Adaptations to altitude: A current assessment

The high-altitude Andean and Tibetan Plateaus offer natural experimental settings for investigating the outcome of the past action of evolution and adaptation as well as those ongoing processes. Both Andean and Tibetan high-altitude natives are descended from sea-level ancestors; thus both initially encountered chronic, lifelong high-altitude hypoxia with the same homeostatic "toolbox" that evolved at sea level for responding to brief and transient hypoxia. Yet now they differ phenotypically in many traits thought to be important for offsetting chronic high-altitude hypoxia. Compared on the basis of mean values of five traits, the characteristics of Tibetan high-altitude natives differ more than those of Andean high-altitude natives from the ancestral or unselected response to chronic hypoxia exhibited by acclimatized lowlanders. This suggests that different evolutionary processes have occurred in the two geographically separate areas, although it is not clear why or how those processes differed. Answers to those questions require better knowledge of the prehistory of human populations on the plateaus, as well as information on new phenotypes and the relationship between phenotype and genotype.