CARTA: Childrearing in Human Evolution -- Kristen Hawkes: Grandmothers and the Extended Family

Summary

Kristen Hawkes discusses the critical role of grandmothers in human evolution and childrearing. The presentation contrasts the traditional view of the nuclear family with the significant contributions of grandmothers in certain societies, like the Hadza people in Tanzania. Hawkes argues that grandmothers' support in child-rearing not only allows mothers to have more offspring but has also influenced human life history, including our longer lifespans post-fertility compared to other great apes. This grandmothering hypothesis is backed by mathematical modeling, suggesting it has been pivotal in distinguishing human evolution, contributing to our species' unique social structures and survival strategies, such as big game hunting and male alliances.

Highlights

• Grandmothers play a central role in childrearing, essential for the survival and evolution of human societies. 👩‍👧‍👦
• The Hadza people exemplify traditional societies where grandmothers significantly contribute to family welfare. 🏕️
• Grandmothers allow mothers to have more children by providing care for older offspring. 👩‍👦‍👦
• This support system is distinctive in human evolution compared to other great apes, contributing to longer lifespans. 🦍➡️👵
• Mathematical models show how grandmothering could drive evolutionary changes, such as increased longevity. 📈
• Human male alliances and big game hunting are intertwined with grandmothering, impacting social dynamics. 🤝

Key Takeaways

• Grandmothers have been pivotal to human evolution, influencing our life history and social structures. 🌟
• The grandmothering hypothesis suggests extended post-menopausal lifespans are key to human development. 👵
• Grandmothers provide critical support in childrearing, allowing mothers to have more children. 👶
• Mathematical models indicate grandmothering influenced human longevity and reproductive strategies. 📊
• Male alliances and big game hunting are also crucial in human societies, affected by grandmothering. 🏹

Overview

In her enlightening talk, Kristen Hawkes sheds light on the underestimated role of grandmothers in human evolution. Challenging the conventional focus on nuclear families, she introduces the grandmothering hypothesis, highlighting how grandmothers bolster the survival and development of offspring, enabling mothers to reproduce more efficiently. This support system has been fundamental to defining human social and evolutionary paths, distinguishing us from other apes.

Through studies of the Hadza people, Hawkes illustrates a living example of these dynamics where grandmothers play a substantial role in childcare and family survival. The evolutionary significance of this dynamic is profound, as it not only affects familial structures but also correlates with our extended lifespans past reproductive age. This discovery indicates grandmothers are central to the unique longevity of humans and our broader social systems.

Mathematical modeling supports this hypothesis, showing how grandmothering could have driven evolutionary changes leading to human's unique life history traits. Moreover, Hawkes points out the indirect benefits of such a society where male alliances, big game hunting, and resource distribution all pivot around these fundamental family contributions, showcasing a complex interplay between familial and societal evolution.

Chapters

• 00:00 - 02:00: Introduction and Context This chapter serves as an introduction, providing the necessary background and setting the stage for the topics to be discussed. Accompanied by music, it aims to engage the audience and prepare them for the content that follows.
• 02:00 - 09:00: The Role of Grandmothers in Childrearing The chapter discusses the significant role grandmothers play in the upbringing and childrearing of their grandchildren, providing historical and cultural context to their involvement.
• 09:00 - 15:00: Grandmother Hypothesis and Human Evolution The chapter begins with an illustration of a Hodza couple and their child, setting the stage for a discussion on human evolutionary theory.
• 15:00 - 19:00: Mathematical Modeling and Life History The chapter titled 'Mathematical Modeling and Life History' discusses the significance of including all relevant figures in a narrative or study, particularly highlighting the role of a grandmother in the life of a baby. This discussion takes place in the context of modern foragers who live on wild food, providing a contemporary parallel to the environments in which our genus originally evolved.
• 19:00 - 27:00: Impact on Male Behavior and Social Dynamics The chapter discusses the significance of grandmothers in child-rearing and their broader contributions to human life history, highlighting the importance of relationships outside the immediate family, including those involving men.
• 27:00 - 31:00: Concluding Remarks and Implications The chapter discusses the role of social relationships in shaping the life and behaviors of individuals, particularly in traditional communities. It emphasizes the importance of the central figure's interactions with other men and how this affects his perception of life and child-rearing. These patterns, although observed in traditional socio-ecologies, also resonate with modern settings.

