Those caucasians are probably Eurasian Steppe Nomads. I mean parts of Eastern Europe have Asian DNA from various Asian nomads (huns, mongols, etc)
The Hunnic migration from east Asia to east Europe took place much later (post-AD) than the dating of the Tarim mummies (pre 1000BC). Which means Caucasoids were long present in Chinese prehistory and ancient history several centuries, maybe millennia before the first Huns started moving towards Europe. Heck, that's even a long time before the possibility of Romans coming to China. Bulk of the Mongolic-Turkic migration to central Asia and eastern Europe took place around the Medieval period.
Agree with the above however the Tokharians didn't become extinct they
sought sanctuary with the Pashtun tribes in afghanistan/pakistan and were assimilated
and that's family history by the way
The Tocharians actually spread out in all directions, and a lot of them merged with the Uighyurs and other Turkish tribes living in present day Xinjiang.
You are also probably identifying the Tocharians with the Kushans, aka Yuezhi. The two groups are probably related.
IMO, Causasoid genetic markers in Chinese are probably due from Tocharians/Yuezhi, though Asian steepe nomads may also be part of it.
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