[gridlockd]

Why do you believe that? Tibetans, for instance, are already genetically adapted to high altitudes where ordinary people may have some health issues. Mountain climbers can adapt to lower effective oxygen levels in a relatively short amount of time.

Again, some of our ape ancestors evolved in a time with far lower oxygen levels. After that, oxygen levels rose higher, but they have been drifting downwards for millions of years since then.