Darwin and the Ascent of Man: Why humans are such hypersocial apes?
April 30, 2009
Sarah Hrdy lecture presented as part of the Darwin Distinguished Lecture Series. These events are sponsored by Arizona State University, Office of the President, College of Liberal Arts and Sciences, School of Life Sciences, and the Center for Biology and Society.
Transcript
Quentin Wheeler: [0:06] Good evening. I’m Quentin Wheeler. I’m university vice-president and dean of the College of Liberal Arts and Sciences. And I’m delighted to welcome you all here this evening for what I am going to describe as an intimate evening with Dr. Sara Hrdy. I say intimate because she has been battling laryngitis all day and will probably be whispering into the microphone.
[0:25] So if you have difficulty hearing, we’re going to sort of test this as we go. We’ll try to increase the volume and get through this. We do have a plan B if necessary and we hope we don’t have to resort to that.
[0:38] It really is a pleasure to welcome Dr. Hrdy here on behalf of the School of Life Sciences and the college. She is a professor emeritus at the University of California at Davis. Among her many distinctions is Guggenheim Fellow and elected member of the California Academy of Sciences, The National Academy of Sciences and The American Academy of Arts and Sciences.
[1:01] She was once described as "one of the most radical evolutionary thinkers of our time." A productive author and scholar, her notable books include, "The Langurs of Abu," "Female and Male Strategies of Reproduction," "The Woman That Never Evolved", selected by the "New York Times" as one of the notable books of the year, "Infanticide: Comparative and Evolutionary Perspectives," and together with Sue Carter and other authors, "Attachment and Bonding: A New Synthesis.".
[1:32] One of her most popular works, chosen by both "Publishers Weekly" and "Library Journal" as one of the best books of 1999 was "Mother Nature: A History of Mothers, Infants and Natural Selection" which won the Howse Prize for Outstanding Contribution in Biological Anthropology. "Mother Nature" has been described as "a critical work that demonstrated that mothers may abort, abandon or even kill offspring they do not have the resources to rear." Her assertion that infanticide is common across the animal kingdom shook biology, especially when it was applied to Homo sapiens.
[2:10] One reviewer of her work summed it up in the following way, "Her feminist reinterpretation of evolutionary theory and data challenged the archetype of the good mother, a natural Madonna and replaced her with a more complex female figure, ambitious, calculating, nurturing, selfish, loving and sexually assertive."
[2:33] Sarah’s most recent work, I’m happy to say, is going to be unveiled here. This is sort of the world premiere tonight with her latest book being launched, "Mothers and Others: The Evolutionary Origins of Mutual Understanding". Adding yet another facet to our understanding of human sociality, examining the social glue and psychological implications of humankind’s long legacy of shared care of infants.
[2:58] Please join me in welcoming Dr. Hrdy. Her talk is entitled "Darwin and the Ascent of Man: Why Are Humans Such Hyper-social Apes?"
[3:08] [applause]
Sarah Hrdy: [3:14] Thank you Dean Wheeler. I’m delighted to see several old friends in the audience. But for the most part, you know, you guys are strangers, which is why I am very grateful that you are human rather than some other species of ape who might be less welcoming to an interloper. And that’s what I am going to talk about tonight. I want to try to understand why humans are such peculiarly hyper-social, other-regarding apes.
[3:57] It was back in 1871 when Charles Darwin proposed that Africa was formerly inhabited by extinct apes resembling today’s gorillas and chimpanzees. And it was, of course, an inspired guess, which is why 200 years later we are still celebrating this man’s highly original and methodical way of thinking.
[4:36] Today, of course, we know that around 15 million years ago the line of apes leading to the African apes split off from those leading to the Asian orangutan. And between five and seven million years ago the line leading to Homo sapiens split from the line leading to chimpanzees and bonobos, which of course is why today there’s a 99% overlap in the DNA sequences of these three species.
[5:20] There are also broad cognitive similarities. For example, this specially-trained chimpanzee in Tetsuro Matsuzawa’s lab in Japan is able to remember the location of numbered sequences that appear briefly on a computer screen and actually does a better job than the graduate students in that department.
