Tibetans' adaptation to high altitude took place over less than 3,000 years, a new genetic analysis shows, in what was the fastest shift in human evolution yet discovered.
When people from lower altitudes move above 13,000 feet - where oxygen levels are about 40 percent lower than at sea level - they tire easily, get headaches, have babies with lower birth weights and have higher infant mortality.
Tibetans have none of these problems, despite lower oxygen saturation in the blood and lower hemoglobin levels.
Comparing the genomes of 50 Tibetans and 40 Han Chinese, University of California evolutionary biologists found that the ethnic Tibetans evolved this unique capability after splitting off from the Han less than 3,000 years ago.
The analysis revealed more than 30 genes with DNA mutations that have become more prevalent in Tibetans than Han Chinese. Nearly half are related to how the body uses oxygen. One mutation in particular spread from fewer than 10 percent of the Han Chinese to nearly 90 percent of all Tibetans.
"We made a list of the genes that changed the most, 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," says Rasmus Nielsen, UC Berkeley professor of integrative biology, who led the statistical analysis.
"This is the fastest genetic change ever observed in humans. 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 in Tibetans is near a gene called EPAS1, which codes for a protein involved in sensing oxygen levels and perhaps balancing aerobic and anaerobic metabolism.
The new findings could perhaps explain some diseases, including schizophrenia and epilepsy, which are associated with oxygen deprivation in the womb, said 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 Han population expanded dramatically to become the dominant ethnic group in mainland China.
The Tibetan branch either merged with the people already occupying the Tibetan plateau, or replaced them.
"We can't distinguish intermixing and replacement," 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."