I am an anthropologist and primate sociobiologist who seeks to understand, step by Darwinian step, how apes could have evolved to imagine and care about what the lives of others might be like. I believe that such questing for inter-subjective engagement laid the foundations for significant later developments such as language and cumulative culture. My focus then is on the prequel to what became the main human feature film, worlds with symbols, words and story-telling, realms beginning to be explored by psychologists like Marc Hauser, ethnographers like Polly Wiessner and literary scholars such as Brian Boyd, Joseph Carroll, Jonathan Gottschall and Lisa Zunshine. What follows is my take on how humans became such “other-regarding” apes.
“Nature red in tooth and claw”, “selfish genes”, and “rational actors” notwithstanding, humans are a peculiarly other-regarding, “pro-social” species. We routinely share and behave in ways that benefit others and find it pleasurable to do so. Our closest relatives among the other apes, chimpanzees and bonobos, with whom we last shared common ancestors some seven million years ago, and still share nearly 99% of DNA sequences, also descend from highly social, manipulative ancestors and possess similar cognitive capacities, yet they are far more single-mindedly self-serving. In this respect, other apes are far more nearly “rational actors” than humans are.
Time and again, anthropologists have drawn lines in the sand dividing humans from other apes, only to see new discoveries blur those boundaries. By now every one of the Great Apes has been observed to select, prepare and use tools, crafting natural objects into sponges, umbrellas, nutcrackers, even pointed sticks for jabbing prey. Such traditions are transmitted across generations so that researchers in the new sub-discipline of “Primate Archeology” (Haslam et al. 2009) are excavating stone mortars chimpanzees used thousands of years ago. Other apes are also born able to scan and imitate the faces of their caretakers, much as human newborns do, and they exhibit rudimentary capacities for attributing mental states to others. Chimpanzees and bonobos who exhibit considerable empathy in some contexts, also sometimes help one another (Warneken and Hare 2007) and may occasionally share food although, in the wild they usually have to be badgered first (Fruth and Hohmann 2002). Apart from language (no one challenges human exceptionalism on this score) remaining outliers distinguishing humans from nonhuman Great Apes mostly have to do with how much further along the continuum of other-regarding impulses humans fall.
Humans are far better than other apes at attributing mental states to others and appear to care much more about what others think and feel. Children as young as two years old who have not yet learned to speak are already concerned about what others think. They are capable of both pride and embarrassment, eager to know how others perceive them. Right from an early age humans are also eager to share with others. Every human society ever studied is characterized by food-sharing and carefully considered gift-giving. Furthermore, when a human does something nice for someone else, the brain centers stimulated are those associated with registering pleasurable sensations. So what brought about this striving for inter-subjective engagement?
In retrospect, there are obvious benefits to attributing mental states to others and intention-reading. No other apes coordinate behavior to achieve common goals the way humans do. But Natural Selection can not favor traits simply because they might be useful and enhance fitness down the line. So how did individuals in the lineage leading to the genus Homo evolve these peculiarly other-regarding impulses? Traits like mutual tolerance, giving impulses and mental attribution had to already be there before higher levels of cooperation, social learning, teaching, cumulative culture and above all language could develop. Furthermore, as psychiatrist Peter Hobson and linguists like Tecumseh Fitch and Tom Givón stress, questing for intersubjective engagement provide important motivations for developing forms of communication that go beyond the signaling of other animals — “watch out”, “there’s danger on the ground” or “food over here”. If even before they master language, children wonder what others think and feel and are capable of inventing imaginary friends with emotional lives of their own, it is because — as poet Daphne Merkin puts it — we humans “long …to find our singular passions reflected in a larger pond than the selves we swim in.”
So just how on Darwin’s earth did Natural Selection come to favor individuals incrementally better at monitoring the intentions and feelings of others? How did the peculiarly human quest for inter-subjective engagement get started? An important first step is to break down nonhuman-human ape differences into their component parts so as to better understand exactly what traits are involved.
In the most ambitious comparative study to date undertaken at the Max Planck Institute for Evolutionary Anthropology, 106 chimpanzees of all ages, 32 orang utans and 105 two and a half year old children were subjected to a specially designed battery of tests allowing Michael Tomasello’s team to compare their sociocognitive capacities (Herrmann et al. 2007). In terms of spatial memory and object permanence, or the ability to discriminate quantities or understand causality, chimps and orangs perform in the same range as human toddlers. The big differences are in the social realm where children test significantly better at learning how to solve a problem from watching a demonstrator. Children are also better at understanding communicative cues like pointing, and better at attributing mental states to others so as to understand what they are intending or trying to accomplish – what psychologists call Theory of Mind. So to explain nonhuman versus human ape differences, we need to identify selection pressures favoring increases in mutual tolerance, social communication and intention-reading.
