In gerontocratic human breeding systems, where old men not only control the marriage options of younger men but monopolize younger women themselves (think Aboriginal Australia), old “silverbacks” continue to exercise influence. This led Frank Marlowe to hypothesize that selection favoring longevity might have operated even more strongly on these old patriarchs than on grandmothers (the “patriarchs hypothesis”).104 Bear in mind, however, that uniquely human ideologies promoting respect for elders long after they have lost their physical edge probably required language. If lifespans were already longer by the time humans acquired language (a proposition I will examine in the next chapter), this brings us back to grandmothers.
THE LUCK OF THE DEMOGRAPHIC DRAW
Apart from human females, no other primates, and very few other mammals, take decades to mature before they begin to breed and then live for decades after their ovaries peter out. Among the rare exceptions, short-finned pilot whales and orcas quit breeding around age 40 but live decades longer.105 Nonhuman female primates who survive long enough to cease menstruating go on to live only a few years afterward, or a decade at most in the case of chimpanzees, who reproduce for the last time around age 42. Even the most long-lived of these females spend only 16–25 percent of their lives as postreproductives, not nearly as long as women, who cease to cycle some time after age 40 and then potentially live on for twice that long.106 The proposal mentioned in Chapter 6 that menopause might have evolved to produce in humans a sort of “sterile caste” to forestall competition between older and younger breeding females overlooks the fact that what is different about human apes is not cessation of reproduction around age 40—that is, menopause itself—but how long women go on living afterward.107
Experienced in childcare, sensitive to infant cues, adept at local subsistence tasks, undistracted by babies of their own or even the possibility of having them, and (like old men as well) repositories of useful knowledge, postmenopausal females are also unusually altruistic. Given the flexibility of forager lifestyles, these ideal allomothers can readily relocate near needy kin—though it is well to keep in mind that the meat a new husband provides members of his wife’s group may also be part of the attraction. Across the societies in the Sear and Mace overview, grandmothers were second only to mothers, and rivaled only by older siblings, in their beneficial impact on child survival. But postmenopausal allomothers also have a drawback: the probability they will eventually grow frail.
Nothing guarantees that a postreproductive woman will survive long enough to be of use. In an unusually well-researched thought experiment, anthropologists Jeffrey Kurland and Corey Sparks used archaeological records from late Paleolithic gravesites to compile demographic parameters and then used these to estimate probable lifespans for foragers under a range of ecological conditions. Under good conditions with low mortality, they estimated that a 20-year-old mother would have a roughly 50 percent chance of having a 40-year-old grandmother alive to help her raise her children. As mortality went up, this chance drops to around 25 percent. Using census data from ethnographies, Kristen Hawkes and Nick Blurton Jones came up with lifespan estimates that fell between those two extremes. Their low estimate was consistent with that for a sample of 20 Efe infants—four had surviving grandmothers. At the high end, 7 of 15 Aka infants had either maternal or paternal grandmothers present.108
Fewer than half of Pleistocene mothers would be likely to have had a mother alive or living in the same group when they first gave birth. The chances of a mother having an older sibling still alive were several times higher than that. The chance of her infant having older siblings or cousins, or having either a father, possible fathers, or a would-be father, or some combination thereof would have been higher still. Depending on the circumstances, some combinations would be more beneficial than others, even though—all other things being equal—those with a helpful grandmother would be better off than those without. In the terminology of five card stud poker, where only the very lucky are likely to be dealt a full house or even a matching pair, having a grandmother nearby was like having an ace in the hole. Given equivalently mediocre hands, a grandmother was often the winning card in the Darwinian game of life—but only for those lucky enough to have been dealt one.
