Human beings need special care while we are young and when we become old. The 2020 pandemic has made this vivid: millions of people across the world have taken care of children at home, and millions more have tried to care for grandparents, even when they couldn’t be physically close to them. COVID-19 has reminded us how much we need to take care of the young and the old. But it’s also reminded us how much we care for and about them, and how important the relations between the generations are. I have missed restaurants and theatres and haircuts, but I would easily give them all up to be able to hug my grandchildren without fear. And there is something remarkably moving about the way that young people transformed their lives to protect older ones.
But this raises a puzzling scientific paradox. We know that biological creatures are shaped by the forces of evolution, which selects organisms based on their fitness – that is, their ability to survive and reproduce in a particular environment. So why has it allowed us to be so vulnerable and helpless for long stretches of our lives? Why do the strong, able humans in their prime of life put so much time and energy into caring for those who are not yet, or no longer, so productive? New research argues that those vulnerabilities are intimately related to some of our greatest human strengths – our capacities for learning, cooperation and culture.
‘Life history’ is the biologist’s term for an animal’s developmental course: how long it takes to mature, how many babies it has and how it takes care of them, and how long it lives overall. A striking discovery in evolutionary biology is that natural selection often operates not just on an organism’s traits as an adult, but on its whole life history. In many cases, a small genetic tweak to the developmental programme of an animal can lead to large and important changes in the way it adapts to the environment.
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.
One idea is that our long, protected human childhood gives us a chance to learn about the varied environments that we might face when we grow up, and so to develop the skills we’ll need to thrive as adults. Human beings can survive and flourish in an exceptionally wide range of places, even including outer space, and we constantly alter and create new environments ourselves. Almost nothing in the room in which I’m writing this even existed in the Pleistocene, yet I’m perfectly at home in it. While chimpanzees and gorillas still live in the same places basically where they evolved, humans quickly dispersed. We move around a lot and always have.
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. Young animals in general, and smart young primates in particular, are especially motivated to learn about new things. Susan Perry, an evolutionary anthropologist at the University of California, Los Angeles, and her colleagues have been carefully tracking a troupe of capuchin monkeys in Costa Rica for 30 years, watching the young monkeys as they grow. They have found that the young are especially exploratory, curious and innovative. They often try new things, even if they’re not very effective or sensible – such as draping a plant over one ear or rolling a new fruit around on the ground instead of eating it. The younger monkeys are also more ‘neophilic’ than older ones, in that they seek out new things and experiences, instead of ‘neophobically’ avoiding them.
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. That might be a bug if you want a system that will exploit effectively, that is, act in an efficient, focused way to achieve its objectives. But it’s probably a benefit from the perspective of learning.
In fact, both neuroscience and computer science suggest that there’s an intrinsic trade-off between exploration and exploitation. Grown-up ‘executive function’ abilities such as inhibition, long-term planning and focus are great for exploitation, but might make you less likely to explore broadly. Computer scientists suggest that the best way to resolve this trade-off is to explore first and exploit later. Childhood in general, and human childhood in particular, seem to be evolution’s way of implementing that strategy. Rather than approximating the sophisticated ways that adults acquire knowledge, new studies show that children can actually be better at exploring and learning than adults.
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.
This relationship between intelligence, learning and caring exists in other animals too. The cognitive scientist Natalie Uomini at the Max Planck Institute in Germany and her colleagues studied the life history of some extremely smart birds, Siberian jays and New Caledonian crows. Although these birds evolved very differently from humans, they also have remarkable tool-use and learning abilities, and many neurons, a mark of a highly developed brain. And, like humans, crow and jay fledglings stay in the nest for a long time. Uomini and her colleagues found that the jays who get to live with their parents for longer become smarter and do better in the long run. The crow parents don’t explicitly teach their young, but they do tolerate the young birds, feed them and give them opportunities to learn by leaving sticks around for them to play with. (So it might not be a bad idea for folks working at home to emulate the crows and just shoo the kids out into the backyard with snacks and sticks for a while.)
A long childhood allows exploration, while a wider set of carers allows that childhood to unfold
Other studies affirm the intimate links between care, intelligence and learning. The evolutionary biologist Emilie Snell-Rood and the medical anthropologist Claire Snell-Rood argue that young creatures, human as well as animal, are themselves sensitive to cues that indicate how much care they will get, and their development can actually change as a result. 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.
For humans, the elders at the other end of the lifespan appear to be a particularly important source of care, and might have played a crucial role in human evolution. The anthropologist Kristen Hawkes has called this ‘the grandmother hypothesis’, and has shown that, in forager cultures, post-menopausal grandmothers are a crucial resource, especially for toddlers. Since human mothers have babies at relatively short intervals, a mum might be nursing a new infant even while the older sibling still needs lots of attention – grandmothers can take over after a nursing infant becomes an equally vulnerable but even more demanding toddler.
It appears that there’s a kind of symbiosis in the human life history: a long childhood allows exploration and learning, while a wider set of carers, including elders, allows that childhood to unfold. Yet these many and varied human carers need to share responsibility and work together to raise those helpless children, and this is a challenge as well as a benefit. Hawkes also suggests – along with Sarah Hrdy at the University of California, Davis, Judith Burkart at the University of Zurich and Michael Tomasello at Duke University in North Carolina – that human social interaction, communication and cooperation might be rooted in how our life history unfolds, too.
