As noted in earlier chapters, primatologists and other physical anthropologists view primate and human growth and development from an evolutionary perspective, with an interest in how natural selection has operated on the life cycle from conception to death—a perspective known as life history theory. Why, for example, do humans have longer periods of infancy and childhood compared with other primates (Fig. 16-11)? What accounts for differences seen in the life cycles of such closely related species as humans and chimpanzees? Life history research seeks to answer such questions (e.g., Mace, 2000; Hawkes and Paine, 2006). Life history theory allows us to predict the timing of reproduction under favorable circumstances. It begins with the premise that there’s only a certain amount of energy available to an organism for growth, maintenance of life, and reproduction. Energy invested in one of these processes isn’t available to another. Thus the entire life course represents a series of trade-offs among various life history traits, such as length of gestation, age at weaning, time spent in growth to adulthood, adult body size, and length of life span. For example, life history theory provides the basis for understanding how fast an organism will grow and to what size, how many offspring can be produced, how long gestation will last, and how long an individual will live. Crucial to understanding life history theory is its link to the evolutionary process: It’s the action of natural selection that shapes life history traits, determining which ones will succeed or fail in a given environment. Although it isn’t clear if life history theory works in contemporary human populations (Strassman and Gillespie, 2002), it serves as a useful guide for examining the various life cycle phases from evolutionary and ecological perspectives.
Most life cycle stages are well marked by biological transitions, such as those that occur at birth and puberty. Biological markers associated with life cycle changes are similar among higher primates, but for humans there’s an added complexity: They occur in cultural contexts that define and characterize them. Puberty, for example, has very different meanings in different cultures. A girl’s first menstruation (menarche) is often marked with ritual and celebration, and a change in social status typically occurs with this biological transition. Likewise, menopause is often associated with a rise in status for women in non-Western societies, whereas it’s commonly seen as a negative transition for women in many Western societies. As we shall see, collective and individual attitudes toward these life cycle transitions have an effect on growth, development, and health.