CLIMATE AND BODY SHAPE IN HOMINID EVOLUTION

The very broad pelvis of small early hominids (AL 288-1, STS 14, etc.) has previously been interpreted in obstetrical and biomechanical terms. However, neither of these considerations can explain the subsequent decrease in maximum pelvic breadth relative to stature in larger more recent hominids. It is shown here that this increase in relative linearity of the body with an increase in body size is consistent with basic thermoregulatory principles. Specifically, to maintain a constant surface area/body mass ratio, absolute body breadth should remain constant despite differences in body height. Variation among modern humans supports the prediction: populations living in the tropics vary greatly in stature, but show little variation in body breadth. In contrast, populations living in colder climates have absolutely wider bodies, and thus lower surface area/body mass, regardless of stature. All African early hominids-small australopithecines as well as the very tallHomo erectus KNM-WT 15000-have absolute body breadths within the modern human tropical-subtropical range; variations in relative body linearity are due almost entirely to variations in stature. A later hominid from a cold temperate climate (the Kebara 2 Neandertal) has an absolutely wide body, similar to living higher latitude populations. Thermoregulatory constraints on absolute body breadth, together with obstetric and biomechanical factors, may have contributed to the evolution of the rotational birth process and secondary altriciality with increased body and brain size inHomo erectus. Thermoregulatory considerations also suggest that AfricanH. erectus would most likely have been limited to relatively open/dry environments, while australopithecines could have inhabited either open/dry or closed/wet environments. © 1991 Academic Press Limited.