To my mind, existing explanations for other-regarding impulses –such as the need for in-group solidarity in the interests of defeating out-group enemies — leave unaddressed these initial steps towards prosociality and fail to explain why other apes such as chimpanzees did not spend the last seven million years becoming more cooperative as well. This is one reason why in Mothers and Others I focus on one of the most obvious, albeit too often overlooked, differences between humans and other apes, the peculiar way that bipedal apes living in small-scale hunter-gatherer societies rear young.
Among chimps, gorillas, bonobos and orangs, mothers are extraordinarily possessive of new babies, in fur-to-fur, body-to-body contact, not out of touch even for a moment, for the first six months or so of life. Infants are suckled for up to seven years. Once weaned however, nonhuman ape youngsters provision themselves. By contrast, human babies born in traditional societies are passed around among other group members from the first day of life, and in some groups are held by allomothers (typically kin — fathers, older sibs, aunts, cousins or grandmothers) for much of the time, even suckled if the allomother holding them is lactating. Within months, long before children are weaned, infants are fed as well by allomothers who deliver soft or pre-masticated foods, kiss-feeding the baby by pushing the mash in with their tongues. Supplementation of children’s diets continues for many years.
Even though human infants are bigger at birth and take nearly two decades to mature, allomaternal provisioning means that mothers can wean babies sooner than other apes. With toddlers buffered from starvation, mothers breed again sooner, potentially increasing lifetime reproductive success but also forcing mothers to become even more dependent on alloparental assistance. In Mother Nature I proposed that increasingly contingent maternal commitment produced the curious combination of passionate maternal love with ambivalence evident in human mothers. Although rarely seen in primates with exclusive maternal care, high levels of ambivalence in mothers short on social support is common in other cooperatively breeding primates. Humankind’s deep legacy of cooperative child-rearing also had implications for the cognitive and emotional development of youngsters growing up dependent upon allomothers (the main focus of Mothers and Others) as well as implications for allomothers (Burkart et al. in press).
None of us has a machine to go back in time and observe how infants in the Pleistocene were reared. But what we do have is a growing body of information about exactly who cares for immatures not only among other primates, but also among humans living as hunters and gatherers. Furthermore, it is increasingly clear that in traditional societies with high child mortality, alloparental care and provisioning is not only useful, but critical for child survival. Homo sapiens species could not have evolved without it.
Space does not permit here a review of what sociobiologists are learning about the demographic implications of cooperative breeding and the conditions under which it is likely to evolve — particularly in highly social species with helpless young confronting environmental challenges such as unpredictable resources. Nor does space permit me to review here information from neuroscience, endocrinology and the emerging field of comparative infant development relevant to understanding what the psychological implications of multiple caretakers were likely to be. But here are a few of the highlights. The presence of a supportive kinswomen like a grandmother is correlated not only with increased child survival, but also with increased maternal sensitivity to infant needs, and infants growing up with nearby grandmothers are more emotionally secure and develop cognitively at a faster pace. Having older siblings around also enhances development of social skills. As cognitive psychologists quip, “Theory of Mind is contagious” you catch it from older caretakers. Furthermore, forming attachments to multiple caretakers enhances perspective-taking and conditions children to integrate different perspectives. Even though historians of the family, social workers and psychologists have long known about cognitive and emotional advantages from growing up in an extended family, only recently have evolutionists begun to recognize that such alloparental assistance would have been essential for our ancestors to rear surviving young.
