If you've ever climbed to very high altitudes you may know the difficulties that sometimes occur in such environments. The low partial pressure of oxygen up there can cause headaches, fatigue, dizziness and other unpleasant symptoms. 

If you're a native of Tibet, chances are, you have never experienced such sickness in your life.

Researchers at the University of California at Berkeley found that Tibetans have a different form of the gene most responsible for regulating the blood's oxygen-carrying molecule, hemoglobin. At incredibly high altitudes, most people's blood thickens with increased hemoglobin in response to low oxygen pressure. This thickening is what causes some of the more serious cardiovascular problems and, on occasion, deaths, at extremely high altitudes.

But Tibetan's have a variant of this gene that allows them to survive and thrive in environments 13,000 feet above sea level and beyond. Researchers found that this gene variant comes from the Denisovans, extinct close relatives of Neanderthals. Some scientists call it the "super athlete gene."

Researchers analyzed genomes of Tibetans and Han Chinese (belonging to China's dominant ethnic group) and the Denisovan genome collected from bone fragments found four years ago in a cave in southern Siberia, all supplied by the Beijing Genomics Institute. The Tibetans and the Han Chinese had a common ancestor, but the group split around 2,750 years ago. The Tibetan ancestors then moved to higher altitudes. The researchers' analysis with Illumina sequencing machines showed that the gene in question, EPAS1, contains a nearly identical mutation in 90% of Tibetan and Denisovan genomes. Less than 10% of the Han genome contained this mutation.

This is the most likely reason as to why Tibetan women encounter fewer problems with pregnancies at high altitudes, and why most Tibetans can travel to such altitudes without any cardiac problems at all.

Then the question remained-how did Tibetans get this form of the EPAS1 gene? The most logical answer researchers can think of is introgression, or inbreeding.

As per Rasmus Nielsen, a biologist on the team that studied the Tibetan gene, the possibility of Tibetans having this variant gene without inbreeding is next to none. The team published its findings in Nature on July 2.

"This is the fastest genetic change ever observed in humans," said Nielsen in a statement released by UC Berkeley. "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." Thanks to natural selection, the useful adaptation rapidly passed on.

Scientists hope this discovery helps distinguish more genes that play roles in protecting the body from low oxygen environments, especially genes involved in diseases (e.g. schizophrenia, epilepsy) linked to prenatal oxygen deprivation.