Sexual reproduction and time for growth allow new organisms to develop differences from their parents—the offspring may display plasticity. The term plasticity means a potential for change in the phenotype of the individual caused by a change in the environment (MascieTaylor & Bogin, 1995; Stearns, 1992). The fitness of a given phenotype varies across an environment's range of variation. When phenotypes are fixed early in development, such as in mammals that mature sexually soon after infancy, environmental change is positively correlated with high mortality. Examples are mice, rats, and other rodents, which may mature only a few weeks after birth. However, fewer than one out of a thousand newborns survive to maturity.
The process of growth and development is arduous, often prolonged, and generally hazardous. The high rates of death for sex cells, fertilized eggs, embryos, fetuses, newborns, and young attest to these hazards. The pattern of human growth is one of the most prolonged of any mammals and this entails many hazards. In traditional societies of both historic and prehistoric eras, including hunter-gatherers and horticulturists, about 35% of liveborn humans die by age seven years, and less than 60% reach adulthood (Bogin, 1999; Lancaster & Lancaster, 1983). Despite the risks for death, more human beings reach adulthood than any other animal species. The next most successful species after humans is the chimpanzee, but only 35% of live-born chimpanzees reach adulthood. Part of the success of humans is due to the prolonged process of human growth, which allows for greater plasticity in development.
Social mammals such as some carnivores, elephants, and primates, prolong the developmental period by adding a juvenile stage (a period of feeding independence prior to sexual maturation) between infancy and adulthood. Adult phenotypes develop more slowly in these mammals. They experience a wider range of environmental variation, and the result is a better conformation between the individual and the environment. This plasticity leads to an increase in evolutionary fitness, meaning that more offspring can survive to reproductive age. In large mammalian species without a juvenile stage, less than 10% of live-born offspring survive to reproductive age, while between 12% and 30% survive in the social mammals with a juvenile growth stage (Pereira & Fairbanks, 1993).
Human beings have a juvenile stage and add a childhood stage of growth between infancy and the juvenile period. Evidence for the evolution of the childhood growth stage is provided in Bogin (1999). Childhood allows for an additional four years of relatively slow physical growth and behavioral experiences that further enhance developmental plasticity.
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