We take it for granted, but a long childhood is what makes us Earth’s most complex animal | Brenna Hassett

Children spend the same amount of time growing up as bowhead whales – yet they live for hundreds of years

One of the things that makes Homo sapiens so unique as a species seems so mundane, so everyday, that we rarely stop to question it. But seen from the perspective of every other animal on the planet, our long childhood is an extreme outlier. We remain children longer than any of them. To put us in perspective, we spend about the same time growing up as bowhead whales – perhaps 25-odd years. However, bowhead whales are many times our size and can live for hundreds of years; we’re not taking between five and 10% of our lives out to be children, but almost a quarter.

Over time, our species has evolved to move the markers of what biologists call “life history” – milestones like birth, growth, maturity, death – into a radically different arrangement to other species. We do not live for ever, but comparatively we are for ever young. So much about our bodies, minds and the way we build our social and physical worlds is arranged to accommodate this long, bright teatime of growing up. And if we trace the evolutionary choices our species has made, we can see we have repeatedly chosen to invest in the slow growth of the next generation in ways no other animal has managed.

Our strange childhood begins at the beginning: it has made the way we mate and bond strange, too. For about 90% of animals (almost all except birds, in fact) monogamy has been dismissed as unworkable. But our monogamous ways have given us genetic co-investors (you might also know them as “dads”) who in pair-bonded species are often important long-term providers. This means less competition for mates, something we can see written right into our bodies, as males and females in our species are a similar size – we’re no silverbacks or giant-fanged baboons fighting for short-term partners.

It’s even evident in our genitals: primates who engage in competitive mating have genital flourishes like very large testes or additional penis length, bones or even spikes. Humans are remarkably unremarkable in this area – if we were mouse lemurs, for instance, we would have testes the size of grapefruits to produce competitive amounts of sperm needed to win the mating game.

We pour so much physical investment into our babies that they nearly kill us; if not on the inside, then definitely in the process of coming out. Evolutionary theory used to hold that the reason human birth is a big risk is because of an inherent conflict between our upright-walking pelvis and a big-brained baby, but we have now studied enough other primates to see that they do things similarly, without dying in childbirth so often. Instead, it may be the exceptional amount of resources we invest in our babies in utero that causes the problem in the first place. Human babies are born fat – ours are 15% lard, compared to the sleek 3% of a chimpanzee – and big-brained, which takes a ton of resources. Killer pregnancy conditions like pre-eclampsia and gestational diabetes may result from our reproductive system’s uniquely baby-led pregnancy that can prioritise the baby at the expense of the mother.

Once our babies are born, we take them off the breast far sooner than any of our primate relatives. We don’t physically mature until years after other great apes – even ones bigger than us like gorillas – but we breastfeed for just two to four years, which is nothing compared with the seven to eight years an orangutan breastfeeds. This seems to be because we want to spend longer in the later stages of childhood. Rather than being infants, we take our time in the part of childhood where we are surrounded by friends, family and our societies. We take our time when the largest number of people can possibly be talked into investing in us – and doing the most important thing a child can do: learning.

We primates have to invest not just in physical growth but in building the relationships and skills that allow us to survive. Here is the secret of childhood: we need all this extra time to learn to be a better monkey. Female chimps learn how to make galago-stabbing spears from their mothers (male chimps don’t bother), and bored Japanese macaques learn how to hot-tub. And us? Well, we have a lot to learn too.

We are taking that time to invest in our children, building them up physically but also teaching them the skills they need to survive. And our children demand so much during this period that we have created a parenting ally no other species has managed: grandmothers. In every other animal – bar a few whales – females have reproductive cycles throughout their entire lives. Us? We call time halfway through women’s lives, giving us an adult with no dependent offspring of her own to invest in bringing the next – very needy – generation along.

This brings us to the final question. We can see that we have evolved to stay younger, longer; that we have found ways to invest in our children no other animal has ever dreamed of. But where do we go from here? For some lucky children, the answer is more of what childhood has always demanded: more learning, more training, more time to be a child.

Throughout human history, we see that parents have found new ways to invest in children, giving them longer time as dependants; from an Akkadian boy sent to scribe school 4,000 years ago, to the kids getting an internship financial boost from the bank of Mum and Dad today. How long we let kids be kids for is a decision for society. For my grandma, school until 16 was considered an indulgence; two generations of investment along, and I was still doing a degree at 30. In Afghanistan, girl children are out at 11, and not so long ago we told the children of miners and farmers that 12 was good enough; now we know they can do just as well with the extended training that used to be the privilege of the rich.

The real question for this forever-childhood our species has spent millions of years developing is not so much why we want it, but who we let benefit from the remarkable evolution of human investment.

  • Brenna Hassett, PhD, is a bioarchaeologist and author of Built on Bones: 15,000 Years of Urban Life and Death and Growing Up Human: The Evolution of Childhood

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Brenna Hassett

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