After a century of suppression in anthropology, the truth is coming out. Our ancestors were the apes who took the road of co-operative breeding. Whereas chimpanzees, bonobos, gorillas and orang-utans never let their babies go until they can fend for themselves, human mothers show astonishing willingness to hand their babies around. We are the great ape that does babysitting; that's why we're here.
Natural selection could not have favoured this development unless our Homo foremothers trusted the people around them to care for children not their own, that is, to act as alloparents, and then to return baby safely to mum. The people an ancestral hominin mother could trust were her own female kin: sisters, aunts, older daughters and, above all, her own mother. So humans did not evolve in "patrilocal" hunting bands of male kinsmen exchanging wives from outside groups. The old "matrilineal priority" theorists of the late 19th to early 20th century, derided by the Bronislaw Malinowski school of social anthropology, were on the right track after all.
Most appropriately in Charles Darwin's bicentenary year, Sarah Hrdy, doyenne of Darwinian feminists, sets the record straight. While rigorously applying the logic of the "selfish gene", with a wide interdisciplinary literature at her fingertips, Hrdy has a history of debunking sexist stereotypes. In her first book, The Woman that Never Evolved (1981), she reflected on what she learned as a young fieldwork primatologist about promiscuous female monkey and ape behaviour. What might female sexual strategies, designed to confuse males about paternity, teach us about the evolution of human female sexuality? Her follow-up volume, Mother Nature: A History of Mothers, Infants, and Natural Selection (1999), has become the comprehensive reference for primate maternal strategies.
Mothers and Others goes further into the implications for an ape that evolved reproduction so costly it required social support for mothers. Locating the origins of genus Homo - a two-million-year timescale - in strategies of co-operative breeding, Hrdy accounts for the evolution of our special hypersocial tendencies. She proposes the evolution of emotional modernity in Homo erectus, a species with two thirds of our brain size, 1.5 million years before language, symbolism and ritual arose.
Here she interweaves her thesis with the work of Michael Tomasello. For Tomasello, intersubjectivity - a willingness to share what I am thinking with you, and seek to know what you are thinking of my thoughts - is a uniquely human psychological trait, present even in babies. Other great apes, especially Machiavellian chimpanzees, are great mind-readers, but lack any intention of co-operating to allow their own minds to be read.
Precisely when an evolving hominin mother lets others take her baby, says Hrdy, selection pressures for such two-way mind-reading are set up. The mother must be socially adept to elicit support and judge the motivations of the alloparent; the baby, once handed over, must be monitoring carefully "where's mum gone?", at the same time probing for signs about the intentions of the new carer; while the alloparent adopts a quasi-maternal role. An array of behaviours sprang up to help this triad of mum, baby and allocarer keep in contact: mutual gazing, babbling, kissfeeding. Hyperpossessive great ape mothers never needed such elaborate bonding mechanisms. The only other primates heard to babble are tiny South American marmosets and tamarins, whose breeding systems are fully co-operative, involving shared care and provisioning of infants.
Hrdy's timeline corresponds to the "environment of evolutionary adaptedness" coined by John Bowlby, the author of attachment theory. Hrdy takes issue with the idea that continuous care and contact mothering is "natural" for humans. Bowlby's is essentially a great ape model. Continuous care and contact with infants may be a last resort for mothers who have no safe alternatives.
A typical African hunter-gatherer camp provides many such safe alternatives. In the Tanzanian Rift Valley, Hadza babies are held by alloparents 85 per cent of the time in the days after birth; Efe foragers of Central Africa have an average of 14 different caretakers in the first few days, both female and male; comforting and breastfeeding are regularly shared among mothers.
While related women are the likeliest candidates for allocare, men and boys are involved too. By contrast with the old Man the Hunter model of a sexual contract of perfect fidelity from mummy in exchange for reliable provisioning from daddy, the co-operative breeding hypothesis involves men as hunters who were not always reliable provisioners, and possible rather than certain fathers. Socially approved means for lining up extra "fathers" are found from the Arctic to the Amazon.
The variability in levels of paternal effort in men is remarkable, with more variation within our species than across all the other primates. How can something so important be so variable? Hunter-gatherer studies reveal the social and residential flexibility that allows kin to step in where fathers have let their children down. Through such compensatory mechanisms, the level of care and food received by all the children in a Central African camp works out roughly equal. Children just aren't allowed to suffer poverty among foragers, who grow up with the sense of the world as a "giving" place.
The most likely answer to how natural selection generated allocare is with the mother's mother. The "grandmother hypothesis" developed by Kristen Hawkes, James O'Connell and Nick Blurton-Jones has launched the fightback for early human society centred on matrikinship. Patrilocal models assumed that females in evolving human lineages had no female kin to help. Older women in foraging groups were hardly considered.
Hawkes and colleagues, already suspicious of the reliably provisioning dad theory, began to notice the high-calorie incomes provided by Hadza grandmothers. This answers a classic puzzle of sociobiology: why do women live so long after they have finished reproducing? A hunter-gatherer woman who reaches menopause has a life expectancy of more than 20 years. With the attention now being paid to the effects of mother's mother on infant survival, it is becoming clear that menopause in humans is the single biggest piece of evidence that we evolved in matrikin circumstances. In Early Human Kinship (2008), Chris Knight documents an institutional prejudice against the truth.
But if emotional modernity is the legacy of co-operative breeding, what will happen to us given today's loss of co-operative breeding under pressure of modern economics and weakened kinship links? Children have evolved to cope with multiple carers, but not with poor, casual childcare. Children's responsiveness to their environment can equally mean lack of responsiveness if the right cues are not input at the right age. Could this put the evolution of empathy into reverse, our hunter-gatherer prosocial characteristics becoming a transient phase after 10,000 years of farming, a few centuries of industrial life and a decade of Facebook? Will our descendants 20 millennia hence still be shaped by our heritage of communal care? Hrdy's lucid and comprehensively researched book takes us to the heart of what it means to be human.
Sarah Blaffer Hrdy is professor emeritus of anthropology at the University of California, Davis, where she specialises in primate behaviour and the origins of sex roles. Since earning her doctorate from Harvard University in 1975, she has been elected to the California Academy of Sciences, the National Academy of Sciences and the American Academy of Arts and Sciences.
To date, she has written five books, numerous publications for scholarly or general consumption and worked on several films.
Her spare time is spent on a farm in Northern California, where she and her husband grow walnuts. They are involved in habitat conservation, and together have planted about 55,000 trees. She says: "As the world goes to hell in a handbasket, it is uplifting to see the western edge of Yolo County become more, not less, biodiverse."
Mothers and Others: The Evolutionary Origins of Mutual Understanding
By Sarah Blaffer Hrdy
Belknap Harvard, 432pp, £19.95
Published 12 May 2009