Evolution of the human life cycle.

Social mammals have three basic stages of postnatal development: infant, juvenile, and adult. Some species also have a brief female post-reproductive stage. The human life cycle, however, is best described by five stages: infant, child, juvenile, adolescent, and adult. Women in both traditional and industrial societies may also have a long post-reproductive stage. Analyses of bones and teeth of early hominids who died as subadults suggest that the evolution of the new life stages of childhood and adolescence are not of ancient origin. The current human pattern evolved after the appearance of Homo erectus. It is possible that evidence for the existence of the post-reproductive stage for women will also be recoverable from the fossil record because the hormonal changes associated with menopause have profound effects on bone density and histology of tubular bones. It is hypothesized that the new life stages of the human life cycle represent feeding and reproductive specializations of the genus Homo. © 1996 Wiley-Liss, Inc.