Immunity to high altitude sickness may have come from extinct Denisovans.

It's amazing what they can do with a finger bone.

The variant of the EPAS-1 gene, which affects blood oxygen, is common in Tibetans - many of whom live at altitudes of 4,000m all year round.

But the DNA sequence matches one found in the extinct Denisovan people.

Many of us carry DNA from extinct humans who interbred with our ancestors as the latter expanded out of Africa.

Both the Neanderthals - who emerged around 400,000 years ago and lived in Europe and western Asia until 35,000 years ago - and the enigmatic Denisovans contributed DNA to present-day people.

The Denisovans are known only from DNA extracted from the finger bone of a girl unearthed at a cave in central Siberia. This 40,000-50,000-year-old bone fragment, as well as a rather large tooth from another individual, are all that is known of this species.

The tiny "pinky" bone yielded an entire genome sequence, allowing scientists to compare it to the DNA of modern people in order to better understand the legacy of ancient interbreeding.

Now, researchers have linked an unusual variant of the EPAS1 gene, which is involved in regulating the body's production of haemoglobin - the molecule that carries oxygen in the blood - to the Denisovans. When the body is exposed to the low oxygen levels encountered at high elevations, EPAS1 tells other genes in the body to become active, stimulating a response that includes the production of extra red blood cells.

The unusual variant common among Tibetans probably spread through natural selection after their ancestors moved onto the high-altitude plateau in Asia several thousand years ago.

"We have very clear evidence that this version of the gene came from Denisovans," said principal author co-author Rasmus Nielsen, from the University of California, Berkeley.