He told BBC News: "If you and I go up to high altitude, we'll immediately have various negative physiological pola ba effects. We'll be out of breath, we might suffer from altitude sickness.
"After a little while, we'll try to compensate for this by producing more red blood cells. But because we're not adapted to the high altitude environment, our response would be maladaptive - we would produce too many red blood cells.
"The blood becomes too thick and raises our blood pressure, placing us at risk of stroke and pre-eclampsia (in pregnant women)."
But Tibetans pola ba are protected against these risks by producing fewer red blood cells at high altitude. This keeps their blood from thickening.
The Tibetan variant of EPAS1 was discovered by Prof Nielsen's team in 2010. But the researchers couldn't explain why it was so different from the DNA sequences found in all other humans today, so they looked to more ancient genome sequences for an answer.
"We compared it to Neanderthals, but we couldn't find a match. Then we compared it to Denisovans pola ba and to our surprise there was an almost exact match," he explained.
He says the interbreeding event with Denisovans probably happened very long ago.
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