The Peoples of the Steppe Frontier in Early Chinese Sources by Edwin G. Pulleyblank

“…It is argued that they were Indo-Europeans, speaking a Tocharian language of the type attested in later documents from the oasis states of the Tarim basin. Recent finds of Caucasoid mummies in Xinjiang dating from the early second millennium BCE onward suggest that this was the time when Tocharian speakers first reached the borderlands of China, bringing with them important cultural elements from the west – metallurgy and the horse-drawn chariot – that played an essential role in the formation of Chinese civilization. …”
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