1Human Evolutionary Biology, Harvard University, 2Institute for Quantitative Social Science, Harvard University
Friday 8:15-8:30, Ballroom C
Humans are one of the few mammalian species showing polymorphism in permanent molar number. The underlying source of this variation, however, remains unclear. Recently developmental biologists have suggested the inhibitory cascade (IHC) model as a paradigm for rodent tooth development, which they also claim generalizes across mammals. Using the Swindler primate dental metric database supplemented with measurements from photographs and histological data, we assess the fit of the IHC model to molar proportions of hominins, extant primates and other large-bodied mammals (n=68 species). Results from analyses of molar morphospace occupation and from the patterns of temporal overlap between first and second molars are incompatible with the patterning mechanism proposed for rodents. Notably, the evolution of low-crowned teeth (irrespective of frugivory) appears tightly linked with the evolution of large second molars and small third molars. Humans, the only species of primate to fit both the morphological and developmental predictions of the IHC model, still differ markedly from the model's prediction of strong covariation between second and third molars. Third molar modularity first appears at the base of the hominin lineage. Surprisingly, the shift to this pattern of covariance occurred substantially before reduction in mandibular corpus length, indicating a non-functional explanation for the origin of third molar agenesis.