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Study traces forefathers’ gift to Tibetans

New Delhi, July 2: An enigmatic group of extinct early humans believed to be related to the Neanderthals might have bequeathed modern Tibetans their unique ability to thrive in high-altitude terrain, a genetic study released today has suggested.

The study by an international team of scientists has indicated that a unique set of genetic variations that appear to confer high altitude endurance to modern Tibetans originated in archaic humans called the Denisovans.

Scientists studying human origins believe the Neanderthals and Denisovans are descendants of an older species called Homo heidelbergensis that split 300,000 years ago into the Neanderthals that dominated Europe and the Denisovans that reigned across parts of Asia. About two years ago, researchers in Germany sequenced the Denisovan genome, using a tiny fragment of a finger bone discovered earlier in Denisova cave in a mountain in southern Siberia.

In the new study, scientists compared sections of the Denisovan genome with sections of the genome representing present-day people in Tibet and Han Chinese. The findings to be published in Nature on Thursday suggest mating between extinct archaic humans and modern humans played a role in helping modern humans adapt to new environments.

“The exchange of genes through mating with extinct species may be more important in human evolution than previously thought,” Rasmus Nielsen, a scientist at the University of California, Berkeley, who led the study, told The Telegraph.

The Tibetan plateau — often called the roof of the world — stands over 4000 metres above sea level where the atmospheric oxygen pressure is 40 per cent lower than at sea level. Scientists believe genetic variations have helped Tibetans adapt to the high altitude through physiological differences in how their bodies react to the low oxygen concentrations.

The study by Nielsen and his colleagues suggest that a set of genetic variations that allow such adaptation originated in the Denisovans and slipped into the ancestors of modern Tibetans through introgression — the exchange of genes between different species.

The scientists speculate that the gene exchange between Denisovans and modern humans may have occurred some time between 30,000 and 40,000 years ago. The genetic variations may have existed in a low frequency in modern humans until they moved into Tibet. But once there, the process of natural selection would have favoured individuals who possessed these variations who would be more likely to survive and reproduce in the inhospitable high altitude.

“Such studies may help plug gaps in our understanding of the origin of high altitude endurance in humans,” said Kumarasamy Thangaraj, a scientist at the Centre for Cellular and Molecular Biology, Hyderabad, who specialises in population genetics but was not associated with the research.

It is unclear how the genetic variations linked to high altitude endurance emerged in the Denisovans themselves. “We may hypothesise that Denisovans were also adapted to high altitudes,” Nielsen said.

The 2012 study of the Denisovan genome has indicated that the Denisovans shared more sets of genetic variations with populations from islands in southeastern Asia and Australian aborigines than with populations elsewhere in Asia.