Instant Genius - The evolution of human childhood, with Dr Brenna Hassett

Episode Date: July 17, 2022

Anthropologist Brenna Hassett, author of Growing Up Human, explains why our super-long childhood is so weird compared to the rest of the animal kingdom. Once you’ve mastered the basics with Instant Genius, dive deeper with Instant Genius Extra, where you’ll find longer, richer discussions about the most exciting ideas in the world of science and technology. Only available on Apple Podcasts. Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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So you can experience exceptional sound at home. Music just as the artist intended. Visit name audio.com to learn more. From BBC Science Focus magazine, this is Instant Genius, a bike-sized masterclass in podcast form. I'm Sarah Rigby, online staff writer at sciencefocus.com. I'm talking to Dr Brenner Hassett, an anthropologist. psychologist and archaeologist at University College London, and author of Growing Up Human. In this episode, she explains just how weird human childhood is compared to the animal kingdom.

So just to start, could you please give us a brief explanation of what your book is about? So I have written a book, Growing Up Human, the Evolution of Childhood, which is very much about the evolution of an incredibly strange species that has done some incredibly strange things, all of which are evolutionary adaptations, choices that our ancestors have made down the line. And these choices have led to us having an extraordinary long time spent as children. So let's start just before the story of childhood. Let's start with how we choose our partners in the first place. So we often say that humans are monogamous, but is that really the case?

The way I like to think about childhood is that essentially childhood is that essentially childhood is a period when you're investing in the next generation. And the whole book is essentially a story of how we make that investment and how that investment sort of gets handed down. What humans have done is basically given ourselves lots of extra time to make lots of extra investment. And that actually starts even before you get a baby, which is remarkably strange. So one of the things that humans appear to have done is gone for a style of mate choice that almost no other animal does. So it's something like 90% of the animal kingdom have looked at monogamy or peer bonding, you know, one partner for the rest of life, and gone,

no, that's not for us, thanks, absolutely not, except for birds, birds are very, very into monogamy. And it's only about 5% of mammals have gone, yes, we will definitely settle with the one. Primates, however, have gone over the odds, about 15% of them chosen what turns out to be an incredibly rare form of mate choice. And humans, despite what you may think from reality television or tabloid newspapers, are actually pretty committed to peribon. So we count as a monogamous species, even though monogamy may not always be what you think it is. I think there's a little bit in the book about extra pair paternity, which actually humans are much better at than something as sneaky as like a mouse lemur. So, you know, important to remember. But,

But monogamy is essentially something that seems to have gotten us help down the line. Having a pair in primate species seems to be a really important part of how you get an extra investor. Even before sort of you have a baby to invest in, one of the things that seems to happen in primates who have pairs is they have dads. They have dads that, for instance, carry the baby. some of them for years. Marmosets, very good at dad carrying. This is like, you know, their number one contribution. And it's, it may be that that's actually a really important evolutionary contribution. So you said that in primates it's about 15% of species that are monogamous, whereas in birds it's about 90%. So why are we so different from so many of our closest relatives?

Well, that is a very good question. So I think one of the ideas is, is that monogamy has a couple different rationales. People used to say, well, if you have a pair, then the male of the pair, or the small gamete haver, if you will, if you think of egg and sperm, small and large gametes, that's how biologists like to refer to people. So the small gamete haver knows that the partner is, you know, that those kids are theirs,

and they won't be tempted towards infanticide. And so lots of people have theorized that we must have had lots of, like other primate species do. When a new male comes in, they actually kill all of the babies so that the females will go back to being ready to have more babies, which will be their genetic material. So people have theorized that that's in the human sort of ancestral condition. But we do see a lot of primates that don't have infanticide. The one problem with this theory is it really supposes that male monkeys know what paternity is. And that's kind of a big cognitive leap when you consider, again, thinking of reality TV, that not all people seem to understand this. So we're not too sure that the theory is accurate.

