
There are people who have adapted to high altitudes in the Andes and in
Tibet. I suspect that it took them several generations to make the
adaptation because Europeans had difficulty making the adaptation. They
had to send the women to a lower altitude when they were pregnant in order
to insure sucessful childbirth.


Another factor you should consider is the X-ray opacity of the atmosphere 
in case of stellar flares, the uv opacity is also important because uv 
radiation can kill or damage microbes, plants, and animals. 