If you traveled to Tibet, called the roof of the world, you'd have a tough time catching your breath because the average altitude there is thirteen thousand feet! But Tibetans thrive there! It turns out they're the only modern humans to possess a gene they may have inherited from a now extinct species of ancient human called Denisovans.

When we look at another group of people living at high altitude, in the Andes Mountains of South America, they produce higher levels of hemoglobin in their blood. They also have larger chests that accommodate larger lungs, but Tibetans don't exhibit these physiological adaptations.

Recent studies reveal Tibetans possess a unique form of the EXPAS1 gene. In others, this gene responds to low oxygen levels by increasing hemoglobin production. But the opposite happens in Tibetans. They do not have elevated levels of hemoglobin. This is counterintuitive since in theory, increasing hemoglobin increases oxygen transport in the blood which helps in high altitude where the air is thinner. But it's also true that too many blood cells thickens the blood, and increases stroke risks. So, scientists are unsure exactly how this Tibetan variant of the EXPAS1 gene protects them.

The newest study tells us this gene may have come from an extinct species of humans. The Denisovans' fossils were only discovered four years ago in Southern Siberia. But the Neanderthals also have a similar version of the EXPAS1 gene, so the source is unclear. What's fascinating is that modern humans may owe some of their beneficial traits to ancestors who interbred with ancient humans that helped them to survive and spread across the planet.