There are other theories, essentially, that the contribution to the next generation having that dad around for carrying, for contributing to infant care is really, really useful. And there's also ideas that females who roam, females who travel a lot, need to be pair bonded because otherwise the males can't find them. And nothing happens. So it could be a mix of all sorts of evolutionary prompts, but it does seem pretty certain that that is what we've ended up with. And so when two humans are, you know, monogamously together, usually, and then they're getting ready to have a baby, humans, we can get pregnant all year round. Why don't, humans have a mating season like other species? Well, that's a really interesting question. So a lot of species opt to have their kids either once a year or, you know, if they're a fast reproducing species a couple times a year, when, you know, they know that food is around. So a lot of primates are seasonal breeders and will pick the time when their kids are most annoying slash dependent for when there's the most food. So fruit season, for instance, if you're a lemur in Madagascar,

then fruit season is short and special and you need to have your kid when there's the most fruit available. Humans actually, weirdly, are seasonal breeders. So for anyone who's ever looked around a classroom and wondered why they have to share their birthday with so many other children, there is actually a reason for that. There's a reason there are so many Leo's and Virgoes in the world. Humans have a little tiny residual seasonal breeding effect. Of course, we can control our foods support, you know, sources, far better than a lemur. But when it comes to the things, that signal seasonal changes like light, those change with latitude. And we can actually see that as latitude changes, we have a stronger and stronger effect of seasonal breeding. So the farther

north or the farther south you get, the bigger impact on whatever human breeding season. But of course, humans do something else besides stare at the sun and wait for seasonal signals to have babies. They have cultural practices. And that is one of the major drivers of all of our evolutionary things, is essentially that we have this extra tool to drive evolution. And in our case, the New Year's Christmas midwinter holiday season for humans seems quite frequently to trump any seasonal considerations. And in humans, it seems more so than any other species, really, we have a lot of difficulty with pregnancy and giving birth. So why do humans have such a hard time giving birth? That one is definitely a sort of evolutionary anthropology cliffhanger. So that's one of the

major questions. But I think people are still arguing about for a long time we decided we knew. For a long time, we decided that human birth was a problem because we had tried to do too many things evolutionarily. We tried to walk upright, which required a nice, stable pelvis. And that had to be a certain shape and size, but we also wanted big brains, which required that the baby that would come out of this pelvis also be a certain size, and that size is large. So when you look at the size of a baby head versus a mother pelvis, in most primates, there's plenty of room. Not so in humans. In humans, that's a very tight fit, and we have to actually perform a kind of, you know, like twist, like Tom Daly on a diving board twist just to get out alive.

And that's, you know, that's a lot to ask. So for a long time, we called this the obstetric dilemma. And we decided that this was just the price we paid for our lifestyle. However, it turns out we're not the only primates that make this twist. Now that we have much better sort of imaging capacity and we can actually see these things, especially in captive animals, chimpanzees do it too. chimpanzees have plenty of room to turn a baby and to get a baby on. However, they still turn. They still make this little twist. And so we're actually probably not looking at quite the same

restraining factors. And one of the issues might actually be how much we invest in kids when they're in utero. So the whole book is about investment. And one of the things we really, really do is we have to pump that little baby full of calories straight through. When we're pregnant, it's just an amazing calorie suck, not as much as you thought. You can't actually eat for two, apparently. That's folk wisdom, which is unfortunate. But you do actually need to put calories into the baby. And our reproductive system basically allows our babies to draw so much nutrition from us that it triggers problems that other animals don't even have. So things like preeclampsia, gestational diabetes. These are conditions that actually kill us. Other animals don't get them.

