Edge chipping patterns in posterior teeth of hominins and apes

Chip scars in fossil teeth are a lasting evidence that bears on human evolution. Chip dimensions in posterior teeth of hominins, apes and white-lipped peccary (Tayassu pecari) are measured from published occlusal images. The results are plotted as D/D_m vs. h/D_m, where h, D and D_m denote indent distance, chip width and mean tooth crown diameter. The hominin species follow a similar pattern where D/D_m monotonically increases up to h/D_m ≈ 0.3. The behavior for the apes is characterized by two phases. In the first, h/D_m monotonically increases up to h/D_m ≈ 0.26 while in the second (h/D_m ≈ 0.26 to 0.42), D/D_m experiences a drastic change in behavior. The interpretation of chip morphology is assisted by results from controlled spherical indentation tests on extracted human molars. This study shows that in addition to the commonly recognized chipping due to cusp loading, a chip may also initiate from the inner wall of the tooth's central fossa. Accordingly, it is suggested that the chipping in hominins generally initiates from a (worn) cusp while that in apes involves cusp loading up to h/D_m ≈ 0.26 and fossa loading thereafter. The behavior for T. pecari is much similar to that of the apes. The fossa chipping is facilitated by a consumption of hard, large-size diet (e.g., plants, roots, barks and nuts) and presence of broad central fossa, conditions that are met in apes. Finally, a simple expression for the critical chipping force P_ch due to fossa loading is developed.