Kristen Hawkes is a Distinguished Professor of Anthropology at the University of Utah.
Sandra Aamodt: What makes humans different from other primates?
Kristen Hawkes: Michael Tomasello argues that it’s shared intentionality, our interest in being in the same game with other people. Social relationships have this weight for us. Even when we're alone, we're not alone. We've got all this social stuff going on in our imagination—interaction with others and the history of the things we've done together or might do together. That's what I would say is really different about us compared to our primate cousins, who are smart, gregarious, social critters but don't do that.
By far the best hypothesis we've got for where this difference comes from is Sarah Hrdy’s. She shows why these appetites for shared intentionality likely come from our childrearing practices, which are different from any other apes’. A human infant does not have Mom’s undivided attention and complete commitment as a birthright because she's got other kids who are still dependent. She has these other kids because she has help, and my favorite hypothesis is it all starts with grandmothers.
Sandra Aamodt: How do grandmothers come into the story?
Kristen Hawkes: My focus on grandmothers started with observing the Hadza hunter-gatherers in Northern Tanzania. None of us were thinking especially about old ladies, but we were interested in the details of how you live on wild foods and so what everybody was doing. We found that old women were producing more food than women of childbearing age, especially foods that kids are too small to get, like digging up deeply buried tubers. That made us think about ancestral populations in similar environments. If kids couldn’t feed themselves after weaning as other ape infants do, moms would have to share food with them longer. If she did that, older females without infants could help, and with that help moms could have next babies sooner. All of that would mean more descendants for helpful grandmothers.
Other ideas about where our life histories come from start with the idea that kids have so much stuff to learn to be a human adult. That makes growing and furnishing big brains seem to be the center of the human story. But where I would put my money is that we got grandmothers first, which made increased longevity pay, evolving an ancestral ape-like life history into a human one, and these other characteristics then followed in our evolution. Several studies in ethnographic and historical populations have shown that having a living grandmother improves the grandchildren's welfare. But grandmothers could have given our genus distinctive longevity, and the foundation for shared intentionality, long before we got to be modern Homo sapiens.
Sandra Aamodt: Are we the only female primates that continue to live after we can't reproduce anymore?
Kristen Hawkes: Female fertility ends at about the same age in all of the great apes—including us—but most of them get old and frail during the childbearing years. We have all the eggs we're ever going to have as a five-month-old fetus, and after that we start losing them. There’s no difference in the rate of loss from birth to age 47 between chimpanzees and humans. The difference is that it's rare for a chimpanzee female to live into her 40s. But even in hunter-gatherer societies where mortality is relatively high, if you make it to adulthood, your chances of living past the childbearing years are way better than of dying earlier. We have the ancestral pattern of ovarian aging, but women remain healthy and productive to much older ages. Once you've got those long lifespans, then selection favors continuing to grow longer, and get bigger, before you reach adulthood, so a lot of these other pieces fall into place. The weird thing about our lineage is the grandmothering because we're the only ones who do it.
We just published this modeling paper. The thing that's so cool is it's a simple model without any hunting or learning or brains or pair-bonds or any of that stuff. All it takes in the model is a little bit of grandmothering to turn an ape-like life history into a human-like one.
Sandra Aamodt: I’ve also heard our long childhoods proposed as the explanation for why we developed pair-bonding.
Kristen Hawkes: Back to Darwin, certainly throughout the 20th century, the hunting hypothesis was the dominant suggestion about what happened in our lineage. As ancestral populations saw the ecology changing and the grassland spreading, the forest receding, things drying out, hunting was a much better way to make a living than depending on plants. Bigger brains and more learning would make you a better hunter, and bipedalism allows your hands to be free to use tools. Bigger brains also made birth trickier, so babies had to be born earlier and more helpless, which got in the way of the mom hunting. So mothers paired with mates where the deal was you get the paternity if you bring home the bacon, and that that's why we have nuclear families and sexual division of labor. But that idea started to unravel as the fossil evidence showed that bipedalism occurred millions of years before increasing brain size, so these pieces don't go together that way. Ethnography shows that most of what hunters kill goes to others, not to their own wives and kids.
Sandra Aamodt: In the popular imagination, the hunting hypothesis is still widely believed—especially by men, in my experience.
