The Masticatory Apparatus of Humans (Homo sapiens): Evolution and Comparative Functional Morphology

Data are presented on adult body mass for 230 of 249 primate species, based on a review of the literature and previously unpublished data. The issues involved in collecting data on adult body mass are discussed, including the definition of adults, the effects of habitat and pregnancy, the strategy for pooling data on single species from multiple studies, and use of an appropriate number of significant figures. An analysis of variability in body mass indicates that the coefficient of variation for body mass increases with increasing species mean mass. Evaluation of several previous body mass reviews reveals a number of shortcomings with data that have been used often in comparative studies.

Many dramatic changes in morphology within the genus Homo have occurred over the past 2 million years or more, including large increases in absolute brain size and decreases in postcanine dental size and skeletal robusticity. Body mass, as the 'size' variable against which other morphological features are usually judged, has been important for assessing these changes. Yet past body mass estimates for Pleistocene Homo have varied greatly, sometimes by as much as 50% for the same individuals. Here we show that two independent methods of body-mass estimation yield concordant results when applied to Pleistocene Homo specimens. On the basis of an analysis of 163 individuals, body mass in Pleistocene Homo averaged significantly (about 10%) larger than a representative sample of living humans. Relative to body mass, brain mass in late archaic H. sapiens (Neanderthals) was slightly smaller than in early 'anatomically modern' humans, but the major increase in encephalization within Homo occurred earlier during the Middle Pleistocene (600-150 thousand years before present (kyr BP)), preceded by a long period of stasis extending through the Early Pleistocene (1,800 kyr BP).

Measurements were taken on the upper and lower molars of 37 species of primates and one tupaiid to assess the relative importance of shearing, crushing and grinding features. Significant correlations were found between pairs of allometrically standardized dimensions which measure the same molar function (shearing, crushing, or grinding). Correlations between pairs of dimensions which do not measure the same function are not significant. Second molar adaptations for shearing, crushing, and grinding, as well as the length of the second lower molar, and the total surface of the post-canine dentition are negatively allometric with respect to metabolic rate. Species which take different proportions of fruit, leaves, and insects in their diets have different molar structure. Frugivores have small teeth for their adult body size with poorly developed shearing, crushing, and grinding features on their molars. By contrast, leaf-eating species tend to have large teeth for their adult body size with well developed shearing, crushing, and grinding. The second molars of insectivorous species were found to parallel closely those of leaf-eating species. The two groups are clearly distinguishable from the former on the basis of body size alone: the smallest living primate leaf-eater is on order of magnitude larger than the largest living primate insectivore.