Actually, there are some South American Indian tribes that live at 18,000 feet (2/3) and the capital of Bolivia (La Paz) is over 12,000 feet in elevation. The Indians in question are short and have very large lungs :)
I grew up at about 9000 feet in Utah, - I guess that explains it!
Those Andean Indians have been living at high altitudes for centuries, and have permanent genetic adaptations to low oxygen levels. They also often chew coca, which gives them endurance and tolerance of pain. Climbers who attempt Everest require an extended period of acclimation to the altitude, which they obtain at the base camps. Most people could not possibly make it to the top without oxygen tanks. It would be comparable to being in an aircraft at 30K’ when the cabin lost pressure. Have you heard that Reinhold Messner climbed Everest solo without oxygen? Surely that was one of the most amazing feats of human endurance in history.