title: Human feeding biomechanics: performance, variation, and functional constraints.
abstract: The evolution of the modern human (Homo sapiens) cranium is characterized by a reduction in the size of the feeding system, including reductions in the size of the facial skeleton, postcanine teeth, and the muscles involved in biting and chewing. The conventional view hypothesizes that gracilization of the human feeding system is related to a shift toward eating foods that were less mechanically challenging to consume and/or foods that were processed using tools before being ingested. This hypothesis predicts that human feeding systems should not be well-configured to produce forceful bites and that the cranium should be structurally weak. An alternate hypothesis, based on the observation that humans have mechanically efficient jaw adductors, states that the modern human face is adapted to generate and withstand high biting forces. We used finite element analysis (FEA) to test two opposing mechanical hypotheses: that compared to our closest living relative, chimpanzees (Pan troglodytes), the modern human craniofacial skeleton is (1) less well configured, or (2) better configured to generate and withstand high magnitude bite forces. We considered intraspecific variation in our examination of human feeding biomechanics by examining a sample of geographically diverse crania that differed notably in shape. We found that our biomechanical models of human crania had broadly similar mechanical behavior despite their shape variation and were, on average, less structurally stiff than the crania of chimpanzees during unilateral biting when loaded with physiologically-scaled muscle loads. Our results also show that modern humans are efficient producers of bite force, consistent with previous analyses. However, highly tensile reaction forces were generated at the working (biting) side jaw joint during unilateral molar bites in which the chewing muscles were recruited with bilateral symmetry. In life, such a configuration would have increased the risk of joint dislocation and constrained the maximum recruitment levels of the masticatory muscles on the balancing (non-biting) side of the head. Our results do not necessarily conflict with the hypothesis that anterior tooth (incisors, canines, premolars) biting could have been selectively important in humans, although the reduced size of the premolars in humans has been shown to increase the risk of tooth crown fracture. We interpret our results to suggest that human craniofacial evolution was probably not driven by selection for high magnitude unilateral biting, and that increased masticatory muscle efficiency in humans is likely to be a secondary byproduct of selection for some function unrelated to forceful biting behaviors. These results are consistent with the hypothesis that a shift to softer foods and/or the innovation of pre-oral food processing techniques relaxed selective pressures maintaining craniofacial features that favor forceful biting and chewing behaviors, leading to the characteristically small and gracile faces of modern humans.
authors.affiliation: Zoology Division, School of Environmental and Rural Science, University of New England, Armidale, New South Wales, Australia; Department of Anthropology, State University of New York at Albany, Albany, New York, United States.
authors.forename: Justin A
authors.identifier: 0000-0002-3882-9354
authors.initials: JA
authors.lastname: Ledogar
authors.affiliation: Department of Biomedical Sciences, Texas A&M University College of Dentistry , Dallas, Texas , United States.
authors.forename: Paul C
authors.initials: PC
authors.lastname: Dechow
authors.affiliation: Department of Biomedical Sciences, Texas A&M University College of Dentistry , Dallas, Texas , United States.
authors.forename: Qian
authors.initials: Q
authors.lastname: Wang
authors.affiliation: Department of Biomedical Sciences, Texas A&M University College of Dentistry , Dallas, Texas , United States.
authors.forename: Poorva H
authors.initials: PH
authors.lastname: Gharpure
authors.affiliation: Department of Anthropology, State University of New York at Albany , Albany, New York , United States.
authors.forename: Adam D
authors.identifier: 0000-0002-1807-4644
authors.initials: AD
authors.lastname: Gordon
authors.affiliation: Department of Anatomy, Midwestern University , Glendale, Arizona , United States.
authors.forename: Karen L
authors.initials: KL
authors.lastname: Baab
authors.affiliation: Department of Anthropology, State University of New York at Albany, Albany, New York, United States; Department of Anthropology, Washington University in St. Louis, St. Louis, Missouri, United States.
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Smith
authors.affiliation: Department of Anthropology, University of Vienna , Vienna , Austria.
authors.forename: Gerhard W
authors.initials: GW
authors.lastname: Weber
authors.affiliation: Department of Mechanical & Industrial Engineering, University of Massachusetts , Amherst, Massachusetts , United States.
authors.forename: Ian R
authors.initials: IR
authors.lastname: Grosse
authors.affiliation: Department of Organismal Biology & Anatomy, University of Chicago , Chicago, Illinois , United States.
authors.forename: Callum F
authors.initials: CF
authors.lastname: Ross
authors.affiliation: Division of Anthropology, American Museum of Natural History, New York, New York, United States; Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany.
authors.forename: Brian G
authors.initials: BG
authors.lastname: Richmond
authors.affiliation: Department of Anatomy, Kansas City University of Medicine and Biosciences , Kansas City, Missouri , United States.
authors.forename: Barth W
authors.initials: BW
authors.lastname: Wright
authors.affiliation: Department of Biology, Mercer University , Macon, Georgia , United States.
authors.forename: Craig
authors.initials: C
authors.lastname: Byron
authors.affiliation: Zoology Division, School of Environmental and Rural Science, University of New England , Armidale, New South Wales , Australia.
authors.forename: Stephen
authors.initials: S
authors.lastname: Wroe
authors.affiliation: Department of Anthropology, State University of New York at Albany, Albany, New York, United States; Department of Anthropology, Washington University in St. Louis, St. Louis, Missouri, United States.
authors.forename: David S
authors.initials: DS
authors.lastname: Strait
keywords: Bone strain; Cranium; Evolution; Loading