The Tibet plateau is a land of yaks and sherpas — and rapid evolution.
Over a mere 3,000 years, a blink of an evolutionary eye, Tibetan highlanders have developed a unique version of a gene that apparently helps them cope with life at extremely high altitudes, according to a study published Friday in the journal Science.
FOR THE RECORD:
Tibetan genes: A July 4 article in Section A about a gene that helps Tibetans live at high altitudes said that the Tibetan and Han Chinese populations were calculated to have separated 3,000 years ago. In a quote, this time span was equated to about 40 human generations. Assuming that a human generation is about 20 years, the number of generations should have been about 150. —
The research group, led by UC Berkeley biologist Rasmus Nielsen, found the gene by comparing DNA from 50 Tibetans and 40 neighboring Han Chinese. The two ethnic groups are closely related, with one important difference: The Tibetans live at elevations of 14,000 feet and higher, while the Han population generally lives relatively close to sea level. The genetic variant was found in 87% of the Tibetans and 9% of the Han Chinese.
"The change at this particular position in Tibetan highlanders represents one of the most dramatic examples of genetic change in recent human history," said University of Nebraska evolutionary geneticist Jay Storz, who was not involved in the study. "It really is a great story about how the human gene pool is still being shaped by the forces of natural selection."
The researchers calculated that the Tibetan and Chinese populations separated about 3,000 years ago.
"This is not the distant past," said John Hawks, an anthropologist at the University of Wisconsin. "This is stuff that's happened in 40 human generations."
It makes sense that the harsh environment of the Himalayas promotes fast evolutionary adaptation. High altitude, with its lower levels of oxygen, is associated with reproductive difficulties such as miscarriages, low birth weight and increased infant mortality. In response, Tibetans have adapted in a way that may seem counterintuitive but is remarkably effective: their blood hemoglobin levels do not rise too high.
Scientists still don't know exactly how the low hemoglobin levels help the Tibetans, but they do know that too much hemoglobin makes the blood too viscous, making oxygen distribution more difficult. By maintaining hemoglobin levels about the same as those seen in people at sea level, the Tibetans have avoided this damaging effect.
Still, they must have other adaptations that allow them to thrive at an elevation where each breath of air has 40% less oxygen than at sea level.
Researchers also don't know exactly how the EPAS1 gene (also known as HIF2-alpha) is involved in this picture. However, the gene is known to be involved in the body's reaction when a normal person goes to high elevations, so it seems likely that the Tibetan variant somehow results in a blunted response.
The report follows on the heels of two similar studies that also identified EPAS1 as playing an important role in Tibetan evolution.
Lactose tolerance, which spread across Europe over the last 7,500 years, is another example of relatively fast evolution in the modern human population, but the Tibetan study shows that such changes can occur in less than half that time.
"It's likely that there are many more examples of genes evolving this fast," Nielsen said. "It's just that we managed to catch this one in the act."
Such studies are becoming increasingly common because of the massive amounts of human genetic data now being collected and the complex statistical methods that allow researchers to plumb the depths of the genome.
In years past, researchers had to limit themselves to looking for differences in genes with known functions related to the trait in question. Now that this limitation has been removed, the doors have been flung wide open.
"This genomic approach holds the promise of allowing us to identify genes involved in adaptation that we would never have expected," Storz said.