Our Ancestral Physiological Phenotype: An Adaptation for Hypoxia Tolerance and for Endurance Performance?

There are well known mechanistic similarities in human physiology between adaptations for endurance performance and hypoxia tolerance. By using background principles arising from recent studies of the evolution of the diving response in marine mammals, here we analyze human responses to hypobaric hypoxia based on studies with several different low and high altitude human lineages. As in the evolution of the diving response in pinnipeds, we found ``conservative'' and ``adaptable'' physiological characters involved in human responses to hypoxia. Because the analysis concerns traits within a single species, conservative characters dominate the picture (they define basic human physiology and largely are independent of environmental parameters). Most notably, we also found evidence for adaptable characters forming the foundations for a fairly unique physiological phenotype--a low capacity version favored under hypobaric hypoxia and a high capacity one favored for endurance performance. Because current evidence implies that the human species arose under conditions that were getting colder, drier, and higher (situations in which these traits would have been advantageous), we hypothesize that this physiology is our ``ancestral'' condition.