The probability of different types of help varied with circumstances. So did the kind of help different alloparents provided and how such help was weighted. Children make adept berry-pickers and lizard-catchers but lack the upper body strength and long arms to dig out deep tubers.109 Nor do they come close to being as practiced and single-minded at tasks like gathering or nut-cracking as old women are. But prereproductive babysitters have the merit of availability. Supervision by a nearby adult would have made older children more usable still, freeing mothers to forage more efficiently. And of course after the Neolithic, with all the chores typical of farming societies, children became productive assets in their own right.110
Cousins and older sibs can be good for a snack, but children’s most common allomaternal contributions are as role models and child-minders (usually with adult supervision not far off). (Peabody Museum/Marshall Expedition image 2001.29.416)
WHEN GRANDMOTHERS OUTLIVE THEIR USEFULNESS
Helpful as grannies are, a grim final question remains: What happens when they cease to be useful? Medicare, Social Security, and other features of the modern safety net make Westerners some of the only people (and the only primates) where resources routinely flow from the young to the very old. More often in primates, resources flow from grandparents and parents down to breeding adults and their offspring.111 Inevitably among our ancestors there came a time when even the most helpful old female became too decrepit to provision herself, much less share with others.
In some species, grandmothers voluntarily opt out of competition for food with younger kin, falling in rank, giving way to younger relatives, marginalizing themselves and being marginalized by others. Across human societies, treatment of old people varies from reverence to astonishing callousness.112 Just as in modern America, where children telephone grandparents more times each month if they possess significant heritable resources, old women in foraging societies are more valued when the food they gather is an important component of the diet. Along with women generally, they are less valued in societies subsisting primarily on game brought in by men.
Customs for coping with decrepitude range from reverse solicitude (the young caring for the old) to voluntary euthanasia (as in traditional Japan), from reverence to marginalization, abandonment, or outright execution. As Kim Hill listened, an old Ache man recalled when as a young man he used to sneak up with his axe behind old women who had become a burden on the group. “I would step on them, then they all died, there by the big river . . . I didn’t used to wait until they were completely dead to bury them. When they were still moving I would [break their backs and necks].”113 In other words, it may not be purely altruism that motivates an Ache woman my age to work so hard.
THE ART OF MANUFACTURING ALLOMOTHERS
Given the neediness of human children and the vagaries of a hunter-gatherer existence, humans were fortunate to be so flexible, mobile, and well-equipped to consciously strategize. For unlike marmosets, human children are not chimerically related to several fathers who are also brothers. Nor can they rely on allomothers who are genetically more closely related to them than the helpers are to their own young, the way honeybee grubs can. This is why the special talent human parents have for cultivating future caretaking prospects, even to the point of manufacturing fictive kinship, is so important. The sort of sexual liaisons described in Chapter 5 provide a taste of myriad possible ploys. Once acquired, language and kinship customs equip women with an even wider range of options for manufacturing kin. Humans are expert at forging alliances on their children’s behalf.
Beginning in girlhood, and as they mature, women become increasingly adept at making friends. The roots of such predispositions do not grow out of men’s quest for hunting partners or broth
ers-in-arms. Whether consciously or not, women seek “sisters” with whom to share care of our children. Even the obsession with being popular and “belonging” so poignantly evident in teenage girls, rendering them both acutely sensitive to what others think and also causing them to be competitive and ruthlessly mean in excluding others, may possibly have much to do with forging bonds which in ancestral environments would have been critical for successful childrearing. From adolescence onward, many girls are more concerned with popularity and belonging than with achievements per se, so much so that their “sense of self becomes . . . organized around being able to make, and then to maintain, affiliations and relationships,” and they dread the rupture of friendships and other social ties.114
Some evolutionary psychologists attribute such tendencies to the innate powerlessness of women in ancestral worlds, where they were carried off from their natal communities to breed among less-than-supportive members of another patriline.115 Others see in the human female’s urge to “tend and befriend” a way to obtain support in times of stress (such as during attack by a saber-tooth tiger).116 But neither of these hypotheses explains why women became so much more affiliative than say chimpanzees, who also usually left home to breed and also had to worry about big cats. These psychologists overlook a key difference between women and other apes. Girls as they matured to breeding age and throughout life needed to line up help from more individuals than just their mates. The bonds themselves became the resource to be protected.
