Independently, a group led by Cynthia M. Beall, an anthropologist at Case Western Reserve University, and Yong-Tang Zheng of the Kunming Institute of Zoology in China has detected a genetic change in the same gene in Tibetans and found that it correlated with having less hemoglobin in the blood. Their report was published in the June 22 issue of the Proceedings of the National Academy of Sciences.

Human adaptation to high altitude is a field of obvious interest, but another reason for the appearance of three studies on the same subject in matter of a few weeks may be that the technology to assess which parts of the genome are under selection has only recently become available.

The three new reports agree in finding the Tibetans’ version of the gene has been favored by natural selection. But the 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.

“The separation of Tibetans and Hans at 3,000 years ago is simply not tenable by anything we know from the historical, archaeological or linguistic record,” said Mark Aldenderfer, a Tibetan expert at the University of California, Merced.

Dr. Aldenderfer said that there had probably been many migrations onto the Tibetan plateau, and that there was indirect evidence that pastoralists had entered the plateau from the north-northeast around 6,000 years ago. Earlier genetic studies have found that Tibetans are more similar to northern Han than to those from southern China, and have some admixture of genes from Central Asia, he said.

Geneticists have a more elastic view of dates than do archaeologists, and the estimate of a Han-Tibetan population split at 3,000 years ago could probably have been adjusted to 6,000 if the geneticists had taken any account of any other kind of evidence.

Rasmus Nielsen, a Danish researcher at the University of California, Berkeley, did the statistical calculations for the Beijing study. “We feel fairly confident that something on the order of 3,000 years is correct,” he said. But in a later e-mail message, Dr. Nielsen said, “I cannot with confidence rule out that the divergence time is 6,000 instead of 3,000.”