The evolution of body mass and relative brain size in fossil hominids

Abstract

Abstract The observation that absolute brain size increased over the past 2 million years is one of the few uncontested facts of hominid evolution. There is, however, less agreement about how the size of the brain evolved relative to that of the body. Relative brain size has proven to be difficult to quantify because fossil hominid crania that offer the endocranial 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 calculated for each fossil hominid cranium that has a measured endocranial volume. This paper investigates one such cranial feature, the area of the orbital aperture, and its correlation with body mass in a large sample of extant primates. The results demonstrate that orbital area is correlated with body mass atr=0·987. Predictions of body mass on the basis of orbital area measured for fossil hominids suggest that body mass inHomoincreased 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 bothAustralopithecusand archaicHomo, with the values forAustralopithecusexceeding those of the living hominoids, and archaicHomoexceeding those ofAustralopithecus. The step-like differences among these taxa stand in contrast to the observation that there appears to have been no or only a negligible increase in relative brain size through the subsequent nearly 2 million years of evolution in the genusHomo. The most dramatic changes occur with the appearance of modernHomo sapiensat 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.