[5:54] But there are also remarkable differences. In the most ambitious comparative study to date, undertaken by Michael Tomasello’s group in Leipzig at the Max Planck Institute for Evolutionary Anthropology there, they used a specially designed battery of socio-cognitive tests to compare 106 two-and-one-half year old children with 105 chimpanzees and 32 orangutans.
[6:36] The remarkable thing is that in terms of special abilities, these animals tested very much in the same range. In quantitative abilities like many versus few, again, same range. Causality, if for example you take a stick and try to reach through to get something, what happens to the thing on the other side? Very much in the same range.
[7:04] Where the big differences showed up was in social learning, watching a demonstrator do something and learning just by observing in a social situation. Communication, learning by pointing and theory of mind, able to understand what someone else has in their mind to attribute intentions to them. These were the big differences.
[7:37] Another area of difference, of course, is that humans are far more eager to share than other apes are and they do so right from a very early age. Even though other apes, chimpanzees for example, will occasionally help one another and bonobos will share food. But it is much more like power-aided theft then the kind of considered gift giving that anthropologists document for all human societies.
[8:19] So why should humans differ so much from other apes in these respects? Humans are not only inclined to share with others, they are far more interested in their mental states and intentions and such other regarding impulses provide the building blocks for cooperation.
[8:48] Intention reading equips people to coordinate with others to achieve a common goal, like these Kyopo tribesmen. They waded into the river waist-deep to beat this timbo plant with their clubs and they are releasing poison into the river so that women and children can come along, wading with baskets to harvest this tremendously rich food source of stunned fish that would not be available to any other ape.
[9:26] Like most social scientists, I initially assumed that such gifts arrived as a kind of neural correlate of having such big brains, brains that are three times the size of a chimpanzee or of the Australopithecines that Don Johansson and Bill Kimball study.
[9:52] However, I now follow Franz Dewall and Peter Hobson in thinking that things besides big brains and also language -- most people think that language is what is doing it. I think something else had to be going on first. And as Hobson wrote not long ago, before language there was something else, more basic in a way and more primitive and with unequal power in its formative potential that propelled us into language.
[10:29] Even apart form language, humans differ from other apes in their perpetual questing for inter-subjective engagement. Yet the recent discoveries by neuroscientists that even comparatively small-brained primates like macaques posses mirror neurons, and especially there is new evidence for imitative capacities in newborn chimpanzees, evidence that makes it unlikely that sapien brains by themselves are going to explain our pro-social inclinations.
[11:09] Many of you will be familiar with the work that Andrew Meltzoff did years ago, showing that right from birth, humans can imitate others, scanning their facial expressions and copying them when they stick out their tongue and make funny faces.
[11:24] Later in 2002 Meltsoff observed that, "The neuro-cognitive machinery of imitation lies at the origins of empathy and developing a theory of mind." But when he wrote that, Meltzoff, like most of us, still assumed that humans were the only newborn capable of imitating funny faces that way.
[11:50] Now, thanks to the work of Yamakoshi in Japan, Kim Bard and others in the emerging field of comparative infant development, we know that newborn chimpanzees as well may respond in the same way, scanning the faces of caretakers, and when the caretaker sticks their tongue out, they may stick theirs out.
[12:16] In other words, the basic wiring for this kind of imitation is there. Something else has to be involved in the differences that we’re seeing between humans and other apes. Over the course of their development though, human babies become more and more interested in and adept at monitoring the intentions of others, imitating and engaging them and seeking to understand what they are thinking and what that person is feeling.
[12:52] By contrast, chimpanzee youngsters -- after their initial enthusiasm -- lose interest by about 12 weeks. So why do humans remain so interested in the subjective states and intentions of others, getting better and better at mind reading and social learning? And why are humans so inordinately pro-social and interested in helping others?
