Evolutionary hypotheses for human childhood

Abstract

The origins of human childhood have fascinated scholars from many disciplines. Some researchers argue that childhood, and many other human characteristics, evolved by heterochrony, an evolutionary process that alters the timing of growth stages from ancestors to their descendants. Other scholars argue against heterochrony, but so far have not offered a well-developed alternative hypothesis. This essay presents such an alternative. Childhood is defined as a unique developmental stage of humans. Childhood is the period following infancy, when the youngster is weaned from nursing but still depends on older people for feeding and protection. The biological constraints of childhood, which include an immature dentition, a small digestive system, and a calorie-demanding brain that is both relatively large and growing rapidly, necessitate the care and feeding that older individuals must provide. Evidence is presented that childhood evolved as a new stage hominid life history, first appearing, perhaps, during the time of Homo habilis. The value of childhood is often ascribed to learning many aspects of human culture. It is certainly true that childhood provides ''extra'' time for brain development and learning. However, the initial selective value of childhood may be more closely related to parental strategies to increase reproductive success. Childhood allows a woman to give birth to new offspring and provide care for existing dependent young. Understanding the nature of childhood helps to explain why humans have lengthy development and low fertility, but greater reproductive success than any other species.

Extension of cortical synaptic development distinguishes humans from chimpanzees and macaques

“…It has been proposed that this ontogenic extension/inclusion allows a longer period of interaction between human infants and their social environment and forms the main basis of human intelligence (Gould 1977;Johnson 2001;Langer 2006). Notably, 5-7 yr of age also appears to correspond to a shift in cognitive maturity, such as increased self-regulation (Bogin 1997). Our data suggest that the timing of synaptic development in the human PFC has undergone a parallel extension.…”

“…Humans are born in an altricial state and undergo both rapid and extended brain growth compared with that of macaques and chimpanzees (Leigh 2004). A related view is that human ontogeny has acquired a novel phase, between 3 and 7 yr of age, when human infants are not being breast-fed but are still highly dependent on adults (Bogin 1997). It has been proposed that this ontogenic extension/inclusion allows a longer period of interaction between human infants and their social environment and forms the main basis of human intelligence (Gould 1977;Johnson 2001;Langer 2006).…”

Evolution of Human Parental Behavior and the Human Family

“…To be sure, there are thoughtful evolutionary analyses of the human family and the dynamics of human parenting (e.g., Belsky et al, 1991;Draper & Harpending, 1988;Hewlett, 1992;Hewlett et al, 2000), but, with the exception of Alexander (1990;, these have not been considered in terms of their comparative uniqueness or in terms of the suite of the other unusual features of H. sapiens (see Table 1). Among these unusual features are male parenting -humans are the only species of terrestrial primate that lives in multimale, multifemale groups with fathers -and a development period that is considerably longer than would be expected based on life history parameters of other mammals and primates (Bogin, 1997;Geary, 2000;Hill & Hurtado, 1996;Hill & Kaplan, 1999;Lancaster & Lancaster, 1987). Accompanying the long developmental period, of course, is extended and oftentimes extensive levels of PI, although in many preindustrial societies much of this investment is provided by the extended kin network, not simply the biological parents (Pasternak et al, 1997).…”

“…(3) Concealed ovulation and continuous, nonreproductive sexual activity (Alexander, 1990). (4) Children have a very long developmental period, relative to other comparably sized mammals and primates, and are highly dependent on adult caregiving (Bogin, 1997;McHenry, 1994a). (5) Menopause (Alexander, 1990).…”

Anterior tooth growth periods in Neandertals were comparable to those of modern humans

“…Our extended growth periods appear to result from investment in rapid postnatal brain growth at the expense of somatic growth (1,2,4,5) and the time required for extensive learning before attaining reproductive age (3,6,7). Large brains allow complex behavior, the selective advantages of which accrue over the extended human lifespan (8).…”