Dental development and the evolution of life history in hominidae

Development of the dentition is critically integrated into the life cycle in living mammals. Recent work on dental development has given rise to three separate lines of evidence on the evolution of human growth and aging; these three, based on several independent studies, are reviewed and inte- grated here. First, comparative study of living primate species demonstrates that measures of development (e.g., age of emergence of the first permanent molar) are highly correlated with the morphological attributes brain and body weight (as highly as r = 0.98, N = 21 species). These data predict that small-bodied, small-brained Australopithecus erupted M, at 3-3.5 years and possessed a life span comparable to that of a chimpanzee. Second, chronologi- cal age at death for three australopithecines who died at or near emergence of MI is now estimated as -3.25 years based on incremental lines in teeth; this differs substantially from expectations based on human growth schedules (5.5-6 years). Third, developmental sequences (assessed by the coefficient of variation of human dental age) observed in gracile Australopithecus and great apes diverge from those of humans to a comparable degree; sequences become more like modern humans after the appearance of the genus Homo. These three lines of evidence agree that the unique rate and pattern of human life history did not exist at the australopithecine stage of human evolution. It is proposed that the life history of early Homo matched no living model precisely and that growth and aging evolved substantially in the Hominidae during the last 2 million years.