Traditionally, anthropologists argued that humans cooperated in order to hunt more effectively. But recent studies of forager cultures suggest that the benefits of hunting might be exaggerated – actually the grandmothers quietly digging up roots and tubers potentially provided many more calories than those mighty hunters. In fact, ‘cooperative breeding’ (as it is unromantically called) might require even more sophisticated coordination and cooperation than hunting – and it has even more obvious evolutionary benefits. In forager cultures, ‘alloparents’ often swap babysitting and even breastfeeding duties. (The experience of lockdown offered another reminder that coordinating food is a cinch compared with coordinating childcare.)
Multiple caregivers are challenging for babies, too. Your biological mum might be primed to care for you, but how do fathers, grandparents and alloparents get on board? Before they are a year old, human babies are already very socially adept. They make eye contact, point to get other people to pay attention to objects, imitate the actions of others, and generally engage in what’s called ‘joint attention’. All this baby charm helps to inveigle the adults into taking care of them. But once it’s in place, it might allow for other kinds of social interaction and cooperation. The social skills that help babies get cared for have obvious evolutionary advantages, but they can also then underpin our distinctive grown-up human cooperation. The same social sensitivity that ingratiates us with potential caregivers can later ingratiate us with bosses or colleagues.
So, childhood learning, exploration, creativity and social skill depend on the fact that we have adult carers around, including grandparents. How about the other end of the story – do older people, in particular, contribute to our distinctively human intelligence beyond just taking care of the kids?
Both childhood and old age appear to be critical periods in the life history of our species when it comes to the transmission of human culture – another thing that is distinctive, if not unique, about human beings. More than any other animal, we pass on information and knowledge from one generation to another. This process has led to the enormous changes in the way we live now compared with the Pleistocene. Older people might have a special place in that process. During human evolution, highly skilled, root-digging grandmothers helped to provide calories for those hungry young toddler brains – but they fed those brains important information, too.
Some of the most striking evidence in support of this idea comes from the orcas. Orcas are among the only other mammals with post-menopausal grandmothers. Orca children and grandchildren stay close to the older females, even after they mature themselves. But they’re also among the few species that have clear cultural traditions, passed on from grandmother to grandchild. As with human grandmothers, those traditions involve food – though the orcas pass on preferences for shrimp or seals, instead of latkes or spaghetti. In particular, different orca groups eat different kinds of food, some go for krill and others for fish. Recent work shows that those food traditions seem to be passed on from the elders. In fact, the grandmothers will actually lead the pod as they hunt for food, and young orcas do much better when they have a living grandmother, especially when food is scarce. Like human grandmothers, the orcas help the helpless young to survive.
The anthropologist Michael Gurven at the University of California, Santa Barbara and his colleagues have also argued that elders play a special teaching role in the context of human evolution. They combined mathematical modelling and cross-cultural observation to relate life history, culture and teaching. For humans to thrive, they must master complex skills, such as foraging, hunting, cooking, child-rearing and tool-making. Many of these skills require years of practice; typically, hunters don’t reach their peak until they are in their mid-30s at least. To learn a complex skill, you also need patient teachers who can pass on their accumulated wisdom and technique.
In some ways, we are at our most human before puberty and after menopause
But there’s a catch, vividly illustrated in the work-at-home world. It’s hard to simultaneously teach someone else to do something, and to do it effectively yourself. (Sunday pancakes take twice as long when the kids help.) Gurven and his colleagues found that, mathematically, the best evolutionary strategy was to have the old teach the young. Let the peak, prime-of-life performers concentrate on getting things done, and match the younger learners with older, more knowledgeable, but less productive teachers. They analysed more than 20,000 observations from more than 40 different locations around the world, and found that this was the precise pattern in many different contemporary hunting cultures. The grandparents, in their 50s or 60s, weren’t as strong or effective hunters as the 30-year-olds, but they were more likely to be teachers.
From this perspective, too, bugs can be features. 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.
So, childhood and old age – those vulnerable, unproductive periods of our lives – turn out, biologically, to be the key to many of our most valuable, deeply human capacities. They nurture and facilitate our exploration and creativity, cooperation, coordination and culture, learning and teaching. In some ways, we are at our most human before puberty and after menopause. In those times of our lives, we have the luxury of focusing on learning and teaching, instead of the four Fs (feeding, fighting, fleeing and … reproducing) that occupy most other adult primates (and are, understandably, so important to humans in our middle years).
Caring for those vulnerable humans at either end of life lets all of us flourish. The COVID-19 crisis has made us realise the importance and difficulty of this kind of care. But it should also make us wake up to the fact that we weren’t doing a very good job of taking care of the young and the old, even before the virus – and even (or especially) in the richest countries on Earth. Childcare and eldercare workers have little pay and less status, even if we are beginning to see how ‘essential’ they really are – as evolutionary biologists could have told us. Care is economically invisible, and childcare centres and nursing homes are an underfunded patchwork in most developed societies. Worse, we isolate children and older people from each other, and from the rest of us. Perhaps, in the aftertime of the virus, we can begin to appreciate the young, brilliant and fragile human learners, as well as their wise, vulnerable, older human teachers – and genuinely bring the grandchildren and grandparents back together.