"This is the fastest genetic change ever observed in humans," said study leader Professor Rasmus Nielsen of UC Berkeley.

"For such a very strong change, a lot of people would have had to die simply due to the fact that they had the wrong version of a gene."

The mutation found in Tibetans is near a gene called EPAS which codes for a protein involved in sensing oxygen levels and perhaps balancing aerobic and anaerobic metabolism.

Professor Nielsen used computational methods to mine genomic information and discover genetic changes driven by natural selection as humans and animals have adapted to new environments.

Changes in the frequency of DNA mutations proved to be critical.

"You look for rapid evolution in genes because there must be something important about that gene forcing it to change so fast," Professor Nielsen said.

"The new finding is really the first time evolutionary information alone has helped us pinpoint an important function of a gene in humans."

As a result of the mutation, Tibetans have none of the problems facing other ethnicities living at high altitudes -- despite lower oxygen saturation in the blood and lower haemoglobin levels.

Professor Nielsen used genome data produced by the Beijing Genomics Institute to determine the genetic changes associated with these physiological changes.

"We're looking for footprints of past selection to find something functional in our genome," said Professor Nielsen.

The analysis revealed that the common ancestors of Tibetans and Han Chinese split into two populations about 2,750 years ago, with the larger group moving to the Tibetan plateau.

That group eventually shrank, while the low-elevation Han population expanded dramatically.

Today, the Han Chinese are the dominant ethnic group in mainland China.

The Tibetan branch either merged with the people's already occupying the Tibetan plateau, or replaced them.

"We can't distinguish intermixing and replacement," Professor Nielsen said.

"The Han Chinese and Tibetans are as different from one another as if the Han completely replaced the Tibetans about 3,000 years ago."

Although Tibetan and Han Chinese genomes are essentially identical, some 30 genes stood out because of dramatic differences between the Tibetans and the Han.

"We made a list of the genes that changed the most," said Professor Nielsen, "and what was fascinating was that, bing!, at the top of that list was a gene that had changed very strongly, and it was related to the response to oxygen."