Progress in understanding hominoid dental development.

Teeth preserve a record of the way they grow in the form of incremental markings in enamel, dentine and cementum. These make it possible to reconstruct cellular activity and the timing of dental development in living and fossil primates, including hominids. They also provide a way of exploring the mechanisms that underlie morphological change during evolution and the nature of the relationship between ontogeny and phylogeny. All living great apes are dentally mature by about 11 y, irrespective of their body mass. While the early period of root formation in living great apes is shorter than in modern humans, enamel takes approximately the same time to form, irrespective of how thick it is. In general, differences in the total time taken to form enamel seem not to be due to differences in the rate at which enamel and dentine are secreted, but rather to faster or slower rates of differentiation of ameloblasts and odontoblasts and therefore to the number of secretory cells active at any one time during tooth formation. Tooth size, especially height, may influence the sequence of appearance of tooth mineralisation stages. The space available in the jaws may also have an influence on both the timing of tooth bud/crypt appearance and the sequence of gingival emergence. When each of these potential influences on dental development are carefully considered, and incremental markings used to calibrate key events, the developing dentition can provide an estimate of the period of dental maturation in fossil hominoids. However, the influence of body mass on the period of dental development among primates remains unclear. The earliest hominoids, dated at around 18 Mya, may still have had modern monkey-like maturational profiles, and the earliest hominids, dated between 1.8 and 3.7 Mya, modern great ape-like maturational profiles. Exactly when the extended or prolonged modern human-like maturational profile first appeared remains debatable, but the most secure suggestion might be at the time of the appearance of the earliest archaic Homo sapiens, when brain size and body mass were finally both within the ranges known for modern humans. But at present we should not reject the hypothesis that an extended, modern human-like, maturational profile arose more than once during human evolution in parallel with an increase in brain size.