Developing Models of Premolar Molarization in Primates

Molars
A Paranthropus dentition compared to a modern human. Photo by Bob Campbell.
By Kes Schroer
February 02, 2011

What makes a mammal? We lactate, are homeothermic, and have hair. We also have diverse tooth types – incisors, canines, premolars, and molars. The evolution of these complex teeth helped mammals exploit a wide variety of habitats. Variation in this complexity provides a way to reconstruct evolutionary relationships and interpret dietary adaptations. Biomedical and evolutionary-developmental research has elucidated substantial portions of the development of molars, but the development of the premolars remains elusive. In primates, premolars are particularly variable, ranging in shape from the tiny, sharp canines of the callitrichids to the enlarged, grinding platforms of the hominins.

My research interest is molarization – the process by which premolars come to look like molars. Outside of our own genus, Homo, most hominins had molarized premolars. One species, Paranthropus boisei, has an extreme form of this morphology, with premolars three times the size expected for its body size. Studying molarization in living primate populations provides insight into how and why Paranthropus developed such unusual teeth.

Several research venues provide models for contextualizing premolar variation. With colleagues at the Max Planck Institute for Evolutionary Anthropology and the Smithsonian Institution’s National Museum of Natural History, I am exploring geometric-morphometric analyses of premolar crown and root structures. With researchers at the National Zoo and here at CASHP, I am exploring the material properties of the diets of primates with molarized morphologies. Scientists at the University of Helisnki’s Evo-Devo Laboratory have also developed an inhibitory cascade model, which estimates the expression of genetic activators and inhibitors at work during postcanine development. Integrating these interdisciplinary models will help elucidate the evolutionary origin of molarization in fossil hominins.