The Physical Characteristics of Humans
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Sexual Dimorphism

In many species, the two sexes are signficantly different in form. In some primate species, gorillas, for example, males are much larger and bulkier than females--in some cases, roughly twice the size of females.

This is the case also in the earliest Hominid species. Sexual difference or dimorphism is expressed in terms of a large size differential between males and females, much as it is in gorillas. On the right are thigh bones (femurs) of adult A. afarensis (3.5 million years old); the larger bones are male and the smaller female. At left, we see contrasted the skulls of male and female A. robustus (top) and Homo habilis (bottom), the first human species, which lived approximately 2 million years ago.

In living mammal species, size differentials of this scale are usually associated with male competition for "harems" of females--for example, in gorillas or sea lions. Indirect evidence of this kind is nearly all that survives in terms of clues about the social behavior of extinct Hominids. Interestingly, this huge size differential between males and females disappears in the human line roughly a million years ago, in Homo erectus. Does this change in anatomy reflect a significant change in behavior? Again, anatomy often provides clues about social behavior.


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