People living in Tibet may have adapted to life at extremely high altitudes thanks to genes they inherited from an extinct human line, scientists say.

A genetic lineage dating back to extinct early humans, known as the Denisovans, helps modern day Tibetans survive at altitudes of almost three miles where oxygen levels are 40 percent less than those at sea level, the researchers say.

Tibetans possess a gene known as EPAS1 that helps their bodies regulate responses to low levels of oxygen, the researchers report in the journal Nature.

"It's also been called the 'super athlete gene,' because we know that certain humans that have a special version of this gene have a better performance with certain types of athletics," says geneticist Rasmus Nielsen of the University of California, Berkeley.

Tibetans have adapted because they carry the gene from a prehistoric human type that had evolved the ability to adapt to high-altitude environments, the researchers say.

They speculate that as modern humans moved out of Africa where they evolved 100,000 years ago, they encountered ancient human species such as Neanderthals and the Denisovans, interbreeding with them and exchanging genetic material.

"We think we have very good evidence that [the gene] came from Denisovans," Nielsen says, explaining that DNA patterns seen around the EPAS1 gene match those found in fossil remains of the Denisovans, an early sister human group to the Neanderthals.

As humans moved into higher altitudes, the researchers suggest, those with the Denisovan gene were able to thrive while those without didn't do as well.

Over multiple generations, the EPAS1 gene would become the norm rather than the exception in the population, they say.

Although some other modern groups also possess Denisovan DNA, they don't posses the EPAS1 mutation patterns found in Tibetans, the researchers report.

"We think modern humans inherited this haplotype from Denisovans a long time ago, but it was of more use to the Tibetans, and so spread among their population," Nielsen says.

A similar pattern of genetic mutation may exist in other modern Asian peoples adapted to high altitudes, such as the Sherpas of Nepal and certain populations in Mongolia, they add.

Many genetic traces of past human species remain in modern humans, they point out.

"We exchanged genes with a lot of other lineages that existed 100,000 years ago or 50,000 years ago," Nielsen says. "We are in some sense mongrels made of DNA from many, many different lineages of hominins."