Individual specimens of Homo erectus vary widely but some trends are visible over time. These artist's reconstructions of three faces from skulls show variations over a period of about a million years. The oldest, approximately 1.5 million years in age, is on the left, and the most recent, approximately .5 million years old, on the right. Seen together, these specimens show the developmental trends within the species. Over time, the face evolved toward a flatter, more vertical plane with a larger and more rounded cranium as brain size increased. It is important to remember that reconstructions such as these are accurate only as to bones and the larger muscles which attached to them; features such as nose structure are only partly inferrable from surviving evidence; ears, hair, skin color and texture, etc. are conjectural. Among early Homo erectus, males are much larger than females [that is, sexual dimorphism was expressed in a significant size differential]. By about a million years ago, however, the size ratio of the sexes changes and becomes roughly the same as in modern humans--females are approximately 80% as large as males. Scientists speculate that this size change in later H. erectus signals important behavior changes in the species--that is, H. erectus anatomy at this point suggests that a human-like social structure had emerged. If that is the case, this may be one of he most important milestones in human development. We are unlikely to find confirming evidence about social behavior in the fossil record, however, simply because evidence of this kind is seldom preserved.
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