I think guinea pigs do and one or two dogs. But I mean, these are incredibly rare conditions that, you know, are not, they seem like an adaptive dead end. You know, reproduction is the key to the species reproducing. You don't really want to hit a door there. But apparently we think it's worth it because we want to invest in our babies so much that we grow them very, very big. And we have the capacity to basically make them hugely big, so much so that it is a problem when they're trying to come out. And then once our babies do come out, they are so dependent on their parents for so long. So why does it take them so long to be able to do anything for themselves? This is a sort of fascinating. So there are two types of animals that you can be,

sort of in an evolutionary terms, biological terms. One is. ultritional, which is the helpless, cute little think of a, you know, a kitten, boring with the eyes and ears shut, stumbling around. And one is precocial. So you could think of like a giraffe, which you know, pretty much ready to run in, you know, the first hours. Or something like a spider, little spiderling is basically just a mini spider ready to go seconds after hatching. And we are firmly in the altricial category. We are useless and we are more useless than, you know, even our other primate relatives. So other primates, they basically are usually born, even the ones who aren't born so ready. So a lot of the great apes aren't great shakes and walking or moving

around. But most of them can at least cling. And if you have ever held a newborn human child, they cannot cling. They cannot lift their neck. Their neck is a problem. You know, and we are, we are incredibly useless. And this may again be because we spend so much time, investing in utero, but we can't really get them to the size we want because we spend all of our time instead of making them growing them so that their muscles and everything are competent and their skeletons are a little bit more finished and everything's a little bit more done. We put our money down on really expensive brain tissue. So all those calories were pumpkin or kids, they go to building brains, not stronger babies.

And you said that human babies, they're rubbish at gripping things. Even though we don't cling to our mother's fur anymore, is that why babies have the instinct to grab stuff? I'm not actually sure. That's a fascinating question. I think there are a couple, there are a couple things that are left over in almost all mammals that we all have. And there's a couple reflexes that babies have that are just locked in motor-wise and they actually lose them after several months. So things like you've ever seen a small child lock their legs when the both of their feet are supported. You know, that's a very strange muscle response. But it's not because, you know, we were originally able to stand. It's just that's some great legacy we've got

left over. And so human childhood, well, we kind of, it depends on our cultures, but we often think of it as lasting until we're about 18 years old, which is such a long time, isn't it? Are there any other species that have their childhood as a longer fraction of their life than us? I think you'd be hard-pressed to find one, and I think that's one of the things that's so unique about humans is we have a childhood that's sort of similar in length to something like a bowhead whale. Bowhead whales, however, live several hundred years. We know this because someone actually found a bowhead whale in the 2000s that still had a harpoon from the 1800 stuck in it. It had obviously gotten attempted wailed or how you phrase that. And so it's very hard to study whales, especially an animal that outlived you. But I mean, the bowhead whales, it takes about 25 years for them to get to their full kind of bowhead whale competency.

And there's actually parts of a human skeleton that don't finish and form their final kind of shape until you're almost 30. most of your skeleton is sort of done ish around 20-ish later for boys, boys are always behind. But, I mean, we are basically living like we are a species that lives for hundreds of years, but we're not. We've just had this, we've decided to put all of our money down on this huge, long, reproductive investment phase. Right. And what parts of the human body is it that keep growing or keep changing until we're about 30? That is actually, that is the main. medial end of your clavicle. So your collar bones, where they meet your sort of chest plate bone, is actually your sternum. All of your bones have basically end caps. So they start off as like little

sticks, all the ones that sort of, and those little end caps don't really fuse on. So if you happen to be like me, an anthropologist, who's bread and butter is looking at the skeletons of very dead things, you get used to the idea that you can look at these bones and work out what stage of growth they're in just from the appearance of the actual bones. So obviously things like size, people might imagine, you could tell the bones of a kid from an adult. But especially for things like when we get to species where we don't know what size the adults would have been, those are the clues that we use in pale anthropology to actually reconstruct how something grows. Right. I see. Wow. Okay. And why do we lose our teeth?

I love this question. Someone actually, one of my students, about a billion years ago asked me this question. And it was such a perfect question because it's such a big answer and such a tiny one. So the tiny one is because we haven't got room for our full set of teeth when we're babies. We're too small, and our teeth wouldn't fit. So we have a small set and a big set.

