Sex and the mandibles of early hominins

Determining sex from human mandibles (as you will do in another part of this lab) depends on a series of characteristics that tend to differ between male and female humans. But those same features do not necessarily vary in the same way in every population of people living today. The pattern of sexual dimorphism in the human mandible has evolved over time, and therefore varies.

When we look at the mandibles of earlier hominins, we see that they vary in a different pattern compared to recent humans. With a fossil mandible, it can be very difficult to determine whether it represents a male or female. Any determination must depend on the variation known to exist within the ancient population.

We can compare early hominins to other primates, and we find that again, the pattern of sexual dimorphism is somewhat different. Chimpanzees, bonobos, gorillas and orangutans have a clear pattern of canine size dimorphism. Males have larger canines, females smaller. The lower third premolar also is somewhat dimorphic in shape, and even more so in size and wear pattern due to the presence of the large upper canine. With substantial body size dimorphism in gorillas and orangutans, the male mandibles are noticeably larger than female mandibles. All of these primates have a bar on the posterior side of the mandibular symphysis, called a simian shelf. The size and robusticity of this feature and other parts of the mandible reflect sex.

Early hominins do not have the same extent of canine size dimorphism as other hominoids, but the males do tend to have larger canines than females. In early hominins like A. afarensis, this dimorphism is marked in both projection and diameter of the canines, and the lower third premolars also vary in shape and orientation between males and females. In later hominins, who accentuate the large chewing teeth, the canines still have some size dimorphism in their diameters, but this loses its utility in the robust australopithecines.