The world had been awash in news about how we can see the evidence in our DNA of ancient humans mating with Neanderthals and their close relatives, the Denisovans. Now in a new study out in the journal Nature, a group of researchers has found the strongest evidence to date that this mating mattered.
Modern Tibetans are incredibly well adapted to the harsh environment of the Tibetan plateau. This place is cold, doesn’t have a lot of resources and has about 40% lower levels of oxygen than at sea level. It turns out that a big reason Tibetans do so well compared to everyone else is because of a version of the EPAS1 gene their ancestors got from the now extinct Denisovans. Most every Tibetan who is well adapted to this environment has the Denisovan version of the EPAS1 gene.
When these researchers compared the EPAS1 gene of Tibetans with other modern humans, Neanderthals and Denisovans, the best match by far was with the Denisovans. The same gene version was also found in a couple of nearby Han Chinese individuals, but there was no sign of it in the Melanesian population which is thought to have the highest levels of Denisovan DNA.
And we aren’t talking just a single DNA difference that might have arisen by chance. No, they found at least 32 specific single base changes scattered throughout the region of the gene that were only shared by Denisovans, Tibetans and those two Han Chinese individuals. The only model that fits this data is that at some point the gene was introduced into the human population by the Denisovans that then spread through natural selection.
Basically, through random mutation, a Denisovan ended up with a version of the EPAS1 gene that allowed them to thrive at higher altitudes. This gene version (or allele) then spread through the Denisovan population.
Next, ancestors of modern humans and Denisovans had babies together. Some of these babies inherited the Tibetan EPAS1 allele. Some of these humans then moved to the Tibetan plateau where life got very hard for people with the everyday version of the EPAS1 gene.
The babies of people lacking the Denisovan allele died young and those that survived suffered from high rates of heart disease. It took very little time for these folks to be replaced by the ever growing number of people with the Denisovan version of the EPAS1 gene.
Scientists have identified a few cases where DNA from now extinct relatives have helped and are still helping modern humans. For example, it looks like certain HLA alleles involved in immunity were introduced by Neanderthals into the human gene pool and helped these humans deal with their new environments. Same thing with the skin color gene BNC2. But none have been as obvious or compelling as this EPAS1 allele. It is becoming more and more obvious that part of who we are comes from genes introduced by now extinct relatives of modern humans.
And this isn’t just a fascinating look into our human ancestry. It might actually help people who want to immigrate to Tibet.
Given how hard it is for foreigners to be healthy on the Tibetan plateau, it might be worth a simple genetic test to check who has the right genes to thrive there. Certainly there will be Han Chinese with the right allele. Genetic testing can help people know the environments they can move to and do well!
More than One Way to Conquer High Altitudes
Of course, Tibetans are not the only people living at high altitudes. People have been doing the same in the Andes and for even longer in the Ethiopian highlands. None of these three groups adapted in the same way though.
Tibetans most likely acquired the version of EPAS1 they needed to survive from the Denisovans. But the same is not true for either Andeans or Ethiopians. Their EPAS1 gene is not the same as the Tibetan EPAS1 allele.
So we see no evidence thus far of a scenario where Denisovan-human breeding released a low level of altitude-tolerant EPAS1 alleles into the human population that was selected for whenever people ended up high in the mountains. Instead, it looks like different groups of humans have different versions of different genes that help them survive high altitude. Evolution and natural selection are a messy business!