Progress in understanding hominoid dental development

Teeth preserve a record of the way they grow in the form of incremental mar kings in enamel, dentine and cementum. These make it possible to reconstruc t cellular activity and the timing of dental development in living and foss il primates, including hominids. They also provide a way of exploring the m echanisms that underlie morphological change during evolution and the natur e 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, irrespecti ve of how thick it is, rn general, differences in the total time taken to f orm enamel seem not to be due to differences in the rate at which enamel an d dentine are secreted, but rather to faster or slower rates of differentia tion of ameloblasts and odontoblasts and therefore to the number of secreto ry cells active at any one time during tooth formation. Tooth size, especia lly height, may influence the sequence of appearance of tooth mineralisatio n stages. The space available in the jaws may also have an influence on bot h the timing of tooth bud/crypt appearance and the sequence of gingival eme rgence. When each of these potential influences on dental development are c arefully considered, and incremental markings used to calibrate key events, the developing dentition can provide an estimate of the period of dental m aturation in fossil hominoids. However, the influence of body mass on the p eriod of dental development among primates remains unclear. The earliest ho minoids, dated at around 18 Mya, may still have had modern monkey-like matu rational 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 de batable, but the most secure suggestion might be at the time of the appeara nce of the earliest archaic Home sapiens, when brain size and body mass wer e finally both within the ranges known for modern humans. But at present we should not reject the hypothesis that an extended, modern human-like, matu rational profile arose more than once during human evolution in parallel wi th an increase in brain size.