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One of the tribes in my world exists among the mountains of which the tallest is almost 16,000m. A major city for the tribe exists on the mountain at around 8000m. Therefore, what internal - or external - adaptions would the body need to make to live at that height? I have already decided that these people would be taller and thinner people. Also, would that be acceptable for living in those conditions?

Is your world having the same level of gravity and atmospheric distribution as the Earth, meaning that atmosphere will be as thin as Earth's at the same altitude? If yes, your city would be in glacier level, with no vegetation and permanent snow. I'm not saying that having a real city is impossible there, but you may want to think about logistical problems. – Alexander – 2017-04-04T00:44:52.383