Now consider information from other primates that cooperatively rear young. Experiments with tamarin monkeys by Marc Hauser’s team at Harvard, and with marmosets by Judith Burkart and Carel van Schaik at the University of Zurich, reveal that these tiny-brained cooperatively breeding monkeys are far readier to pull a rope delivering food to others than are large-brained chimpanzees in comparable tests. These generous “other-regarding” impulses come into play even if the recipient is unrelated. In the wild as well, cooperatively breeding tamarins and marmosets tend to be mutually tolerant and helpful to one another, as well as unusually good at coordinating their behavior in subsistence tasks. Among these tiny-brained and distant primate relations, relations, the father (or another male the mother mated with) carries the babies (usually twins) much of the time, except when the mother is nursing them. Allomothers also provision the babies around the time of weaning such that in some species of tamarins, 90% of the infants’ first solid food comes from alloparents. Chronic food-sharing spills over into generosity in other realms as well. Adults routinely vocalize to call the youngsters’ attention to novel, or particularly palatable, food items while intervening to prevent them from eating toxic foods — behavior that comes close to teaching (Rapaport and Brown 2008; Burkart et al. in press).
In Mothers and Others I summarize such evidence and invite readers to join me in a well-documented thought experiment. Take a primate with the cognitive and manipulative potentials and rudimentary empathy and Theory of Mind typical of all Great Apes, and rear that creature in a novel developmental context where his mother’s commitment is contingent on how much social support she has and she and her infant depend on care and provisioning from multiple caretakers. The resulting phenotype will be a youngster adept at perspective-taking, far more so than any other ape under natural conditions would be. Then subject this novel ape phenotype to novel selection pressures such that infants best at monitoring the mental and emotional states and intentions of others, and also best at learning from them, are going to be those best cared for and best fed. This then leads to directional selection favoring traits like enhanced mutual tolerance, social learning, social communication and perspective taking — precisely the traits that comparisons between humans and other apes require us to explain.
No one knows when cooperative breeding got started in the hominin line. Evolutionary anthropologists Kristen Hawkes and James O’Connell have hypothesized that our ancestors began relying on alloparental care and provisioning by the beginning of the Pleistocene, 1.8 million years ago, and I concur with their reading of the paleontological evidence. If correct, this means that long before behaviorally modern humans capable of symbolic thought, art and language emerged, and even before big brained anatomically modern Homo sapiens with fully sapient 1350 cubic centimeter brains in the last 200,000 years, these cooperatively breeding ancestors would have been well on their way to “emotional modernity”. Emotionally modern apes would have grown up keeping track of others and inordinately interested in the feelings and lives of others, even those out of sight, or far away.
Acknowledgements: This essay is based on a lecture originally presented at the Darwin Festival, University of Cambridge, July 5-9, 2009.
Selected References, including recent additions since Mothers and Others
- Burkart, J.; E. Fehr; C. Efferson; and C. van Schaik. 2007. Other-regarding preferences in a nonhuman primate: Common marmosets provision food altruistically. PNAS 104: 19762-66.
- Burkart, J.; S. Hrdy; and C. van Schaik. 2009 (in press). Cooperative breeding and human cognitive evolution. Evolutionary Anthropology.
- Fruth, B. and G. Hohmann. 2002. How bonobos handle hunts and harvests: Why share food? In Behavioural Diversity in Chimpanzees and Bonobos, edited by C. Boesch, G. Hohmann and L. Marchant. Cambridge: Cambridge University Press, pp. 231-243.
- Haslam, M. et al. 2009. Primate archeology. Nature 460, 339-344.
- Hauser, M.; M.K.Chen; F. Chen and E. Chuang. 2003. Give unto others: Genetically unrelated cotton-top tamarins preferentially give food to those who altruistically give food back. Proc. Roy. Soc. London (Series B) 270:2363-70.
- Hawkes, K.; J.F. O’Connell; N.G. Blurton Jones, et al. 1998 Grandmothering, menopause and the evolution of human life histories. PNAS 95:1336-39.
- Herrmann, E.; J. Call; M.V. Hernandez-Lloreda J.; B. Hare and M. Tomasello. 2007 Humans have evolved specialized skills of social cognition: The cultural intelligence hypothesis. Science 317:1360-66.
- Hrdy, S.B. 1999. Mother Nature. New York: Pantheon.
- Hrdy, S.B. 2005. Evolutionary context of human development: The cooperative breeding model. In: Attachment and Bonding, a new synthesis, edited by C.S. Carter et al. Cambridge: M.I.T. Press, pp. 9-32.
- Rapaport, L.G. and G.R. Brown 2008. Social influences on foraging behavior in young primates: learning what, where and how to eat. Evolutionary Anthropology 17: 189-201.
- Sear, R. and R. Mace. 2008. Who keeps children alive? A review of the effects of kin on child survival. Evolution and Human Behavior 29:1-18.