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 a trade-off between the need to increase the efficiency of bite force production and the need to maintain the structural integrity of the face.  This may have implications for interpreting cranial form in other vertebrates.

Details regarding the construction and analysis of finite element models of OH5, Sts 5, MH1 and the chimpanzees are provided elsewhere (Smith et al., 2015a,b; Ledogar et al., 2016) and the finite element analysis of AL 444-2 followed the same procedures.  Briefly, a watertight, tessellated surface model was converted into a mesh of tetrahedral finite elements. The model was assigned the material properties of bone and loaded with forces simulating the jaw adductor muscles. Nodes at the two articular condyles and a bite point on either the M²or P³were constrained from moving, producing reaction forces at those nodes. The reaction force at the bite point is the bite force.  Maximum principal, minimum principal and von Mises strains were recorded at selected nodes throughout the model, but are available here at every node in every model.

Smith AL et al.  2015. Biomechanical implications of intraspecific shape variation in chimpanzee crania: moving towards an integration of geometric morphometrics and finite element analysis. Anat. Rec. 298, 122-144.

Smith AL et al.  2015. The feeding biomechanics and dietary ecology of Paranthropus boisei. Anat. Rec.298, 145-167. 

Ledogar JA et al.  2016. Mechanical evidence that Australopithecus sedibawas limited in its ability to eat hard foods. Nat. Comm.7, 10596.

Finite element models and results can be opened in Strand7 finite element analysis software.