The period between 300 AD about 750 AD is sometimes termed the “Buddhist Age.” The reason for this is is that this was the period when Buddhism was established in China, and, was still a force in mainland South Asia. It is also when Buddhism was arguably the dominant religion in much of Central Asia. In fact, Buddhism probably arrived in China mostly through this route, via the city-states of the Tarim basin.
A point of interest for many in the public is that some of these Tarim basin Buddhists looked very “Western.” That is, they had European features and coloring. The reason for this is that their ancestors were the eastern edge of the Indo-European migrations on the steppe. Many of them famously spoke Tocharian languages, an extinct branch of the Indo-European languages. But others spoke Iranian languages. Iranian not in that they came from Iran, but that they were descended from proto-Iranians of the steppe.
A few years ago there was a discussion on this weblog and elsewhere about very recent admixture dates for the western and eastern admixture components in the Uyghurs. That is, after 1000 AD. This struck many as too recent. I think perhaps I have an answer for what happened.
The admixture plot to the right has Uyghurs with some ancient and modern samples. The Uyghurs are about 50% East Asian, as you’d expect. But the balance of their ancestry is split between Indo-European populations, and those of Neolithic Iran (with a slight bias toward the former).
Were the Indo-Europeans who arrived in the Tarim basin 4,000 years ago different from their cousins to the west? I doubt it. Rather, the “Iranian” ancestry came later. And I think I know how it came: the modern Uyghur language is actually descended from a dialect of Karluk Turk. It is not related to the ancient Uyghur language. After 1000 AD the process of Islamicization that occurred in the Tarmin basin expanded from the western edge, in Kashgar, until the final Buddhist Uyghur kingdom of Turfan in the east fell.
A simple model of elite diffusion suggests Islamicization and language-shift occurred without demographic perturbation. But another alternative is that the switch to Islam and the Salar language resulted in gene flow from Central Asia proper, into the Tarim basin. The people of Central Asia were Turkicizing at this period, but their origin was as Iranians. Many of these people were an admixture of people from the Pontic steppe, and, the Iranian agriculturalists.
If in the future someone does a temporal transect of the Tarim basin, I predict that one thing they will note is the sharp rise in Iranian agriculturalist ancestry over time, with a particular jump around 1000 AD (though the Sogdians were also a presence in these city-states).