As a grandfather, I am well aware that the conventional wisdom is that I have outlived my usefulness as far as evolutionary theory goes. Once you have had offspring and raised them to an age where there are independent and capable to having offspring of their own, you have pretty much exhausted your biological usefulness. This leads to one speculation as to why our bodies, after a certain age, tend to fall apart. It is because there is no selection pressure to develop mechanisms keep it going.
But the fact remains that people do live longer than is strictly necessary for evolution to function and this article argues that older people can still serve an evolutionary purpose.
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.
Childhood seems to be designed to enhance learning – so extending that period would be a good strategy for a species that needs to learn more.
Like the research on the young capuchins, studies from my lab and others suggest that young humans are especially motivated to explore their environment – or, what we tend to call ‘getting into everything’. When babies play with things, they do it in ways that seem designed to give them the maximum amount of information about how those things work. Young animals, and especially human children, are also notably impulsive, random and risk-taking.
To exercise these impressive early learning abilities, you need elders who can protect and nurture you. Curious children depend on caring adults. So, the other part of the life-history change was the evolution of more caregiving, especially including the caregiving that elders can provide.
When young animals, including humans, detect that they are cared for, they take their time growing up, and invest in large brains and the learning that goes with them. Indications that care is in short supply might lead to a different ‘live fast, die young’ pattern of development, one that is less intelligent but requires less caregiving and is better adapted to a harsh environment.
Just as the impulsiveness, curiosity and noise of children might contribute to exploration and compensate for their other inabilities, the older humans’ expertise, patience and storytelling skills might compensate for loss of speed and strength. Several studies suggest that we get happier or at least more content in our 50s, and stay that way as long as we remain healthy. Losing the single-minded drive of our middle years might contribute to this happiness, and actually make us better suited to the role of carers and teachers, guardians of tradition and bearers of wisdom.
It seems that all these life-history developments interacted to create the coevolutionary cascade that led to the remarkably swift emergence of Homo sapiens. A longer, smarter, more social childhood, as well as an extended old age, lets you develop more skilled adults. In turn, these adults can produce more calories and afford more care and cooperation, and so allow for an even longer, smarter and more social childhood in the next generation.
One always has to be a little careful about evolutionary explanations for things. If not backed up good evidence, they can come across as glib and superficial.
Still, it is good to feel that we older people may not be that useless after all.