CARTA: Childrearing in Human Evolution -- Kristen Hawkes: Grandmothers and the Extended Family Transcription

• 00:00 - 00:30 e [Music]
• 00:30 - 01:00 so the the the setup before me has already prepared you for for where I'm
• 01:00 - 01:30 going to start here this this is a hodza couple and their youngest child and I don't know whether it was true before you walked into this room but for a lot of people The View that they have of what happened that was special in our lineage was the construction of of the nuclear family in which we had dads um supporting moms and Offspring and a whole bunch of other things followed from that sexual division of labor and other people have already prepared you for the argument I'm going to make although we kind of have dueling model
• 01:30 - 02:00 here to say we're we're leaving out an important part of the story if we don't include this very important person so that uh woman is the grandmother of the baby at the center and that's that's the older sister of the baby these are modern people that's all we've got left you know is us moderns but these are people who are foraging for a living living on wild food in in an environments that that's the best modern analog we have for the one that are genus evolved in and I'm going to put on
• 02:00 - 02:30 the table uh some reasons to really uh think that actually what this woman grandmother contributes to child rearing is Central to what's happened in our lineage but not only her contributions to child rearing but her contributions to our life history that have made relationships outside this family unit really critical including these guys so
• 02:30 - 03:00 there are other hods of men and the relationship between the the central hunter in the picture here and those other men uh turns out to be a really important part of how he thinks about his life and how he spends his time and how child rearing Works in uh communities like this and in fact I would say not only in traditional socio ecologies but some of these things I think will start to seem familiar now when when I went we started
• 03:00 - 03:30 this project with the hodza my collaborators Jim oconnell and and Nick blurton Jones um started studying the behavioral Ecology of these these people who live in in Northern Tanzania I didn't have grandmothers on my mind neither did they one of the reasons that we were especially interested in in this ethnographic opportunity is that the the archaeological record of our genus if we get very early in time in fact the beginning of the archaeological record
• 03:30 - 04:00 is the bones of big animals and stone tools and these guys hunt these big animals so figuring out how that works how those animals are handled how uh deposition patterns are produced was especially an interest of Jim oconnell who was the archaeologist on the team and yet there we were looking at how people spent their time what they got for it and here were these old women the so the the women that I'm showing there in their 60s when
• 04:00 - 04:30 this Photograph was taken and because we were monitoring how people spent their time and what they got for it there it was in the data the economic productivity of these old women was really floating the boat in a way that we just had not anticipated we were also astonished to discover that little kids were amazingly productive foragers at at very young ages but those those tubers that the old ladies are cooking there are the starch
• 04:30 - 05:00 staple for this population throughout the year and digging them is something that it takes adult strength and and and sort of engineering skill to manage maybe not quite adult strength but little kids who are just weaned can't do it and so they have to depend on somebody else they depend on their moms but then when their moms have new babies then Mom's attention is partly on that
• 05:00 - 05:30 newborn and and that means that these kids' welfare as we could measure it depended on the work of grandmothers and so here is again this woman in her 60s you know showing her stuff there how complicated it is to to dig these these tubers that like to get underneath the rocky soils and so it requires engineering and strength to do it and this package of the role that these older women were playing in in becoming
• 05:30 - 06:00 especially important economically when kids were weaned their moms could move on and have a new baby because grandmother was there to subsidize the requirements of the older kids putting that together with what we can see in the age structure of of um of these folks this age structure is a signature of of our species you know when you hear I was just reading a paper saying this again just recently that because the
• 06:00 - 06:30 life expectancy is now so much longer than it was before it's only now that people are living past 40 but here uh is is this this is uh the just the female half of the age structure and the um each of those bars is uh the length of the bar is a fraction of the population that's in that in that bar and the green bars are the women in their childbearing ears and those those those yellow bars
