Mechanical compensation in the evolution of the early hominin feeding apparatus

Abstract

Australopiths, a group of hominins from the Plio-Pleistocene of Africa, are characterized by derived traits in their crania hypothesized to strengthen the facial skeleton against feeding loads and increase the efficiency of bite force production. The crania of robust australopiths are further thought to be stronger and more efficient than those of gracile australopiths. Results of prior mechanical analyses have been broadly consistent with this hypothesis, but here we show that the predictions of the hypothesis with respect to mechanical strength are not met: some gracile australopith crania are as strong as that of a robust australopith, and the strength of gracile australopith crania overlaps substantially with that of chimpanzee crania. We hypothesize that the evolution of cranial traits that increased the efficiency of bite force production in australopiths may have simultaneously weakened the face, leading to the compensatory evolution of additional traits that reinforced the facial skeleton. The evolution of facial form in early hominins can therefore be thought of as an interplay between the need to increase the efficiency of bite force production and the need to maintain the structural integrity of the face.

Keywords: australopith; diet; finiteelement analysis; ingestion; mastication.

Figure 1.

Colour mapping of von Mises strain in FEMs of chimpanzees (a–f), gracile australopiths (g–i) and a robust australopith (j) during maximal bites on the upper third premolar. Chimpanzee crania were intentionally selected to be morphologically different from each other [19] and are labelled according to whether or not they represent the extreme positive or negative ends of the range of variation along three principal components of shape. Colours correspond to strain magnitude, with white indicating strains greater than 1000 microstrain. (Online version in colour.)

Figure 2.

Plot of von Mises strain generated during maximal (a) premolar (P³) and (b) molar biting (M²) using bilaterally symmetrical muscle forces, recorded from 14 homologous locations across the craniofacial skeleton of FEMs of chimpanzees, and gracile and robust australopiths. Vertical grey bars represent the range of variation seen in chimpanzees, while horizontal coloured lines represent the values observed in individual hominin FEMs. (Online version in colour.)

Figure 3.

Line plots of von Mises strain were recorded along six transects across the face of each model during maximal P³ biting. Transects are shown (a) on A.L. 444-2, and line plots of the strain data in each model are shown separately by transect (b–g). Grey areas represent the range of variation seen in chimpanzees, while coloured lines represent the values observed in individual hominin FEMs. (Online version in colour.)