"Usually, if you or I or any nonadapted person went to high altitude, we would increase our hemoglobin levels to compensate for the low amount of oxygen."
But high hemoglobin levels have been linked to complications such as hypertension and chronic mountain sickness, Simonson said.
These negative effects could have led to a genetic mutation among Tibetans that "prevented them from making as much" hemoglobin, she noted.[...]
Several variants of genes associated with high-altitude living, such as those that process oxygen, were found in Tibetans but not in their low-living neighbors. That includes the two genes that are strongly associated with low hemoglobin production.
Tibetans also tend to have wider blood vessels and take more breaths per minute.
Link | Photo: Lynn Johnson, National Geographic
UPDATE: If you're curious about the identity of the rifle held by the Tibetan man in the picture, James Rummel is holding a discussion on it.