| Reduced oxygen availability and cold ambient temperatures pose severe environmental challenges to organisms that live at high altitude. These challenges may be so severe as to form natural selective pressures that mold the physical and physiological constitution of high altitude populations. Living humans from high altitudes exhibit relatively short limbs as a response to the cold climatic conditions associated with highland environments. They also have enlarged chests as part of a respiratory adaptation to high altitude hypoxia. Adaptations to high altitude stress, however, have never been examined either in human prehistory or in nonhuman primates. I examine skeletal responses to high altitude stress in modern humans and macaques. Specifically, I compare body proportions and thoracic shape in the skeletons of modern humans from high and low altitudes in the Andes, in six species of macaques from different climates and altitudes in Asia, and in two groups of rhesus macaques that have inhabited different climates for the past 60 years.; Human skeletons from altitudes above 3500 meters in the Andes have body proportions that resemble human populations that are known to be cold-adapted, while body proportions in individuals from 2500 meters are similar to closely related groups from sea level. Climate and altitude also influence some degree of interspecific variation in the limb proportions of macaques. Morphological responses to changing climatic conditions, however, develop as long-term adaptations, at least among rhesus macaques transplanted to Cayo Santiago, Puerto Rico. Compared with lowland individuals, human skeletons from moderately high altitudes in the Andes have wider chests and longer ribs with less curvature, presumably as a respiratory response to high altitude hypoxia. Thoracic shape among macaques, however, is correlated with climatic conditions rather than altitude. Comparisons of thoracic shape variation in the skeletons of modern humans and macaques suggest that modern humans have developed unique morphological traits that enable them to thrive at extreme altitudes. |
