Konner then explores the genetic and neurological foundations of basic temperament and gender identity and of formative behaviors such as infant attachment and the acquisition of language, and he describes the interrelationship between the biology and psychology of puberty. Unlike animals that hurtle from infancy to puberty, the humans who have escaped the risks of infancy but not yet embarked on the risks of adulthood experience a sort of mini-transformation during the “five-to-seven shift,” and emerge with markedly enhanced powers of cognition into a period of slow growth. This prolonged halcyon phase, sandwiched between the confusion of early life and the intensity of adolescence, seems evolutionarily designed to imbue children with the culture that our enormous brains make possible—the culture that our species (almost) alone can claim.

The sine qua non of culture is socialization, a process we share with many other species. For mammals, it begins with an extreme bond between mother and offspring—a bond that has existed since early in the age of the dinosaurs, when even the infants of egg-laying mammals could feed directly from their mothers’ bodies and demand attention by crying. (Mammalian young cried at high pitches that their mothers could hear but reptilian predators could not.) Although the mother-child bond forms the core relationship, we are cooperative breeders. There is “ample evidence,” developed most prominently by the pathbreaking anthropologist Sarah Blaffer Hrdy, that “human mothers have always gotten help” from fathers, grandmothers, older siblings, and other relatives. Still, some evidence suggests that kinship is not the be-all and end-all it is often believed to be. Research on the !Kung hunter-gatherer society, for example, shows no particular advantage to having a full complement of parents and grandparents, and in cases in which children have few kin, other adults apparently take up the slack, supporting the idea that indeed, it takes a village. Crucially, the many years that human females live after menopause confer a unique advantage on the species, in that grandmothers are almost always involved in child care, allowing their children, particularly their daughters, to produce more and healthier children.

Konner is especially interested in play, which is not unique to humans and, indeed, seems to have been present, like the mother-offspring bond, from the dawn of mammals. The smartest mammals are the most playful, so these traits have apparently evolved together. Play, Konner says, “combining as it does great energy expenditure and risk with apparent pointlessness, is a central paradox of evolutionary biology.” It seems to have multiple functions—exercise, learning, sharpening skills—and the positive emotions it invokes may be an adaptation that encourages us to try new things and learn with more flexibility. In fact, it may be the primary means nature has found to develop our brains.