For Tibetans, a Genetic Leg Up

Even before Tenzing Norgay summited Mt. Everest with Edmund Hillary in 1953, it was widely known that his people, the Sherpas, had something that the rest of us did not.

On May 22 of this year, Apa Sherpa made a record 20th ascent of Mt. Everest, further driving home the obvious -- that these eastern Nepalese of Tibetan stock are superbly adapted to their lofty Himayalan home, which has about 40 percent less oxygen in its air than we lowlanders enjoy at sea level.

Sherpa climbing near Mt. Everest.
Image © Bartosz Hadyniak/iStockphoto
Now scientists have found evidence that, over the thousands of years that Tibetans have lived on the Tibetan Plateau, natural selection has been working on their genes, causing evolutionary changes that enable Tibetan peoples not only to survive but to thrive at altitudes of 13,000 feet or more.

Three independent studies published in recent weeks have identified strong positive selection among Tibetans in genes that are involved in dealing with hypoxia, a deficiency of oxygen reaching the body's tissues.

One study, led by Xin Yi of the Beijing Genomics Institute in China, compared the genomes of 50 Tibetans living at 14,000 feet with those of 40 Han Chinese living in Beijing (altitude <200 feet). The team discovered about 30 genes that differ significantly between the two groups, even though the two groups split only 3,000 years ago, the team says. (As Nicholas Wade reports in a Times article, other experts dispute that figure, arguing that the split occurred thousands of years earlier.)

The greatest divergence the team identified appeared in one variant of a gene known as HIF2a, or hypoxia-inducible factor 2-alpha. Only 9 percent of the Han Chinese bore this variant, while 87 percent of the Tibetans did. The researchers believe this variant, along with others known to be involved in the body's response to hypoxia, give Tibetans that high-altitude genetic boost.

Most of us lowlanders suffer if we try to live at extreme altitude. To compensate for the low oxygen levels, our bodies produce more red blood cells, which help transport oxygen to the tissues. But this can lead to an overabundance in our blood of hemoglobin, the oxygen-carrying pigment, which in turn can lead to chronic mountain sickness and other altitude-related ills.

Tibetans who have the HIF2a variant feature a lower level of red blood cells -- a level more akin to that of lowlanders -- which apparently helps them avoid mountain sickness. Since they're not making more red blood cells, how they compensate for the lack of oxygen remains a mystery.

Two other studies seeking genetic clues to Tibetans' adaptation to high altitude have also reported that the HIF2a gene has undergone recent natural selection. One was led by Cynthia Beall of Case Western Reserve University and the other by Tatum Simonson of the University of Utah School of Medicine.

That three studies on the same subject should appear within weeks of one another is a sign of how far the field has advanced of late. Until recently, finding unequivocal evidence of human evolution was challenging if not impossible. But scientists now have at their disposal a suite of new tools that allow them to quickly turn up alterations in the genome between individuals and even between large groups.

What's next? Perhaps scientists will discover genetic variations that have helped Arctic populations cope with extreme cold -- something the Sherpas can handle pretty well, too, by the way.

User Comments:

Cool! Survival of the fittest!

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