Scientists have often wondered how woolly mammoths survived and thrived in the frigid climes of the far north in Earth’s last ice age. The hemoglobin in elephant (and human) blood cannot easily transfer oxygen to other cells in the body at low temperatures. Instead, the blood’s hemoglobin holds onto its oxygen in icy extremities and the tissue eventually dies; that’s the main reason we get frostbite. There must, then, have been something special about mammoth hemoglobin.
Clearly, the people of Tibet must have evolved quickly to tolerate a life spent living at the top of the world. How quickly? A study out in this week’s Science, which compared Tibetans to Han Chinese to see the differences in their DNA, says that the two groups may have diverged no more than 3,000 years ago. If natural selection has changed Tibetans in such a short time, it would be the fastest known example of human evolution. But not everybody is buying this time line.
As DISCOVER noted when a similar study by another team came out in May, natives of the Tibetan plateau seem to survive the altitude because their bodies make less hemoglobin. It’s somewhat counter-intuitive:
In theory higher levels of haemoglobin would be beneficial, because this would improve oxygen transport. But high levels could make the blood thicker and less efficient at carrying oxygen, says Jay Storz of the University of Nebraska-Lincoln [New Scientist]. (Storz writes the accompanying commentary in Science.)
Looking at the differences in genes that regulate that, the team found vast differences between the Han and the Tibetans, with one version appearing in 87 percent of Tibetans studied but only 9 percent of Chinese. However, the assertion by the scientists at the Beijing Genome Institute—that their findings mean the two group broke apart just three millennia ago—has ruffled archaeologists who believe that the Tibetan plateau has been continuously occupied for much, much longer: more like 7,000 to 21,000 years.
For more about all of this, check out Razib Khan’s post at Gene Expression.
Gene Expression: Very Recent Altitude Adaptation in Tibet
Gene Expression: Tibet & Tibetans, Not Coterminus
80beats: Found: The Genes That Help Tibetans Live at the Top of the World
DISCOVER: High-Altitude Determines Who Survives in Tibet
Image: Wikimedia Commons
Tibetans not only occupy one of the most extreme locations on Earth, they’ve been doing it for thousands of years. This week in a study in the journal Science, scientists have for the first time picked out the particular genetic features that allow these people to survive in the low oxygen levels of the Tibetan Plateau, which is around 15,000 feet above sea level. Curiously, the way they have evolved to survive is unlike that of other high-altitude dwellers around the world.
The American and Chinese researchers doing the study started by keying on 247 genes that looked like good candidates—they tended to change across populations, and seemed to play a role in controlling a person’s blood oxygen level.
Then they analyzed segments of DNA that include those 247 genes in 31 unrelated Tibetans, 45 Chinese, and 45 Japanese lowland people whose DNA was genotyped in the HapMap Project. By identifying regions that had a characteristic signature of being strongly altered by natural selection, they were able to identify relatively new gene variants that had swept through highland Tibetans, but not Chinese or Japanese lowlanders [ScienceNOW].
Ten of the genes turned out to be particularly promising, with two, called EGLN1 and PPARA, showing up in the Tibetans who had the lowest levels of oxygen in their bloodstream.