Evolutionary Anthropology 17 zy The Role of Time and Timing in Hominid Dental Evolution GABRIELE A. MACHO AND BERNARD A. WOOD Exactly when during evolution hominids acquired their extended extra-uterine growth period is a contentious issue. In order to shed light on the tempo and mode of ontogenetic changes during hominid evolution, research has focused on the pattern and, to a lesser extent, the rate of growth observed in the developing den- tition of extant and extinct hominoid taxa. From these data, the absolute timing of events has often been inferred, either implicitly or explicitly. Differences in patterns of growth, especially of the eruption of teeth, are reasonably well documented among hominoids. However, data on the absolute timing of dental developmental events are much more scarce, rendering tentative all inferences about timing from patterns alone. Such inferences are even more tentative when they involve inter- preting ontogenetic trajectories in extinct species such as Plio-Pleistocene homi- nids, which almost certainly had unique patternsof maturation.In order to contribute to the debate about possible relations between pattern and timing in the developing dentition, we have collated informationthat specifically relatesto the absolutetiming of developmental events in extant and extinct hominoids and, hence, also to the rate at which processes occur. In doing so, we have attempted to identify both develop- mental constraints and possible heterochronic processes that may have led to the extended growth period characteristic of humans. There appears to be growing evidence that evolution toward an extended hominid ontogeny did not follow a path that can be described as a simple heterochronicevent. zyxwvut Time and the relative timing of events are important factors in bio- logical studies, for they have the po- tential to permit inferences about the evolution of both morphology and life history. The best known category of study that invokes time as an influence on morphology is allometry.’ In an earlier issue of zyxwvutsrqp Evolutionary Anthro- pology, Smith2 demonstrated the im- portance of time and the timing of dental developmental events as tools to help us understand changes in life- history variables that have occurred during hominid evolution. These vari- Both Gabriele Macho and Bernard Wood are in the Hominid Palaeontology Research Group at The University of Liverpool. bawood@liverpool.ac.u k Key words: ontogeny, life history, heterochrony, crown formation, eruption, enamel formation ables include, for example, gestation length, age at weaning, and sexual ma- turity. Such inferences are possible only because evidence suggests that dental eruption sequence is correlated with significant life history parame- ters among mammals and that the ab- solute timing of events can be reasonably deduced from eruption patterns. A case in point is that, at one extreme, rapidly growing, short-lived mammals such as insectivores erupt all their molars before their deciduous dentition has been shed, whereas slowly growing mammals such as hu- mans generally delay the eruption of their molars. However, differences be- tween closely related species or genera are often subtle. As Harvey and Clut- ton-Brock3 pointed out, “...it is desir- able that analyses should be carried out at a higher level than the species” zyxw (p 561). zyxwvu 5-8 The difficulty of describing life history precisely enough, and of de- ducing the timing of developmental events from patterns is compounded when one is dealing with extinct taxa. It is a virtual certainty that the ontoge- netic trajectories of those taxa are not exactly paralleled by those of extant species. Demonstrably,that is the case with regard to Plio-Pleistocene homi- nids and modern humans.4 Most re- searchers would probably agree that even early African Homo erectus did not yet exhibit an extended infancy precisely like that seen in modern hu- mans. Thus it comes as little surprise that there is an ongoing debate about the nature of the ontogeny of early hominid taxa. Many discussions about the devel- opment and life history of early homi- nid taxa have centered around the conceptual framework within which to interpret the data rather than focus- ing on the data. An example is the dis- agreement between Alan Mann and Holly Smith. Both have studied devel- opment in early hominid taxa. Mann and his colleague^^-^^ maintain that the pattern of dental development is within the limits of modern human variation and therefore implies a “modern human” timing of develop- mental events. On the other hand, Smith’s zyx interpretations2,7,l3-19 favor the conclusion that the early hominid pattern is more like that of apes than modern humans, thus suggesting a more pongid-like rate of development for these early hominid species. This debate has not only highlighted the problem of inferring time from pat- tern, but has also led researchers to investigate variations in dental devel- opment within and between species, as well as between sexes.20-22 Such studies are invaluable for establishing the extent of intraspecific plasticity during development, but they have