It took less than 3,000 years for Tibetans in the Himalayan mountains to evolve and thrive on the “Roof of the World,” where the air contains so little oxygen that most lowlanders quickly suffer serious high-altitude sickness, the scientists report.
In the long history of life on Earth, evolution has most often come slowly. It took more than 150,000 years for the first Homo sapiens to evolve into today’s modern humans, while many single genetic traits can take tens of thousands of years to evolve.
During a two-year collaborative project, an American group headed by Rasmus Nielsen, a computational geneticist at UC Berkeley, and a large team of genome decoders in China examined the 20,000 genes in the complete genomes of 50 Tibetans who live in villages nearly 3 miles high, and they then compared them with all the genes of 40 ethnic Han Chinese who live in lowland Beijing.
The result, Nielsen and his colleagues report: More than 30 genes in the ethnic Tibetans have undergone adaptive mutations within the past 2,750 years – about 100 generations – since Han Chinese lowlanders settled in Tibet’s lofty mountains. Most of those genes haven’t evolved at all among the ethnic lowlanders, the scientists found.
The Tibetans whose genes were studied live in two separate villages – one at 14,100 feet and the other at 15,100 feet.
More than half the genes that had evolved determine how the body uses oxygen, and one gene in particular has mutated in 90 percent of the Tibetans, and now enables them to live with less oxygen than their lowland relatives, the researchers found. That mutated gene appeared in only 10 percent of the Han Chinese.
The mutated gene among the Tibetans is located in their genome next to one known as EPASI1 that stimulates production of red blood cells and increases the concentration of hemoglobin that carries oxygen to the rest of the body. It has been a major factor in the evolution of the Tibetans in their adaptation to high altitude life, the researchers found.
Their report is published today in the journal Science, along with a commentary by Jay F. Stolz, a geneticist at the University of Nebraska who researches similar issues in animals living at varying altitudes.
“It’s really a great study and a solid analysis,” Stolz said in an interview. “It shows one of the most rapid adaptive genetic changes we’ve ever seen in human evolution – a striking and dramatic change.”
In a phone interview, Nielsen said evolution must have somehow worked to make the Tibetans adapt to living in the high mountains.
“It is what has let them adapt to living with such limited oxygen at such high altitudes,” he said.
Nielsen, who recently moved from his native Denmark to the department of integrative biology at Berkeley, is also a member of a team of Danish scientists who in February reported decoding the genome of an ancient human in Greenland from a single tuft of his hair – a man who died some 4,000 years ago after the migration of his people from Siberia about 5,500 years ago.
The Chinese team includes more than 30 scientists at the Beijing Genomics Institute, which is headquartered in the city of Shenzhen and is now one of the world’s most advanced centers for gene analysis. The group is known most recently for sequencing the genomes of species as widely diverse as Arabian camels and Chinese silkworms.
This article appeared on page A – 4 of the San Francisco Chronicle
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