People live in mountainous regions around the world, but there are no permanent settlements above about 17,000 feet and most of the highest villages lie below 15,000 feet, at which point, birth rates plummet and it becomes difficult to maintain good health.
Genetic studies are starting to explain how some populations manage to thrive at altitudes that make many people fill loopy and ill. In 2010, Beall's group identified a gene called EPAS1 that's mutated in Tibetan highlanders, allowing them to breathe in more air per minute than other people. They also produce higher than normal levels of nitric oxide, which dilates the blood vessels, all while maintaining low hemoglobin levels.
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Other groups manage to live at extreme heights without the same adaptations that Tibetans have developed. Andean highlanders, for example, simply increase hemoglobin concentrations like almost everyone else, despite the added stress of pumping thicker blood. Ethiopian highlanders seem to have their own ways of coping with extreme elevations, including genetic variations that allow them to tolerate hypoxia, according to a sequencing study published this year.
"By comparing Tibetan, Andean and Ethiopian highlanders, what we see is that there are whole populations exposed to the same stress of altitude but with different patterns of responses, and they're all successful," Beall said. "That gives us a sense of how human biology is much more flexible than we thought. We are enormously adaptable."