Evolutionary and functional significance of hominoid tooth enamel

The purpose of this investigation is to evaluate enamel thickness in extant and extinct hominoids, The material used in this study spans the evolution ary history of this group, from 20 million years ago to the present. The ob jectives of this investigation are to test three hypotheses: (1) the Loadin g Hypothesis: loading areas of the crown have thicker enamel than non-loadi ng areas; (2) the Phyletic Hypothesis: differences in enamel thickness prov ide a basis for determining evolutionary relationships; and (3) the Functio nal Hypothesis: differences among hominoids result from adaptations to diff ering dietary and ecological habitats, that is from folivory to frugivory t o hard object feeding and from tropical to forest to savanna habitats. Thin sections were prepared and polished to approximately 100 mu m in thickness . Each section was then enlarged and digitally captured to the computer. Im age processing and analysis software, SigmaImage(TM) (was used to measure t he sections. Subsequent statistical analysis was conducted with SigmaStat(T M) and SPSS(TM) statistical software programs. The data provides statistica l support for all hypotheses, In particular, the data support the proposal that "thick" enamel is the ancestral condition for the great apes and human clade, Therefore, Pongo would have retained its enamel thickness from the common ancestor of the great apes and Gorilla and Pan would have secondaril y reduced enamel thickness to "thin," The common ancestor of the hominids, the australopithecines, would have "thick" enamel. The "hyper-thick" enamel of the australopithecines would be a derived character for this clade due to increased crushing and grinding and adaptation to savanna habitat, Homo would have secondarily reduced enamel thickness to "thick," Evolutionary bi ology of enamel differs markedly in hominids from that found in other homin oids and primates. Increased enamel thickness involved both increases in ab solute thickness of enamel and crown size in response to increase masticato ry loading.