Infancy and childhood: Dependence, play, learning
Humans are no exception to biological evolution. In humans, part of what evolved during evolution were new life history stages of growth and development that allowed for greater flexibility in reproductive strategies than those of other species, living or extinct. No living species of mammal, including the nonhuman primates, has all the characteristics of human growth, development, and maturation. In common with all mammals, humans share the «infancy» stage of life history, the period of feeding by lactation, and the «adulthood» stage, the period of reproduction. In common with the social mammals, such as most primates, elephants, and some carnivores (wolves, lions, hyenas), humans share the “juvenile” stage of development, characterized by feeding independence but reproductive immaturity. In contrast to all other living species, humans have two unusual periods of development, childhood and adolescence.
«Childhood» is defined as the period of time after infancy, from about the age of 3.0 to 6.9 years, when infants are «weaned», that is, infant feeding by lactation has ceased, but the youngster is still dependent on older people for care and food provisioning. In other species of mammals, including monkeys and apes, after individuals are weaned they have to get their own food. Children also have a moderate growth rate of about 6 cm per year. The childhood stage ends with the eruption of the first permanent molar and incisor teeth and the virtual completion of brain growth in total weight. The evolution of childhood may explain, in part, the human-specific traits of relatively early weaning, prolonged growth and development between birth and adulthood, complex kinship systems, language, marriage, intergenerational transfers of wealth and political power, relatively slow aging, as well as senescence and greater longevity. Each of these characteristics of modern humans evolved, mostly likely, at different times since the split between ape and human ancestors occurred about 6–7 million years ago.
This is the concept of «mosaic evolution» which was originally proposed in 1954 by Gavin de Beer in his analysis of the birdlike dinosaur Archaeopteryx and later applied to the evolution of Australopithecus, in 1975. Childhood is, in essence, the first piece of the mosaic of human life history evolution, including the evolution of human culture. [Barry Bogin]