A combination of mutations is found in Tibetans and Denisovans.
By John Timmer | Ars Technica
ON THE WEB, 4 July 2014
The Denisovans, relatives of the Neanderthals who inhabited Asia before modern humans arrived, are known only from a scattering of small bones and a wealth of DNA data. So far, all of that originates from a single Siberian cave (called Denisova, naturally). Like the Neanderthals, the Denisovans interbred with those modern humans once they arrived. But the modern populations who have the most Denisovan DNA are far from Siberia, occupying southern Asia and some Pacific islands.
Now, a tiny fragment of Denisovan DNA has also been found in a group that’s much closer to Siberia: the Tibetans. And all indications are that it helps them adapt to the extreme elevations of the Tibetan plateau.
Large parts of that plateau are 4,000 meters (2.5 miles) above sea level. The populations native to the area have lower infant mortality and higher birth weights than people who have relocated to the area. In addition, the Tibetans have acclimated to the altitude without relying on increased red blood cell counts, which is how most other people respond after spending time at altitude. Higher red blood cell counts mean a more viscous blood, which creates its own health hazard, so this difference is also likely to be very advantageous.
With the advent of molecular genetics, it became possible to determine what is different in the TIbetans’ DNA that accounts for their comfort at altitude. Several recent studies have done just that and come up with a variety of genes that appear to be behind it. Many of these studies have identified a gene called EPAS1 as a likely candidate for helping the locals handle the altitude.
Now, a large international team of researchers has gone back and taken a closer look at EPAS1, sequencing the version of the gene in 40 Tibetans and 40 of their close relatives, the Han Chinese. Looking at the 130,000 bases surrounding the gene, they find that there are many differences in this region between the two populations—many more than you’d expect to have occurred in the short time these two populations have been separated. In fact, in a small core area, there are five closely linked changes that are distinct to Tibetans, far more than are likely to accumulate even under the strongest evolutionary selection.
(Nearly distinct to Tibetans, at least. There are two Han individuals, one from Beijing and one from southern China, that also appear to carry a copy of the Tibetan version of this gene.)
To get a better perspective of how this DNA showed up in Tibetans, the researchers started looking at other human genomes from around the world. None of the people from the 1,000 Genomes project had anything that looked like the Tibetan sequence. But a search of the databases turned up one human population who did: the Denisovans. In fact, in the core sequence near EPAS1, the Denisovans shared all five of the changes found in the Tibetans. The similarity extended outside this core region, as well.
In general, the Denisovan contribution to the DNA of Han Chinese is so low that it’s been difficult to identify with any certainty. And the population that has the most Denisovan DNA, the Melanesians, doesn’t have the Denisovan version of the EPAS1 gene. But the new results clearly indicate that even a small, difficult-to-detect contribution can have dramatic effects on the populations who receive it.
This isn’t the first case where versions of genes that were picked up from archaic humans appear to be helpful; other examples involve immune function and skin coloration. This makes a lot of sense, given that the Neanderthals and Denisovans had been living in some environments for tens of thousands of years before modern humans showed up. The authors of the new paper conclude, “we are now also starting to understand that adaptation to local environments may have been facilitated by gene flow from other hominins that may already have been adapted to those environments.”