• 06:30 - 07:00 at the top are the women who are past their fertility this is the hza we look at all kinds of other examples of human populations and we see essentially the same thing even if we look at agrarian economies in in the 19th century they look like this if you're lucky enough in this population as a little Hoda girl to make it to adulthood you have more than a 75% chance of living past your fertility and if we look at at the standing crop about
• 07:00 - 07:30 a third of the adult women are past the childbearing years the combination of how childbearing works in the how much how important grandmothers are to how well kids do and their mother's ability to move on and have a new baby sooner is in Striking contrast to what we see in the other great apes we're part of that radiation well the best data for for the other great apes are come from chimpan es and there's the same kind of age
• 07:30 - 08:00 structure built from a life table for the female part of the chimpanzee population now if if you're a a little chimpanzee kid you if you're an infant you depend on your mom uh until you're weaned and then well she's still an important um figure in your life but now you get your own lunch and Mom moves on and has has a new baby well that pattern of independent mothering and that that
• 08:00 - 08:30 there isn't anybody uh that that in general chimpanzees apes um in general almost never live past their fertile years a few very lucky ones make it but most females die in their uh childbearing years now this suggested that this pattern of fertility and mortality that characterizes us and this pattern of childbearing are intimately connected and about the same time we were trying
• 08:30 - 09:00 to put these pieces together uh Eric charnov a theoretical biologist was building models of life history variation and one of his models was about the mammals how much variation there is across the mammals some live very short Lives die young have babies fast some are much slower and trying to account for why that's so he built an optimization model to explain how natural sele ction would shape these
• 09:00 - 09:30 things and in his model the thing that ran the speed of a life history was adult mortality in his 93 book he included this figure now this is a figure just for the primates because the same general pattern holds for our order as it does for the mammals in general and he was showing how if you know what adult mortality is if you know what the average adult lifespan is can predict
• 09:30 - 10:00 when the age of first birth will be and he showed that that relationship held across the primates well the circled point there is us and he didn't wasn't thinking at the time but wait a minute that um that adult lifespan includes this big part in which there is no baby production and yet that average adult lifespan is a thing that could account for why we mature so late we have such low adult mortality
• 10:00 - 10:30 and putting all of those pieces together gave us a picture that at least made internal sense we could account for the things about our life history that distinguish us from the other living hominids uh that that if if grandmothering is is the secret that our postmenopausal longevity and our early weaning and our late maturity all made a
• 10:30 - 11:00 coherent package in terms of of of trav's model could also that package could account for why when our genus appears it gets out into all these environments that no homones have ever been in before and maybe account for some things in the earliest archaeology a very ah promising collection of pieces that all seem to be going together and yet of course um you know still have lots of colleagues to
• 11:00 - 11:30 persuade even that is something they ought to yeah maybe maybe maybe um but of course many people will say could it really happen that way and we don't have a time machine but this is where mathematical modeling maybe if it doesn't come to the rescue it becomes really important and luckily this uh mathematical biologist Peter Kim got interested in the question Peter Built an agent B model in
• 11:30 - 12:00 which we took advantage of Char's work and this the regular relationships among these pieces of life history as we look across the the the mammals generally but as we look across the primates and um build a build this agent base model which we start out with an aplike life history and then we add grandmothering and we see what happens if if longevity can uh mutate what will happen and this is what happens on the
• 12:00 - 12:30 left you see what happens without any grandmothering so this is time passing on the on the x- axis on the Y is average adult lifespan and on the left there's no grandmothering the average adult lifespan of all the lineages that don't go extinct remains in that apik range there are seven that move up to get slightly longer but they actually go extinct and what's happening there is a a conflict between the sexes the the the males are trying to push for
• 12:30 - 13:00 greater longevity that actually the females can't support if we had grandmothering then so look on the right that's 30 simulations with grandmothering and what happens in 20 one out of the 30 as you see there is they move from that a apik equilibrium to the humanlike one there's a 22nd one that's maybe on its way there but what what the know is at least with the
