Beijing Genome Institute’s calculation that the Tibetan and Han populations split apart only 3,000 years ago is less likely to be accepted. Archaeologists say they believe that the Tibetan plateau has been inhabited for at least 7,000 years and maybe for as long as 21,000 years.

There is similar flexibility in the estimates of population sizes. The Beijing team calculates that at the time of divergence there were only 288 Han Chinese and 22,642 Tibetans. These estimates have bewildered archaeologists, given that rice cultivation in southern China started 10,000 years ago and that there was an extensive civilization by 3,000 years ago. Dr. Nielsen said that the figure of 288 people was meant simply to indicate a bottleneck in the Han population, meaning a time when it was very small, and that this bottleneck could just as easily have occurred 10,000 years ago.