The study by the McDonald Institute of Archaeological Research at the University of Cambridge found evidence of periodic human presence as far as 20,000 years ago, media reported.

They also found signs that around 5,200 years ago, people started to establish year-round settlements at up to 2,500 meters above sea level with the early settlers depending on millet, a frost-sensitive grain long planted across northern China that would not have thrived at higher altitudes.

However, about 3,600 years ago, people started making permanent settlement at higher altitudes reaching over 3,000 meters above sea level and this happened after barley was introduced to the settlers of this region.     

Barley agriculture could provide people enough food supplies even during winter, as barley is frost hardy and cold tolerant, it grows very well on the Tibetan Plateau even on Monday, researchers said.
    
The research was published in the journal Science.

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