The dance of love and lore between grandparent and grandchild is at the center, not the fringes, of our evolutionary story.
On an evolutionary timescale, Homo sapiens emerged only quite recently. Yet in that short time, we have evolved a particularly weird life history, with a much longer childhood and old age than other animals. In particular, we’re very different from our closest primate relatives. By at least age seven, chimpanzees provide as much food as they consume, and they rarely live past 50 – there’s no chimp equivalent of human menopause. Even in forager cultures, where growing up is accelerated, children aren’t self-sufficient until they’re at least 15. What’s more, even in communities without access to modern medicine, if you make it past childhood you might well live into your 70s. We live some 20 years longer than chimpanzees and, except for a few whale species, particularly orcas, we are the only mammals who systematically outlive our fertility.
The extended childhood is especially puzzling because, as parents know, children are expensive, and that was true long before college tuition and summer camp. Adults have always had to feed and protect the young, and early human brain development uses up a tremendous amount of energy – more than 60 per cent of four-year-olds’ calories go to the brain at rest, compared with around 20 per cent for adults. Humans also have babies every couple of years, much more frequently than chimps, so they stack up even more of those helpless, hungry-brained children.
Chimpanzee mothers do almost all the childrearing. But humans evolved exceptionally extended and varied sources of caregiving to deal with their costly babies, including fathers who take care of the kids, post-menopausal grandmothers, and ‘alloparents’ – other people who help to raise children. Prairie-vole dads, orca-whale grandmothers and rhesus-monkey alloparents also help to raise babies, but these kinds of care are rare among mammals. No other species except humans appears to have all three kinds of care.
These changes in life history evolved at the same time as dramatic changes in human brains and minds. We have many more neurons than other primates. And we developed striking abilities to learn and invent, communicate and cooperate, and create and transmit culture. New analyses of fossil records show that humans evolved their large brains and distinctive capacities in parallel with their longer childhood and old age. Our unique human vulnerabilities somehow emerged in concert with our unique human strengths. Just how are these two kinds of changes related? Researchers from biology, psychology and anthropology have recently begun to work together in order to answer these questions – answers that help to explain what makes us distinctively human.