[13:24] Several possible explanations have been proposed. But first I want to talk about the "mind reading mums" hypothesis. Mothers need to anticipate the wants and the capacities of their offspring because an empathetic awareness of the infant’s point of view both physical and psychological will enhance the mother’s ability to teach the child.
[13:48] I want to consider for a moment though the case of nut cracking. Wherever available, primates make use of tree nuts and you can see here this Kung woman in the Kalahari cracking open these incredibly hard-shelled mongongo nuts. It takes years to learn how to do this well, but this is a staple source of food for these people.
[14:20] Of course, their chimpanzee counterparts are also deriving large amounts of calories during the times of year that they are available, they may get 3,500 calories a day from cracking open these kula nuts.
[14:34] And look at these kids, just so attentively watching their mother do it. Surely a chimp mother and her offspring would benefit just as much as their human counterparts do from more efficient learning and teaching with such a valuable subsistence skill at stake.
[14:59] Yet only two instances of deliberate teaching of nut cracking have ever been reported in decades of research among west African chimpanzees. So why haven’t chimps been selected for the aptitudes that make teaching and social learning so effective in the human case?
[15:21] At present the most commonly cited explanation for our pro-social natures invoke the need for highly competitive social creatures to forge useful alliances and to bond with other group members, so as to beat out other groups.
[15:40] Darwin himself attributed our species’ unusually pro-social impulses to the need for people in one group to defeat other groups. "A tribe including many members who were always ready to give aid to other and sacrifice themselves for the common good," Darwin wrote "would be victorious over other tribes."
[16:05] Essays like this one in "Nature" stress that "altruism’s midwife must have been conflict between groups." The idea that generosity and solidarity towards one’s own group may have emerged only in combination with hostility towards outsiders.
[16:25] This has become the explanation of choice for humankind’s peculiarly pro-social nature. That same year a similar news feature appeared in "Science" -- actually several articles on this in "Science" and an article in the "New Scientist" announced a consensus among anthropologists, archeologists, primatologists, psychologists and political scientists that not only is war as ancient as humankind but it played an integral role in our evolution.
[16:59] Now I do not doubt that competition between groups enhances in-group solidarity. There’s a lot of research from economists and social psychologists and animal behaviorists. This is true.
[17:14] However I have trouble seeing how overt conflict between groups explains the initial origins of human hyper-sociality, or explains the emergence of these differences between humans and their common ancestors with chimpanzees in respect to mutual tolerance and cooperative potential.
[17:42] Proponents of the "beat out your neighbor" interdemic hypothesis take for granted that our ancestors spent the last five million years engaged in lethal conflict with their neighbors and compare the "murderous mutual loathing between tribes" to the genocide urges of chimpanzees, which of course do exist.
[18:13] Well, maybe our ancestors were doing this, but maybe not. And if chimpanzees and humans shared a common legacy of inter-group warfare, and if enhanced social learning and increasingly sophisticated capacities to attribute intentions to others could evolve simply from passing through a crucible of genocide and selective group extinction.
[18:47] And if, as we have recently learned, other apes have the requisite underpinnings to imitate, learn from and empathize with others, why didn’t arguably even more dominance-prone, more violence-prone ancestors of chimpanzees spend the last seven million years evolving their own attributes so they could reap the benefits of greater cooperation and wipe out their neighbors?
[19:18] As we ponder this question, it’s important to keep in mind in the face of this consensus just how far from verified many of the assumptions about our continuously war-like past still are. There is a lot of archaeological evidence like this 4,600-year-old burial, the remains of an entire family killed by stones and arrows from Eulau in Germany.
[19:55] Lots of evidence of prehistoric warfare, but almost none of this evidence for socially organized aggression between groups, dates further back than the beginning of the Pleistocene. And none at all comes from the middle to early Pleistocene, probably the most relevant time periods for the emergence of humankind’s pro-social tendencies.
[20:26] Even experts on Paleolithic art, who are specifically looking for evidence of violence, draw a distinction between rock art from the last 10,000 years, which pretty clearly is showing intergroup conflicts, and older Paleolithic art.