And then the big set is a super fascinating answer is that basically mammals have two sets because we grow fantastically and we have these teeth that are incredible sort of evolutionary design to get us the nutrition we need when we need it. So your teeth actually start, they're growing from in utero. Even your adult teeth, the very tippy tops of your front teeth,

will have been growing while you were still a baby in the womb, which is very interesting through a dental anthropologist. But the reason why you actually have two sets of teeth fairly simplistically is that when you are small, you really cannot fit the full adult set that you need in order to get the nutrition to run a full-size human body. Now you probably can, given sort of instant noodles. So when we talk about raising children, we often kind of think of the kind of two-parent unit.

And this is very cultural, but we usually just talk about the children just being raised by those two parents. But then also we talk about the phrase, you know, it takes a village. Did we evolve to be raised by just two parents, two monogamous parents, or did we evolve to be raised by, you know, a large group of adults? I think that's a very interesting question. I think also there's something that we do when we ask whether we evolve to do something, which is to assume that there's sort of one correct way. And I think it's really important to notice that, you know, as modern people,

we have an idea of evolution which says, okay, well, evolution is pointing us towards something. And once we get to that something, it's perfect, it's done. That's the evolutionary solution. And of course that's not true because the only way for evolution to stop is if you're all dead. And that is not a great strategy. So obviously evolution has to change and it means that what we do evolutionarily has to change as well. So I know that people get very sort of bullied as parents about doing evolutionarily correct things, you know, whether they're feeding all natural foods or doing carrying or, you know, I don't think anyone needs to beat themselves up because the store is out of period.

mammoth. I think that's, you know, we can relax on that. So the question of the sort of family units that we're evolved to be in, I think, changes very much with our societies. What we can say, no matter who's doing the investing, is that we have evolved to get as many people as possible involved in the investing, whether that's males, females, aunties, uncles, village, whoever it is, human babies are out there. they're looking for investment. And one of the things we seem to do is to actually breastfeed slightly for less time proportionately than other great apes. So there's quite a lot of debate in sort of many modern parenting circles about what's an appropriate time to breastfeed.

We sort of think that two to four years is about the human maximum. That's the most we know of for most societies throughout history and probably prehistory as well. but something like an orangutan, seven or eight years. So even though we're ish the same size and almost live the same amount of time, we actually take that infancy period where we're just with mom. And we cut that a little bit short, but we still have this huge long childhood, which is a great time for us to make friends and influence people, to convince our large, friendly societies to invest in us as kids,

to learn, to play, and to, to, to, to, get all of the benefit in the book, I call it, social capital from our societies, because human babies, they need that investment. Once we've gone past kind of the childhood phase, we then go into teenage phase. What exactly is, is it? What does it mean to be a teenager? Is this something we've just kind of culturally invented? I think, I think this is, especially for people who are parents or were parents of teenagers. The question why teenagers exist is a fundamental one. But it's a really interesting, it's a really interesting topic. And I think would a lot of people be fascinated to know is that humans are not the only species that have teenagers.

So if you think of teenagers as, well, critters that are essentially, probably physically, almost physically mature, capable of reproducing, sort of almost an adult size, almost adult sort of hormones and things, but not socially mature. And if you, if you think about adolescence as this long, bright tea time of the soul, where, you know, the most important things for teenage animals are basically learning social skills and learning how to sort and sort of transmitting those social skills among their peers. So for instance, gorillas also have teenage groups. They have essentially boy bands. Gorillas have a very competitive social world. So, you know, You think of a silverback in his harem.

That's the sort of guerrilla social structure. And so for young gorilla males, if they stay around while they're sort of getting bigger, it can cause a lot of friction. It can cause competition. So sometimes these males, and maybe a handful of older males, were no longer sort of competing or have been out-competed, will form these sort of guerrilla boy bands from, you know, the guerrilla sort of teenage age of about, you know, 10 or something for five-ish years.

