Mechanical and Thermal Food Processing Effects on Mastication and Cranio-Dental Morphology

Abstract

Chimpanzees spend ~40% of their day chewing fruits, seeds, and tough leaves and pith, while in contrast modern humans spend significantly less time eating (5%), and the foods that they consume are extremely soft and processed. How have these differences, especially the advent and increasing use of foods processing techniques, influenced masticatory effort and ultimately the morphology of the jaws and teeth? This dissertation addresses this question by measuring the effects that early hominin food processing methods (slicing, pounding, and roasting) have on food material properties, masticatory performance and functional integration of the teeth and jaws. Using standard testing techniques, the material properties of plant tubers and meat were quantified. Processing had contrasting effects on the properties of these foods, and were correlated with masticatory performance changes measured in human experiments. Mechanical processing techniques decreased tuber toughness, leading to lower chew force (CF). Roasting further decreased tuber toughness and other material properties, which led to lower comminution efficiency (CE) and CF. In direct contrast to tubers, mechanical processing techniques did not alter meat toughness, yet did increase CF and CE. Roasting the meat also increased CF and CE, likely because of higher toughness and stiffness, coupled with less elastic energy loss. The generation of lower masticatory forces resulting from processing have undoubtedly affected cranio-dental morphology. In particular, it is hypothesized that forces functionally integrate the masticatory system, and reduced forces, especially in modern human populations, lead to malocclusions (dis-integration). An animal experiment was performed to test this hypothesis, and the results indicate that masticatory effort (eating hard or soft foods) coordinates jaw and dental growth. Further testing the hypothesis, the effects of morphology on masticatory function were studied by coupling subject masticatory performance with occlusal scores. Multiple regressions of occlusion and tooth size explained a high proportion of masticatory performance variance (significantly more than tooth size alone), suggesting that occlusal integration does indeed affect masticatory function. Taken together, the results of this dissertation document the significant reductions in hominin masticatory forces and changes in cranio-dental growth and integration that may have resulted from the use of food processing techniques.