Ancient Iron Age DNA found in Tianshan, a part of what is now Inner Mongolia well to the east of the Tarim Basin, has been determined to have a strongly Indo-European profile. This is pretty much the most eastern example of pre-modern Indo-European communities.
The authors then used the f3 statistic to show that the old Shirenzigou individuals are best modeled as derived from a genetic mix between a Yamnaya population as a western source and a North Asian or Korean population as an eastern source. The f3 statistic also confirms the lack of Neolithic farm ancestry among the old Shirenzigou individuals, like those of the Yamnaya culture, but unlike the old individuals of the Srubnaya, Andronovo or Sintashta cultures.
The authors then used the qpAdm software to estimate the Yamnaya ascendance proportions. The result gives a value which varies from 20 to 80% according to the individuals of Shirenzigou. The best modeling gives a genetic mix of three sources: Yamnaya, Oultches
and Han Chinese. While the majority of Shirenzigou individuals have more Oultches or Hezhen ancestry, two individuals (M820 and M15-2) have more Han ancestry.
The strong difference in Yamanaya ancestry (between 20 and 80%) among Shirenzigou individuals suggests that genetic mixing occurred shortly before the formation of the archaeological site during the Iron Age. But other explanations are possible, such as the fact that the region was a crossroads of exchange for millennia, or that the western source was already very heterogeneous.
In conclusion, this study shows that Yamnaya culture or Afanasievo culture has spread south-east to the slopes of the Tianshan Mountains probably as early as the second millennium BC. JC. and thus favors the steppe hypothesis in the formation of the people of Xinjiang.
More discussion from Razib Khan
who notes this money quote:
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.
Razib then explains in his own words that:
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 Salar Turks (the modern Uyghurs revived an ancient ethnonym).
also has good commentary. Davidski explains that:
During the Early Bronze Age, around 2,900 BCE, a population associated with the Yamnaya archeological culture migrated from the Pontic-Caspian steppe in Eastern Europe deep into Asia, as far as the Minusinsk Basin in South Siberia.
This rapid, long-range expansion was likely to have been the first significant migration of a Yamnaya-related group far to the east of the Ural Mountains, and it resulted in the formation of the Afanasievo archeological culture (see here).
The appearance of Tocharian languages in the Tarim Basin, in what is now western China, is often associated with the Afanasievo culture, mainly because of the confirmed presence of European-related populations in the Tarim Basin during the Bronze Age, as well as the likely highly divergent position of the Tocharian node in the Indo-European language phylogeny.
But the Afanasievo people were separated by considerable distance in space and time from the Tocharians, and can't yet be reliably linked to them with archeological or genetic data. So even though the inference that the former are linguistically ancestral to the latter is quite plausible, it's far from certain.
However, thanks to a new paper at Current Biology by Ning et al., at least we now know that a population with significant Yamnaya/Afanasievo-related ancestry was living in the eastern Tianshan Mountains just a few hundred years before Tocharian languages were attested nearby.
The citation to the paper is:
UPDATE July 29, 2019: Eurogenes discusses the alternative theory
that these individuals are ancestors of the Huns with no clear relationship to Tocharians and points out evidence that supports this analysis.