[20:46] There are not a lot of humans depicted, but when you do find humans in Paleolithic art, and when violence is depicted, it tends to be individual homicides or else these poor guys who have been turned into pincushions like this, which looks more like a socially sanctioned group execution, something we do have ethnographic evidence of going on in hunter/gatherer societies.
[21:19] Now, absence of evidence is, I know, not evidence of absence. But surely all these question marks should give us cause for pause. Although there is abundant archaeological evidence and ethnographic evidence for intergroup warfare among complex and stratified hunter/gatherer societies, and certainly lots of it for herders and settled people, there’s relatively little such evidence for band-level hunter/gatherers living at low population densities.
[22:04] Nor is there archaeological evidence for warfare in the Pleistocene. You find other things, murder, maybe cannibalism, but not socially organized aggression between groups. Nevertheless, evolutionary psychologists tend to project evidence from the last 15,000 years of warfare back onto pre-human ancestors.
[22:33] They also lay special emphasis on evidence for lethal intergroup genocidal seeming raids among common chimpanzees while dismissing bonobos as eccentric offshoots even though, of course, the relationship is equidistant between bonobos, chimpanzees and humans.
[23:00] Thus, it is worth keeping in mind that as Sam Bowles -- one of the main proponents of the idea that conflict was altruism’s midwife -- as he candidly acknowledges the computer simulations he and other use to prove that the rewards of in-group amity derive from out-group enmity are based on "artificial" history of early human evolution.
[23:27] He’s not making these claims. These claims are coming from others. In fact, we’re still struggling to construct what the actual histories were and to determine the actual deep history of these creatures.
[23:46] Furthermore, such genetic evidence as we do have points to relatively small, very widely dispersed populations, such that you hear estimates like all humans today probably descended from a founding population of fewer than 10,000 breeding adults.
[24:07] You didn’t have the kind of saturated habitat that these models seem to be presuming. The really big challenge for our ancestors I think probably had to do with avoiding extinction, not because neighboring groups were wiping them out, but because they were having trouble replacing themselves, rearing young that survived, what the paleontologist, Owen Lovejoy, used to refer to as a demographic dilemma.
[24:43] And this hominid demographic dilemma was very much on my mind when I completed "Mother Nature," a book that describes how it takes around 13 million calories to rear a human from birth to nutritional independence, a wonderful figure I love to cite. I think Hilly Kaplan and Kim Hill, we’re indebted to for that 13 million calories.
[25:09] New babies would be born long before a mother’s previous children were independent. And in "Mother Nature" I explained why even though help from the mother’s mate and the meat he provided would have been very important, the vagaries of hunting meant that a Pleistocene father would not always be a sufficiently reliable provider even if he was there to meet the needs of children who need to eat frequently.
[25:44] This led me to propose that care and provisioning from an assortment of group members in addition to parents, alloparents, would have been not only helpful for our ancestors, but absolutely essential.
[25:59] Humans, I argue, must have evolved as cooperative breeders, meaning a breeding system with alloparental, in addition to parental, care and provisioning of offspring. Since Ed Wilson first laid out his ground plan for a unified study of sociobiology, a very robust body of theory has grown up to explain why group members, other than parents would evolve to care for and provision someone else’s young.
[26:40] I’m not going to have time to talk about that theory today. I just want to mention where it took me. So I ended "Mother Nature" saying we must have evolved as cooperative breeders, and then I started to wonder as I encountered work like Tomasello’s and others, could this help explain these peculiarly hyper-social features that we’re seeing in humans? Might our pro-social natures have evolved as byproducts of a lot of shared care and provisioning?
[27:20] "Mother and Others" is actually an attempt to work through, step by step, how this could work in terms of infant development and selection pressures on their caretakers.
[27:35] I don’t have time to talk about the theories behind cooperative breeding but I can tell you that it actually has evolved many times. It’s found among social insects, something I hardly need to remind you of at ASU, which is the world’s capital of work on an extreme form of cooperative breeding, the social insects.