Kristen Hawkes: Exactly. But I think it's also partly the kind of the lives we lead right now. This thing we do where we go to our separate nuclear families and shut the door, that's really weird. In most of human experience, you're around other relatives all the time. The huge weight of that pair-bond in our social ecology is unusual.
Sandra Aamodt: Do you think we’re putting weight on the pair-bond that it can’t support?
Kristen Hawkes: I think we are. Across history, after the introduction of wage labor, when people do not have to depend on their kin, they move away because they want to get away from their families. But I think we are designed for this intense sociality. Intimate face-to-face, skin-to-skin contact is crucial to how we feel and to our health. Hanging all that on a mate, with conflicts of interest inevitable, is bound to be tough. The issue isn’t just conflicts of interest between individuals—even the most intimate of partners or the closest of kin—but conflicts of interest we have with ourselves in our world of too many choices.
Sandra Aamodt: There are many examples in evolution of systems that work well enough but not because they make any coherent sense.
Kristen Hawkes: In evolution you can never start over, and so you end up with these clusters of things that are not the way you’d build the airplane if you started from the beginning. [laughter]
Sandra Aamodt: But somehow it flies.
Kristen Hawkes: Occasionally it soars, and it's so amazing, but not always.
Sandra Aamodt: How far does the importance of grandmothers generalize beyond the Hadza people that you study? Can somebody else do that extra work?
Kristen Hawkes: Somebody has to do it. All kinds of helpers can be implicated—older siblings, mates, other kin, or friends. When you look at modern people (including the Hadza), there are so many other sources of help than grandmother. In our world, single mothers by themselves are trying to do something that we’re not designed to do. No wonder they're having a hell of a time.
There’s all this complicated social construction in modern human communities, including those of hunter gatherers: language and a long history of social institutions. But that wasn’t all there in the beginning, so this question of what got it started can’t begin with all of it happening at once.
Sandra Aamodt: I guess a lot of the problem is that the fossil record is so sparse, it's hard to know in what order all these things happened.
Kristen Hawkes: Exactly. The fossil record is terrible. I say that as someone who consorts with archaeologists [laughs], but the stones and bones are so limited. Still the fossil and archeological records are so important you can’t ignore them either.
Sandra Aamodt: I recently saw a video of a chimpanzee mother and infant interacting in ways that were very reminiscent of humans.
Kristen Hawkes: Matsuzawa's group at the Kyoto Primate Center have good relationships with their chimpanzees, so when they have babies, the experimenters can see everything that's going on between the mom and infant. This is in captivity, so she can put the baby down, which is not characteristic in the wild. But evolution could easily select on and elaborate that behavior because it is there in our lineage to start with. In the right rearing environments, it could mean strong directional selection, if kids who are more interactive are more likely to survive.
Sandra Aamodt: If the environment started rewarding that behavior, you could get more of it.
Kristen Hawkes: Right, and it's exactly that kind of rearing environment that you get if you've got grandmothering. In our recent model, we start out with so few grandmothers, less 1% of the females in the ancestral condition, but it just takes right off. With grandmothering, moms can move on to that next baby before the previous one is independent. And there you are with a rearing environment that puts a premium on infants’ abilities to attract their mother’s—and grandmother’s—attention and care.
Sandra Aamodt: Before we finish, I have a more personal question for you. How has what you've learned about science changed the way you live your own life?
Kristen Hawkes: My longstanding interest in conflicts of interest makes me be prepared to see those, and maybe it makes me a little more charitable about them. I’ve worked on differences between men and women, and I think that makes me more amused by or charitable toward the ways we get in each other's way.
Sandra Aamodt: I think as a culture we often get fixated on irrelevant trivia about men and women and underplay the important differences, like pregnancy.
Kristen Hawkes: We’re doing an interesting experiment that way too. In the history of not only our species but our genus, raising kids isn’t something that moms do alone. That's why friends really matter. On top of that, women who want kids but put it off—for good reason, given the way work is now organized—are going to find they’ve passed their own childbearing ages. We’re still healthy and productive, but that doesn’t stop the ancestral ape-like rate of aging in our ovaries. This attempt to make the nuclear family the center of the story is really hard on everybody.