of Europe (though it is found in Kalash and Yaghnobi), but is found among Uyghurs and among these samples. Tocharians are the most likely descendants of these people, who arrived in the region almost 5,000 years ago.

This explains how the Tocharian languages were so distinct, and, their deep separation from other Indo-Europeans. The Tocharians were isolated and diverged very early. Later they were joined by Iranian groups. Eventually both these were absorbed by Turkic populations, first the Uyghurs, and later the Karluk Turks (the modern Uyghurs revived an ancient ethnonym).