Tibetans,the fastest case of human evolution

At least 30 genes had undergone evolutionary change in Tibetans,helping them adapt to life on the high plateau...

Tibetans live at altitudes of 13,000 feet,breathing air that has 40 per cent 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 at the Beijing Genomics Institute. 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 tolerancethe ability to digest milk in adulthoodamong northern Europeans about 7,500 years ago.

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 fertilityHan Chinese living in Tibet have three times the infant mortality of Tibetans.

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The Beijing team analysed the 3 per cent of the human genome in which known genes lie in 50 Tibetans from two villages at an altitude of 14,000 feet and in 40 Han Chinese from Beijing,which is 160 feet above sea level. Many genes exist in a population in alternative versions.

The gene in question is known as hypoxia-inducible factor 2-alpha,or HIF2a,and the Tibetans with the favoured version have fewer red blood cells and hence less haemoglobin in their blood.

Dr. Aldenderfer,University of California,said that earlier genetic studies have found that Tibetans are more similar to northern Han than to those from southern China,and have some admixture of genes from Central Asia.

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