Tibetans may have inherited their ability to successfully live at high altitudes from an ancient ancestor to our species. Soon after ancient Tibetans mated with a distant species of human, the more modern variety pushed the others to extinction.
University of California Berkeley researchers realized an unusual form of a gene that regulates hemoglobin, became pronounced in ancient Tibetans following their move into the high-altitude environment thousands of years ago. Hemoglobin is essential for the transportation of oxygen in blood.
Low oxygen levels at altitudes of 15,000 feet or more can thicken blood, leading to problems with circulation. Because of this unusual gene, Tibetans are able to live at these great heights without experiencing health problems.
Denisovans, a species of human ancestor that went extinct between 40,000 and 50,000 years ago, remains a mystery to paleontologists.
"We have very clear evidence that this version of the gene came from Denisovans. This shows very clearly and directly that humans evolved and adapted to new environments by getting their genes from another species," Rasmus Nielsen, UC Berkeley professor of integrative biology, said.
When oxygen levels drop, the gene, EPAS1, is set in motion, producing additional hemoglobin. The gene can supply athletes with additional endurance when they are at low elevations, earning EPAS1 the moniker of the "superathlete gene." At high altitudes, most bodies create too many blood cells and concentrations of hemoglobin rise too much, thickening blood, which can lead to heart attacks. In Tibetans, hemoglobin levels are raised just slightly, keeping the native population healthy in the low-atmosphere environment.
"We found part of the EPAS1 gene in Tibetans is almost identical to the gene in Denisovans and very different from all other humans. We can do a statistical analysis to show that this must have come from Denisovans. There is no other way of explaining the data," Nielsen stated in a press release.
Around 87 percent of Tibetans were found to have the high-altitude gene, compared to nine percent of Han Chinese, who share ancestors with the group. No other group of humans is known to possess the gene variant. This includes the people of Melanesia, who possess genes that are five percent Denisovan. That ancient genome was sequenced and published in 2012.
It may be possible that Denisovans in Europe and Asia interbred with modern humans, migrating out of Africa. As humans first moved into what is now China, the wanderers could have split into two groups, with some moving into highlands, becoming Tibetans. The rest would have stayed in the lowlands, developing into Han Chinese.
This discovery is the first to show a gene from a different species of human aiding modern populations in adapting to the environment.
Research into high-altitude living abilities of Tibetans and the role Denisovans may have played in human evolution was published in the journal Nature.
