Just 3,000 years ago, the Han Chinese and the Tibetan people split apart, one expanding into all of modern China and the other settling in the high-altitude, low-oxygen environment of the Himalayas. The groups' genomes are now shockingly different.
Evolutionary biologists at UC-Berkeley undertook a massive genomic analysis of modern Tibetans and Han Chinese, testing fifty genomes from the former group and forty from the latter. Using a set of European genomes as a control group, the study left no doubt that the two ethnic groups came from a relatively recent single common ancestor, as much of the 20,000 or so genes studied were basically identical. However, thirty genes, all involved in processing oxygen and managing metabolism, had massively diverged.
That's where things get interesting, according to project leader Rasmus Nielsen:
This is the fastest genetic change ever observed in humans. For such a very strong change, a lot of people would have had to die simply due to the fact that they had the wrong version of a gene. You look for rapid evolution in genes because there must be something important about that gene forcing it to change so fast. The new finding is really the first time evolutionary information alone has helped us pinpoint an important function of a gene in humans.
The gene mutations occurred near EPAS1, known as the "super athlete gene" because people who have certain variants of it tend to be... well, you can probably guess. EPAS1 codes for a protein that senses oxygen levels and helps out with both aerobic and anaerobic metabolism, and it's likely the genes involved in the Tibetan mutations have similar functions. One hope with this new study is that it's pinpoint oxygen-regulating genes that play a role in diseases like schizophrenia and epilepsy, both are linked with a lack of oxygen in the womb.
As we discussed in a post a couple months ago, genetic adaptations help groups like Tibetans and those living in the Andes mountain adapt to their unforgiving environment. Once you move above about 13,000 feet, oxygen levels are only sixty percent those at sea levels, which can present complications like increased fatigue, headaches, babies born with low birth weight, and an increased infant mortality rate. Tibetans avoid all of these problems despite having much less oxygen and hemoglobin in their blood than their counterparts closer to sea level.
Nielsen's analysis found that Tibetans and Han Chinese, the majority ethnic group throughout the rest of modern China, shared a common ancestor as recently as 2,750 years ago. At that point, most of the original group left their homes and moved to the Tibetan plateau, where their population began to shrink. The minority that stayed behind proved incredibly successful in expanding into the rest of China, growing into a population that is now about one billion strong.
Nielsen says they were looking for which genes had changed the most between the two groups:
We made a list of the genes that changed the most and what was fascinating was that, bing!, at the top of that list was a gene that had changed very strongly, and it was related to the response to oxygen.
Probably the starkest evidence of genetic change between Han Chinese and Tibetans is found in one particular mutation. While only 9 percent of Han Chinese carry the mutation, a whopping 87 percent of Tibetans do, which is an incredibly unusual difference for two groups so closely related. The mutation is associated with lower red blood cell count and reduced hemoglobin in Tibetans, and it also affects the activity of EPAS1, the "super athlete" gene.
[Science]