[27:57] It’s found in roughly 9% of 10,000 species of birds. It occurs in perhaps 3% of 5,400 species of mammals. I still site this figure because it’s all we have. I doubt if it’s right but it’s what’s in all the books.
[28:15] Corporative breeding is especially well-represented among highly social carnivores like these Indian dholes. Pack members will routinely return from hunting, like these wild dogs, with predigested meat in their stomach. Then put the regurgitated meat into the mouths of pups who are waiting there back at the den. Sometimes they’ll provision the mother as well.
[28:48] These carnivores are highly efficient, voracious predators, so it’s just amazing to see grown animals deferring to pups at kills and letting them get away with all kinds of things. As you know, the best way to get bitten by an animal is to try to take their food away. They’re volunteering food to these basically helpless pups.
[29:15] Because young are buffered from starvation by this alloparental provisioning, it permits particularly long periods of post-weaning or, in the case of birds, post-fledging post-dependence. In humans of course, we refer to this developmental phase as childhood.
[29:38] Even though none of the other great apes besides humans exhibit shared care, it turns out that allomaternal infant care has evolved many times in the highly social order primates. It is found in more than half of the 104 primates species for which we have information on this. Mothers are tolerant of allomaternal access, even right after birth. Group members other than the mother are allowed to take and carry another female’s infant.
[30:19] In about 24% of this sample, shared care is accompanied by at least, minimal alloparental provisioning. Although often this is nothing more than one mother occasionally allowing another female’s infant to suckle as occurs in LeAnn Nash’s Galago.
[30:41] Only among humans and in the sub-family Callitrichinae -- which is the marmosets and the tamarins -- do you find alloparental care plus extensive alloparental provisioning. Among these tamarins, mothers routinely give birth to twins and then her mate carries them most of the time except when she’s actually suckling them.
[31:13] Meanwhile other group members, sometimes another male the female might have mated with, or else a want-to-be breeder who entered from outside, or else who might be a sibling from a previous year. A sibling of the infant being fed will catch beetles and frogs to feed to youngsters, especially when they’re being weaned, which is a very vulnerable time for young primates. In some tamarin species, 90% of solid food eaten by youngsters at this phase comes from allomothers.
[31:56] Now 35 million years have elapsed since humans last shared a common ancestor with these tiny-brained South American monkeys. Yet tamarins, marmosets, and hunter/gathers have converged on remarkably similar breeding systems resulting in many parallels, more than I can even tell you today.
[32:23] Unadvertised ovulation -- some people call it concealed ovulation -- I think non advertised ovulation is more precise. Mating through the cycle, even when the female is pregnant, you find these correlations between alloparental assistance and the survival of offspring, rapid reproduction by the mother, short inter-birth intervals, prolonged post weaning dependence and an extraordinary ability to move into and colonize new habitats. Which of course we know humans have been exceptional at doing.
[33:05] Conversely, the absence of allomaternal support tends to be correlated with failures in maternal commitment. Faced with insufficient allomaternal assistance, tamarins and humans exhibit an un-primate-like tendency to abandon newborns.
[33:27] Even though I have been stressing how helpful to one another these creatures are, it’s important to keep in mind that corporative breeding does not mean that everybody is always nice to everybody else.
[33:40] In particular, competition between breeding females among marmosets, for access to resources -- and the resources can include allomaternal care -- leads often to competition between breeding females, so that one female, particularly the alpha female may kill offspring born to rivals. Which is why rivals tend not to conceive, because it’s a waste of their time and energy.
[34:14] Indeed, instead of pregnant females becoming more sensitive to infant needs and more nurturing -- as you might expect because that happens in other mammals -- marmoset females in the last stages of pregnancy actually become particularly infanticidal so as to eliminate rival young.
[34:36] This also helps to explain why alpha females drive out another female should she get pregnant, because she might be infanticidal. So it’s not all hunky-dorey. Sometimes though you do have several females breeding in the same troupe. It seems to depend on a number of circumstances.
