The influence of stature and body form on hominid energy and water budgets; a comparison of Australopithecus and early Homo physiques

Abstract

Despite their bipedal posture, the available fossil evidence indicates that the body proportions of australopithecines were more ape-like than those of modern humans. By at least 1·6 million years ago (m.y.a.) Homo erectus had evolved the taller linear physique typical of many human populations inhabiting hot, open environments. Such body forms are usually interpreted as thermoregulatory adaptations, increasing the area of skin available for both the evaporative and non-evaporative dissipation of body heat. Although surface area to body mass ratio is a major influence on energy exchange, other factors such as the height distribution of the body surfaces and their exposure to direct solar radiation are also important in these climates. Estimates are presented of the combined effect of these factors on the net thermal loads, and associated drinking water requirements, experienced by large naked bipedal hominids of identical body mass with physiques approximating those of Australopithecus and H. erectus. These indicate that the taller more linear Homo body form can reduce daytime (06.00-18.00 h) sweat losses by 21-29% in open equatorial environments. The corresponding reduction in total daytime drinking water requirement is between 15-18%. The findings support the contention that thermoregulatory selection pressures were probably a major influence on hominid evolution.