• 13:00 - 13:30 assumptions in the model there are two equilibria the aplike one and the human-like one and grandmothering is the thing that drives the show between them and once you start in that direction you end up at the humanlike equilibrium so so those pieces all go together grandmothering at least by this argument causes the kind of life history that we have but there's more to the story it's also the case that we're looking at this change in age structure of course not just on the female side
• 13:30 - 14:00 but on the males as well so now once again using the hza to represent humans and chimpanzees and the same sources for for these numbers uh fertility ends at essentially the same age in us as as in chimpanzees it's just that it's rare for a chimpanzee female to live that long but as you can see there not only if we just look at a fertile adult sex ratio those those chimpanzee males are are dying
• 14:00 - 14:30 really fast in in the human case male survival is much higher and also males continue to survive to these older ages and the females who are fertile are still the ones who are under the age of 45 and what that means all those old guys are ahead of the game for the young guys coming in competing for the same um conceptions and if we just look at
• 14:30 - 15:00 operational sex ratio so now we've added inner birth interval into the story those are four of the examples in which um we go from a an aplike longevity to a human-like one and what happens in every case is the operational sex ratio favoring males triples this the the operational sex ratio becomes much more male biased and more male biased sex ratios as Behavioral olist know looking at all
• 15:00 - 15:30 kinds of animals increase the advantages for mate guarding now in in in animals like us male alliances are really critical relationships between males and the other males become really crucial to whether or not a male can effectively make claims on a female cultural anthropologists have been talking about this across human socio ecologies for a long time and recently Lars Rodi has has um tried to remind primatologists this
• 15:30 - 16:00 is a true thing about about human social Arrangements that these relationships among the males within a community are really crucial to what happens within nuclear families in the 8S there were uh the development of models by by people like um Michelle rosaldo and Jane Collier and here is a quote from Jane coler's 88 book looking at what happens with marriage in the simplest human
• 16:00 - 16:30 societies marriage is primarily a relationship between men with respect to women conjugal bonds and their character and what goes on at home depends on what's going on out there and the relationship that this guy has with his Bros really affects how he behaves at home that matters it's a huge thing in Hoda communities a man has to compete for his social standing he has to
• 16:30 - 17:00 demonstrate that he's valuable as an ally and dangerous as a competitor and that draws his effort into public activities and it draws them away from domestic effort now what men do in this in this Society is hunt they also collect vegetable food of course but when they're hunting there are lots of small animals in this environment they could spend a lot of time taking those if they did they could those animals would
• 17:00 - 17:30 mostly when they brought them home go to their wives and kids instead they focus their attention on Hunting the big animals and because they do it means that they fail almost every day on average it takes a month of hunting to get one of those big ones and actually the difference between good and poor Hunters is you know enormous and when when one of those big animals
• 17:30 - 18:00 is taken all kinds of other men join in the the tracking they they come to the kill site everybody comes to the kill site they eat there women children men they carry away meat from the this enormous public event that everybody's been participating in and um the fraction that ends up going to the wife and kids is a small one and rarely at that so there's the there's a way these
• 18:00 - 18:30 pieces go together grandmothers are maybe the the key to our life history what happens with the operational sex ratio makes mate guarding really count conjugal bonds are intention with male alliances and um this relationship that men have with other men affects the way they spend their time drawing public effort away from Dom mtic effort and
• 18:30 - 19:00 that these these rare big game successes are a big deal most going to somebody else that s that stuff that goes to somebody else is subsidizing the the cost of children the cost of child care even though it's not coming from Dad and one of the most exciting things about beginning to see the pieces this way is that there is an archaeological trace of this big game hunting so can put together Theory
• 19:00 - 19:30 empirical data of various kinds to tell a story about what happened in our in our genus thanks [Music]