[34:57] Furthermore, a mother short on allomaternal assistance, let’s say their mate dies or there’s no one else to help them, may terminate investment in her own infant. Along with humans, these Callitrichids are the only primates reported to harm their own infants, which is really the dark side of corporative breeding.
[35:25] Now none of us has a machine to go back in time and observe how our ancestors reared young in the Pleistocene, the period between 1.8 million and 12,000 years ago. All we know is how people still living by hunting and gathering today reproduce and keep their children alive.
[35:47] What we have learned though is that among foragers the average interval between births is three to four years, much shorter than the four-and-a-half to eight year intervals between births in other apes. This is so, even though humans are born bigger and take longer to mature.
[36:11] How is this possible? I think you’ve already probably guessed the answer. It has to do with allomaternal provisioning. Infants once weaned, in other apes, provision themselves, but human children remain dependant on others for many years.
[36:32] Another big difference has to do with the possessiveness of non-human ape mothers in the months after birth. The great ape mother is her baby’s sole source of warmth, locomotion, protection and nutrition, and her dedication is total, a fact that is very well known to poachers, because they realize that the only way they’re going to catch in an infant to sell in a market some place is to kill the mother.
[37:02] No one else is allowed to hold her baby. The mother remains in continuous intimate contact with her newborn 100% of the day and a 100% of the night. If there are mothers in this audience, I think that will sound burdensome. Half a year elapses before she is out of contact, even for a moment. And some, like this mother orangutan, will continue to nurse her baby for seven years.
[37:39] Like many monkeys and pro-simians, but in marked contrast to any of the other apes, human mothers are surprisingly tolerant of postpartum access to their newborns. When this Kung newborn’s mother returned to camp after giving birth, she handed her baby to its grandmother, who massaged and shaped his skull with her hands, as is the custom. Then the baby was passed around from hand to hand among group members.
[38:12] As with the Kung, among the Hassa, the baby is likely to be surrounded by relatives and carried by them, held by others 85% of the time. Among the central African Mbuti, the new mother presents the child to the camp, whereupon she hands the baby to a few of her closest relatives and friends and then they look at him, hold the baby close to their bodies.
[38:35] Far away in the Philippines among the Udka, the infant is eagerly passed from person to person until all in attendance have had an opportunity to snuggle, nuzzle, sniff and admire the newborn. Thereafter he enjoys constant cuddling, caring, loving and affectionate genital stimulation.
[38:56] Among these Central African Efe who are at the extreme end of having a lot of allomaternal care, babies are held by 5-24 allomothers, averaging 14 different caretakers, held by them 39% of the time, right after birth, up to 60% of the time in successive months and according to ethonographer Paula Ivy Hendry, Efe babies with the most allomothers at age one are the most likely to still be alive at age three.
[39:27] Now most of these Efe allomothers are going to be kin. In the case of female allomothers, sisters, aunts and grandmothers are disproportionately represented. In the case of male allomothers -- allomother is just a group member other than the mother so it can be a male -- fathers, brothers, cousins doing most of it, granddads doing surprisingly little direct infant care.
[40:02] Clearly the luck of the demographic draw, just which kin are still alive, good at finding food, not sick, not encumbered with young of their own, this is going to be very important. And luckiest of all are going to be the children with two dedicated parents and an assortment of effective allomothers, perhaps an uncle, and older sibling or two, along with a grandmother.
[40:32] For humans are unusual in having access to this special class of allomothers: post-reproductive females undistracted by babies of their own who are hardworking, experienced and altruistic.
[40:54] Now no one knows when longer lifespan and longer childhoods began to characterize the hominid line. James O’Connell and Kristin Hawks tentatively date these life history changes to the beginning of the Pleistocene, with the emergence of African Homo erectus, simply known as Homo ergaster.
[41:19] And in "Mothers and Others" I lay out my reasons, at least for the time being, of accepting their chronology, and I argue that cooperative breeding must have gotten started at least by this time, though I don’t pretend to be sure.
