Tibetans live at altitudes of 13,000 feet, breathing air that has 40 percent less oxygen than is available at sea level, yet suffer very little mountain sickness. The reason, according to a team of biologists in China, is human evolution, in what may be the most recent and fastest instance detected so far.
Comparing the genomes of Tibetans and Han Chinese, the majority ethnic group in China, the biologists found that at least 30 genes had undergone evolutionary change in the Tibetans as they adapted to life on the high plateau. Tibetans and Han Chinese split apart as recently as 3,000 years ago, say the biologists, a group at the Beijing Genomics Institute led by Xin Yi and Jian Wang. The report appears in Friday’s issue of Science.
If confirmed, this would be the most recent known example of human evolutionary change. Until now, the most recent such change was the spread of lactose tolerance — the ability to digest milk in adulthood — among northern Europeans about 7,500 years ago. But archaeologists say that the Tibetan plateau was inhabited much earlier than 3,000 years ago and that the geneticists’ date is incorrect.
When lowlanders try to live at high altitudes, their blood thickens as the body tries to counteract the low oxygen levels by churning out more red blood cells. This overproduction of red blood cells leads to chronic mountain sickness and to lesser fertility — Han Chinese living in Tibet have three times the infant mortality of Tibetans.
... The biologists found about 30 genes in which a version rare among the Han had become common among the Tibetans. The most striking instance was a version of a gene possessed by 9 percent of Han but 87 percent of Tibetans.
Such an enormous difference indicates that the version typical among Tibetans is being strongly favored by natural selection. In other words, its owners are evidently leaving more children than those with different versions of the gene.
The gene in question is known as hypoxia-inducible factor 2-alpha, or HIF2a, and the Tibetans with the favored version have fewer red blood cells and hence less hemoglobin in their blood.
The finding explains why Tibetans do not get mountain sickness but raises the question of how they compensate for the lack of oxygen if not by making extra red blood cells.
... Genetic differences between Tibetans and Chinese are a potentially delicate issue, given Tibetan aspirations for political autonomy. Dr. Nielsen said he hoped that the Beijing team’s results would carry no political implications, given that it is cultural history and language, not genetics, that constitute a people. There is not much genetic difference between Danes and Swedes, he added, but Denmark and Sweden are separate countries.
July 2, 2010
Nick Wade writes in the NY Times: