As mentioned in the textbook (Gilbert, 2003; p. 408), the human brain grows at fetal rates even after birth. One of the more interesting speculations concerning the consequences of this retained rapid growth rate is that this extrauterine fetal growth rate necessitated the invention of childhood. There are often said to be five recognizable stages of human growth and development: infancy, childhood, juvenility, adolescence, and adulthood. Each stage involves behavior patterns characteristic of that stage as well as preparations for entering the next stage. In childhood, each person learns cultural norms and begins to reflect them, as well. It is a time where the personality begins to become integrated in a reciprocal relationship to society.
Bogin (1997) has suggested that childhood is a new stage of the vertebrate life cycle, interposed between the infant and juvenile stages. Infancy ends with weaning, usually around year 3. The juvenile stage is characterized by feeding independence from adults and the onset of physical maturity. In humans, this usually begins around year 7. Between them, in humans, is a period of childhood characterized by immature dentition, small digestive system, and a rapidly growing brain that demands a high caloric diet. This is a period of time when the human has to be cared for and fed by adults. During this time, the brain is developing faster than any other portion of the body (Figure 1), and is developing much faster than the digestive system required to feed it. The enormous caloric requirements of the developing brain would mandate a prolonged stage where the infant would have to be fed by adults.
The childhood period would allow the brain to develop in an enriched environment. As Childs (1999) has concluded:
Extended exposure of a gradually maturing nervous system to experiences of a variable environment, together with the mental resiliency to continue to learn at all ages, is a recipe for the adaptive agility that has enabled human beings to live in all latitudes and so to exploit the earth's resources to construct civilizations and to be aesthetically creative.
But Bogin argues that this is actually a byproduct of childhood and not its intention. The selective value of childhood would be to improve the success rates of each child surviving to maturity. This would explain why humans have lengthy development and low fertility but yet the greatest reproductive success of any ape species.
Bogin, B. 1997. Evolutionary hypotheses for human childhood. Yrbk. Physic. Anthropol. 40: 63 - 89.
Childs, B. 1999. Genetic Medicine. Johns Hopkins University Press, Baltimore.
Gilbert, S. F. 2003. Developmental Biology, seventh ed. Sinauer Associates, Sunderland, MA