[41:40] Today in the developed world, almost all infants born survive childhood. It’s considered a great tragedy to have your child die. Fewer than 2% will do so, though rates are much higher where people in the developing world lack access to clean water and medical care.
[42:06] Infant mortality rates in the 40-60% range are much more typical when you’re talking about traditional societies. Also not unusual for populations of wild primates and probably our Pleistocene ancestors suffered mortality rates in this range.
[42:24] Fortunately though for our ancestors, flexibility and mobility were hallmarks of forager families. People were able to move away from adversity and gravitate towards opportunities which included the prospect of giving or receiving allomaternal assistance.
[42:44] Among foragers, it is customary in many groups for a man to come and live for a period of years with his wife’s family, hunting on their behalf in a period of bride service until after one or more children are born. This means that inexperienced young mothers, when they are most vulnerable with the first birth, are going to have kin support, which in all primates turns out to be very important.
[43:17] Mothers specifically say that they prefer living with their own kin in a matrilocal setting because there is so much alloparental care available. In this case study of Aka caretaking undertaken by the ethnographer Courtney Meehan, it’s obvious that the mother in the matrilocal setting has a whole lot of alloparental assistance from her kin.
[43:52] In a patrilocal setting, however, there is less alloparental care. But look, the input by the father goes way up, five fold increase as he compensates by helping more. Depending on local ecologies and the luck of the demographic draw, there is no one type of family.
[44:19] My point here is simply that among cooperative breeders, mothers and allomothers alike are very flexible and resourceful and strategic in how they allocate their care. I could by the way show you some similar data from marmosets.
[44:37] Beginning in the 1990s with work like that done by Magdalena Hurtado and Kim Hill, and increasingly in the last decade, behavioral ecologists have begun to pay more attention to childrearing. And recently a growing body of evidence has begun to provide more support for hypothesized links between allomaternal assistance, child survival and maternal reproductive success.
[45:11] Now for those of you who maybe wondering, well, if it’s as important as you say, why didn’t we notice this sooner? I think the answer has to be that until very recently most of the detailed studies of child development were coming from contemporary western societies with very low rates of child mortality.
[45:36] By 2008 when Rebecca Sear and Ruth Mace did -- this is probably a first stab effort, it’s a small sample. But they reviewed the evidence for 45 traditional societies where you had information on the impact of their various allomothers on child survival, and these were all fairly high child mortality societies.
[46:12] The beneficial impact of alloparental care is very clear. The impact of fathers in this sample turned out to vary. It varied between being absolutely critical and seemingly not mattering at all, which is a bit of a shock for many of us.
[46:36] Next to mothers, grandmothers and older siblings had the most consistently reliable impact. I’m showing this slide because it’s what we have. You go to press -- as that cad Donald Rumsfeld, to paraphrase him, said, "You go to press with the literature you have not the literature you wish you had". I think we’re going to do better in the future, but this is what we have for now.
[47:03] In the decades since "Mother Nature" was published, the demographic implications of cooperative breeding have become increasingly well-documented. In this new book, what I wanted to do was just take corporative breeding as granted, then set out to ask, what light all this new information coming in from the emerging field of comparative infant development, neuroscience, sociobiology and psychology -- what light could it shed on the cognitive and emotional implications for an ape growing up dependent on multiple others. That’s really what the book is about.
[47:48] I have time though, only to talk about a few highlights. There’s a lot of developmental information that I’m not able to talk about in this time period. Even without taking child survival or evolution into account, psychologists and social workers studying Western populations have known for a long time that living in extended families affects both maternal commitment and child development.
[48:17] The presence of a maternal grandmother in the house is correlated with increased maternal sensitivity to infant needs, more secure infant attachment to the mother by enhanced cognitive ability at earlier ages. The presence of older siblings is correlated with more sophisticated theory of mind, leading Ruffman and Turner to quip that "theory of mind is contagious." You catch it from your older caretakers. Improved social skills at older ages. Finally multiple caretakers are correlated with an enhanced capacity at perspective-taking and especially to integrate multiple perspectives.
