THE EVOLUTION OF BODY-MASS AND RELATIVE BRAIN SIZE IN FOSSIL HOMINIDS

The observation that absolute brain size increased over the past 2 mil lion years is one of the few uncontested facts of hominid evolution. T here is, however, less agreement about how the size of the brain evolv ed relative to that of the body. Relative brain size has proven to be difficult to quantify because fossil hominid crania that offer the end ocranial measurements: and postcrania that generally provide the body- mass estimates, can only rarely be attributed to the same individual. If it could be established that some aspect of the cranium is strongly correlated with body mass, then relative brain size could be calculat ed for each fossil hominid cranium that has a measured endocranial vol ume. This paper investigates one such cranial feature, the area of the orbital aperture, and its correlation with body mass in a large sampl e of extant primates. The results demonstrate that orbital area is cor related with body mass at r=0.987. Predictions of body mass on the bas is of orbital area measured for fossil hominids suggest that body mass in Homo increased through time, and that body mass sexual dimorphism was possibly somewhat greater for some fossil hominid species than it is in living humans. Combining these body mass estimates with measures of endocranial volume demonstrates increased relative brain size for both Australopithecus and archaic Homo, with the values for Australopi thecus exceeding those of the living hominoids, and archaic Homo excee ding those of Australopithecus. The step-like differences among these taxa stand in contrast to the observation that there appears to have b een no or only a negligible increase in relative brain size through th e subsequent nearly 2 million years of evolution in the genus Home. Th e most dramatic changes occur with the appearance of modern Homo sapie ns at about 100,000 years ago and include a decrease in body mass and an increase in relative brain size that appears to have been driven by selection for smaller body mass. (C) 1996 Academic Press Limited