For most animals, the juvenile, or childhood, stage ends when adulthood begins. For humans and apes and possibly some monkeys, there’s an additional life cycle stage called adolescence. This is a period of extremely rapid growth in humans (the “adolescent growth spurt”) that is not seen in most other primates (Bogin, 1999; 2010). A number of biological events mark the transition to adolescence for both males and females. These include increase in body size, change in body shape, and the development of the testes and penis in boys and breasts in girls. Hormonal changes are the driving forces behind all these physical alterations, especially increased testosterone production in boys and estrogen production in girls. As already noted, menarche is a clear sign of puberty in girls and is usually the marker of this transition in cultures where the event is ritually celebrated.
A number of factors affect the onset of puberty in humans, including genetics, gestational experience, nutrition, disease, activity levels, and stress. In humans and other primates, females reach sexual maturity before males do. An illustration of the “mismatch” effect of diet and other lifestyle factors on puberty is seen in the trend toward a lower age of menarche that has been noted in human populations over the last century (Fig. 16-15) and the tendency for girls who are very active and thin to mature later than those who are heavier and less active.
Socioeconomic factors are also implicated in this trend: In less industrialized nations, girls from higher social classes tend to mature earlier than girls from lower social classes. In general, physical development has accelerated in the past several decades, along with worldwide improvements in public health and nutrition. Although we have emphasized the gradual decline in the age of maturity observed in the last century, there’s a great range of variation within every population. An important lesson from life history theory is that maturation is sensitive to local environmental situations, including diet, health care, and parental care practices. Until the advent of settled living, it’s likely that females became pregnant as soon as they were biologically able to do so, that is, as soon as they had finished growing. This would have been advantageous under conditions when individual life expectancy would have been low. Paleodemographic studies indicate a mortality rate of at least 50 percent in subadults in preindustrial populations; and of the half that survived to adulthood, most did not survive to age 50. Considering the reality of short life spans combined with the long period of infant dependency, producing offspring as early as possible may have contributed to the reproductive success of females, particularly early hominin females. By giving birth as soon as she reached sexual maturity, an early hominin female enhanced her chances of rearing at least one offspring to the point it could survive without her.