157 - Homo Origins Mark an Adaptive Shift Characterized by Constrained Bite Force Production

Poster Board Number: 157
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Abstract Body : Dietary shifts figure prominently in adaptive hypotheses of human origins. The divergence of Homo from gracile australopiths has been described as a trend of decreasing dentognathic size and robusticity, precipitated by stone tool use and/or a shift to softer foods, including meat. However, mechanical evidence supporting this narrative is sparse, and isotopic and archaeological data have led to the suggestion that a shift away from a gracile australopith-like diet would not have occurred in the most basal members of Homo but rather only with the appearance of H. erectus, implying that the origin of our genus is not rooted in dietary change. Recent integrative models of Homo origins stress the importance of identifying adaptive features that originated with the earliest representatives of the genus. Here we use finite element analysis (FEA), a computer-assisted engineering technique, to simulate the craniofacial feeding biomechanics of Homo habilis. We analyzed biting efficiency and structural strength during premolar (P3) and molar (M2) biting in a finite element model (FEM) of H. habilis (based on specimen KNM-ER 1813), and compared the mechanical results with those from previously-constructed models of “gracile” and “robust” australopith species, modern chimpanzees, and recent modern humans. Results demonstrate that H. habilis had a somewhat weaker midface than australopiths and (unlike most australopiths) was susceptible to highly distractive jaw joint forces during molar biting, constraining muscle recruitment and potentially generating damaging levels of tension in the joint. These results suggest that H. habilis was not suited for a mechanically challenging diet that would have required forceful processing by the molar teeth. Modern humans have similar limitations. Our analysis therefore suggests that biomechanical constraints on molar bite force production and their concomitant dietary limitations were already in place with the emergence of Homo habilis, implying that shifts in diet and/or food processing played an important role in the emergence of our genus.