[49:05] Turning now to information from other primates, coordinated caretaking and provisioning of young also has marked cognitive and emotion consequences in other cooperatively breeding primates.
[49:23] Independent experiments done at Harvard by Mark Halesertein and at the University of Zurich by Judith Burkhart and Karl Van Schaick, reveal that cooperative breeding in tamarins and marmosets leads to animals that seem to be far readier to help others and to provide things to them, for example, to pull a rope to deliver food to a tamarin or a marmoset in another cage. Something that in cooperative tasks chimpanzees are loathe to do.
[49:57] These generous, other-regarding impulses come into play even if the other individual was not a relative. In the wild as well, tamarins coordinate knowing with their canines to open tough foods and then they’ll jointly share the fruits of their labor. Primatologist Paul Garbor reports 52 cooperative acts for every aggressive act.
[50:26] Food sharing and provisioning spill over into other social realms as well, such as volunteering information. Adult marmosets routinely vocalized to immatures to call their attention to novel or particularly palatable food items. They may even intervene to prevent immatures from eating a food that the adult already knows is toxic.
[50:51] Teaching in animals is exceedingly rare, rarely documented, but Judith Burkhart argues that the greater inter-individual tolerance typical of cooperative breeders is conducive to enhanced attention to what others are doing, thus to social learning.
[51:14] Obviously it does not make good sense to compare the cognitive abilities of these distantly related little South American monkeys with apes. When compared with other South American monkeys who are not cooperative breeders, Burkhart and Van Schaick found a series of differences. The cooperative breeders were characterized by higher levels of inter-individual tolerance. More pronounced giving impulses and willingness, even eagerness, to share food, and enhanced social learning.
[51:56] I want you to join me in a thought experiment. Take a primate with a cognitive and manipulative potential and rudimentary empathy and rudimentary theory of mind typical of a great ape. Then rear that creature in what -- for a great ape -- is a totally novel developmental context where maternal care is contingent. The infant depends on care and provisioning from multiple others.
[52:33] The resulting phenotype will be more adept at perspective taking, very different already from a chimpanzee. Then subject these novel phenotypes produced by developing in this context to novel selection pressures. Novel selection pressures that I have in mind have to do with the fact that infants best at monitoring the mental and emotional states and intentions of others and also best at learning from them, will be best cared for and best fed.
[53:11] In environments with high child mortality, there will be direct selection favoring inter-subjective questing. These are, of course, just the traits -- enhanced mutual tolerance, social learning, social communication, perspective taking -- that comparisons between humans and other apes require us to explain.
[53:39] Similar arguments can be made for enhanced other-regarding impulses at all mothers at allomothers at older life stages.
[53:49] In conclusions then, no one doubts that humans can be very violent; that they sometimes kill each other and probably always have. Nor am I suggesting that competition between groups was unimportant for shaping traits like ethnocentrism or indoctrinability. Very much in evidence in our species now.
[54:15] But what concerns me is that by focusing so exclusively on overt inter- group conflict, we overlook occupations, such as child rearing, that are at least as important for explaining selection pressure favoring peculiarly pro-social tendencies in a line of ape that was evolving into Homo sapiens.
[54:43] If, as I propose, apes interested in the mental and emotional states of others co-evolved with more costly, slower-maturing offspring and corporative breeding early in the Pleistocene, this means that long before the evolution of behaviorally modern humans capable of language and symbolic thought -- perhaps 80,000 years ago -- and even before the evolution of big brained, anatomically modern humans, maybe 150,000 years ago emotionally modern humans were already questing for inter- subjective engagement. Perhaps as long ago as the beginning of the Pleistocene.
[55:28] Thank you.
[55:30] [applause]
Announcer: [55:35] This lecture is part of the Arizona State University Darwin Distinguished Lecture Series. It’s sponsored by the ASU Office of the President, the College of Liberal Arts and Sciences, the School of Life Sciences, the Center for Biology in Society and is a production of Grassroots Studio.
Transcription by CastingWords