Cortical bone contours in midline sections of gorilla (left) and human (right) mandibles. Labial aspect is to the right. Scale bar is 10 mm. In humans, on average, cortical bone accounts for 49% of the overall area enclosed by the periosteal (outer) boundary of the midline section; in gorillas the average fraction of the total area taken up by cortical bone is about 30%. Because the overall size of the symphysis is significantly smaller than those of great apes, the added bone in the human symphysis does not match the absolute strength in great ape mandibles. Scaled to mandibular length (an appropriate biomechanical size proxy), humans have on average, relatively stronger symphyses [17]. This does not account for the reduced adductor and biting forces that humans produce. Accounting for these factors and the fact that our masticatory demands are reduced due to our practice of extroral food processing, human jaws appear to be much stronger than necessary given the mass and distribution of cortical bone.
