About 4,500 years ago a bunch of indo European language speakers with horses and wheeled chariots swept across a lot of Eurasia. The original culture we call the Yamnaya, it was from the steppes, the descendents of these that made it into western China we call the Tocharians.

Which explains why the Western Chinese share some European features. DNA shows it too, as does the surviving example of their language.

Not popular with the Chinese gov.