Functional morphology, biomechanics and the retrodiction of early hominin diets

A fundamental axiom that underlies evolutionary biomechanics maintains that natural selection has adapted skeletal and dental morphologies to facilitate function in intelligible ways. Because selection is held to result in the improvement of functional design, it is thought possible to predict the adaptive significance of a morphological feature from a design criterion. This rationale has been widely applied to the interpretation of morphological complexes in the paleontological record for which no strict extant analogues exist. In particular, biomechanical models have been used to infer the dietary habits of extinct hominin taxa from aspects of craniodental morphology. Many of these models are hampered by missing data and loosely justified assumptions. Craniodental morphologies may indicate more about what an extinct species was capable of processing intra-orally – and probably more about its phylogenetic history – than the constitution of its diet. Even a dedicated leaf-eating hominin cannot be expected to have possessed bilophodont molars. The dietary retrodictions based on comparative anatomical that have been proffered for Plio-Pleistocene hominin species are reviewed here. We argue that the application of finite element analysis to these fossils has not revealed convincing evidence of specific feeding behaviors. Indeed, the conclusions from these biomechanical models are often incongruent with data from stable light isotopes, microwear and phytolith analysis; nor do they conform with observations on the dietary repertoires and feeding behaviors of extant primates. For example, the sooty mangabey (Cercocebus atys) is a committed hard object feeder, but suffers no deleterious consequences from the “poor design” of its facial skeleton for processing hard foods. Because primates are adept at behaviorally circumventing mechanical problems, it is perhaps useful to consider the breadth of any fossil hominin's feeding repertoire in the context of behaviors that enable mechanical problems to be dealt with prior to ingestion.