And they'll have this adolescence. And this is something that primates seem to spend this time in adolescence where they essentially navigate new social groups if they're moving. So in a lot of species, males, for instance, move groups or females might move groups. And they also spend a lot of time transmitting and learning new skills from their peers. So teenagers teach each other things and they take these things to new groups. And I think it's really interesting that humans seem to have not only given ourselves, you know, until age 18 or something, but for a lot of us who did not hit accepted adult milestones and lived fairly adolescent existences, you know, either doing advanced degrees, in my case,

or, you know, otherwise training or just not quite being able to contribute to the taxman and the way they wish to, you know, that this has now extended at least 10 years in some cases and maybe far beyond. I'm sure there are people out there who would have to say, you know what I'm 60 and I'm pretty sure I'm not grown up. Would you say that what it means to be an adolescent is changing over time? Is it getting longer or shorter? I think that, so this is a really interesting question.

And I think it's one that we can look at actually through even more recent examples and kind of human history. So we can look at how people have been adolescents. in sort of, you know, even in, you know, recent changing societies. And if we think that human evolution is, you know, set up to help us be whoever we need to be for the society that we live in, and it changes the way we play, it also changes the way that we essentially train and learn. And in societies at least sense, we've been able to separate the haves from the have-nots,

we have been able to see that some children get a much longer opportunity, opportunity to be in that state of being invested in rather than having to return that investment. And that's something that we can see all the way from wealthy Acadian parents sending their children to Samarian scribe school, 5,000, 4,000 years ago. So 4,000 years ago, you know, it's like sending your kid to learn Latin. It'll get you a good job. Not sure it will. It did 4,000 years ago. To, you know, we have evidence of, you know, from, again, I studied the bones and teeth from the past. So we have evidence of skeletons from places like Halstatt in Austria, where we can see, you know, children who died very young, whose bodies are very, very overworked

from having to work in salt mines 2,000 years ago. We have had, for some children, the opportunity to extend adolescence for a very long time, but we haven't given it to everyone. Just finally, what three things do you think we all should know about human childhood? Well, I think first, we should definitely know that is very, very long. And should you be one of my parents listening to this, it is totally acceptable if I need to come home and stay in the basement for a little while. On a very serious note, we really do have an incredible portion of time dedicated to investing in our children.

And that's something that's pretty astonishing. I think secondly, we should also note that we are some of the only species on the planet that actually finds another way to invest in our children, and we cut off reproduction for older women. Other species don't do this. Grandmas are totally suspicious, not even reasonable, no other species lets women sort of stop reproductive cycling. And we think that maybe one of the reasons that we have grandmas is essentially because grandma is able to invest two generations down. If she doesn't have any new children of her own, she's not worried about them. She's able to basically extend even more investment into our needy, needy babies.

And then three, the thing that I really hope people get interested in is, of course, the science of paleoanthropology and how we find this out. So we are able to do incredible things. People are sticking teeth and synchrotrons, people are chopping them up, and we can actually trace how we started to get these slow childhoods through different fossil species, from Australopithecus Africanus to homorectus to ourselves today. And I think that this is a fascinating science, and everyone should be paying attention to see basically what we find out next. Thank you for listening to this episode of Instant Genius. That was Dr. Brenner Hassid, If you want to know more about the evolution of human childhood,

check out her book, Growing Up Human. Or, to hear her tell me more about childhood in the animal kingdom, head over to Instant Genius Extra, available only on Apple Podcasts. The July issue of BBC Science Focus magazine is out now. Pick up a copy in store or visit sciencefocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music can be lost through digital sources or poor signal.

Name Audio believes you can have digital precision with analog warmth. Alongside French acoustic specialist vocal, Name creates high-end audio systems combining innovation with craftsmanship so you can listen to music, just as the artist intended. Discover more at nameadio.com.

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