Modern human inhabitants of Tibet owe their ability to thrive in high altitudes to a gene passed to them by our early human ancestors, new research has revealed.
Inhabitants of the Tibetan plateau, at over 4000m, live at some of the highest settlements in the world. But unlike non-natives, who tolerate this environment poorly and suffer high rates of altitude sickness, low birth weigh and pre-eclampsia, these individuals appear uniquely adapted to their homes at high relief. Most significantly, they don't respond to low oxygen conditions by producing excessive amounts of haemoglobin, as other individuals do.
Now scientists know why: they carry a unique form of a gene called EPAS1, passed to them by a primitive human ancestor who left Africa more than half a million years ago.
The discovery, announced this week in Nature, was made by University of California, Berkeley, scientist Rasmus Nielsen. Together with colleagues in China, he and his team compared the DNA sequences of 40 Tibetan natives with 40 Han Chinese.
Consistently, differences cropped up in the EPAS1 gene, which previous studies have shown is strongly activated by exposure to low-oxygen and has been linked previously to a superior ability to deliver oxygen to tissues in athletes.
But the story has a further thrilling twist, because when the researchers ran the genetic sequence of the Tibetan EPAS1 gene through a DNA database of published gene signatures, a direct match turned up. The sequence matched precisely the EPAS1 gene carried by an early human species called the Denisovans, who lived between 600,000 and 40,000 years ago and for which the genetic sequence has been assembled from DNA extracted from a tooth and a finger bone.
Since other modern human populations do not carry this gene variant, the most plausible explanation is that when modern humans arrived in Tibet, they interbred with the Denisovan locals, assimilating into their genetic makeup genes like the Denisovan EPAS1 which could provide a strong evolutionary advantage in the harsh habitat they now called home.
According to Nielsen and his colleagues, "our findings imply that one of the most clear-cut examples of human adaptation is likely to be due to gene flow from archaic hominins into modern humans..."