Grandmothers are not the only aged females who forge loving relationships with children. Her dimming eyesight notwithstanding, this 85-year-old Himba woman peers expectantly into the face of her grandson’s four- to five-month-old daughter as she makes a soft grrrr-sound and gently shakes the baby. The girl looks back tentatively, the more intently as the old woman’s forehead touches hers and she begins to rhythmically sing while patting the baby on the back. Meanwhile, the baby looks away from the old woman, to her mother, and back again to her great-grandmother. (I. Eibl-Eibesfeldt/Human Ethology Archives)
Over generations, devices for manufacturing kin have been culturally elaborated and maintained. “It is intriguing to speculate,” Wiessner writes, “that the roots of human kinship systems might lie in cooperative breeding communities where maternal-like care comes from a number of individuals other than the mother, thereby extending concepts of who constitutes family.”117 Stratagems include honorary naming devices, systems of classificatory kin, as well as customs such as designating “extra fathers.” Females in many species use polyandrous matings to line up possible fathers, while bonobos use sexual gratification as well as grooming and occasional gifts to strengthen social bonds with members of both sexes.118 No species, however, proves as clever or opportunistic and—once language became part of the species repertoire—so endlessly inventive as humans are in the manufacture of partners to share with and alloparents to rely on. If long-lived grandmothers were humankind’s ace in the hole, all these classificatory kin—distant relatives, godparents, possible fathers, namesakes, trading partners, and other manufactured alloparents—became their wild cards.
9
CHILDHOOD AND THE DESCENT OF MAN
All of us long to be at home in the world, to find our singular passions reflected in a larger pond than the selves we swim in.
—Daphne Merkin (2002)
No mammal in the world has produced young that take longer to mature or depend on so many others for so long as did humans in the Pleistocene. Cared for by alloparents as well as parents, these incredibly costly, large-brained offspring grew up slowly and survived in sufficient numbers to produce a founding population that could move into new habitats, rear children there, spread out, and eventually people the globe.
Provisioned not just by their mothers but by other members of the group, even offspring weaned long before they were able to fend for themselves could nevertheless mature slowly without starving. The African hunter-gatherers studied by anthropologists in the twentieth century were already very different creatures from the hominins of the Pleistocene, but the challenges they faced staying alive, staying fed, and rearing their children provide the most realistic models we have for reconstructing the challenges faced by our ancestors. Among the !Kung, girls rarely reached menarche before age 16 or so and usually did not give birth for the first time before age 19 or older. Even more time elapsed before these young women produced as much as they consumed. Yet once we step back to view maturing humans in broad comparative perspective, we see that such prolonged periods of postweaning (or in the case of birds, postfledging) dependence are not, in and of themselves, out of the ordinary for cooperative breeders, even though the sheer extent of dependence is on the long side in the case of modern humans. The really distinctive feature of the human story is not longer childhoods per se but a larger mosaic of life-history traits that derived from cooperative breeding: bigger brains that are metabolically more costly than those of other apes; extended lifespans for females after they pass menopause; and peculiarly prosocial tendencies, especially where food sharing is involved, that distinguish humans from chimpanzees, bonobos, orangutans, and gorillas.
Over thousands of generations, uncounted numbers of prereproductives of both sexes played with, reacted to, distracted, soothed, carried, teased, occasionally fed, sometimes competed with, and (more or less) kept safe and happy the almost incessantly needy babies and toddlers left partially or entirely in their charge. It will be some years—not until she is nineteen or so—before the !Kung girl on the left gives birth herself. This means she will have abundant opportunities to practice caretaking prior to becoming a parent. (Peabody Museum/Marshall Expedition image 2001.29.413)
In this chapter, I briefly consider long childhoods and other life-history traits with origins in cooperative breeding, and then consider how pinpointing their appearance in the fossil record could help resolve the vexed question of just when in the history of the genus Homo such an unapelike mode of childrearing got started, and with it the greater capacities for intersubjective engagement that coevolved with cooperative breeding. Finally, I consider some of the unusual liabilities that these peculiar emotional aptitudes impose upon immatures as socially intelligent and sensitive to separation as all apes are, and speculate about our future evolution.
EXTENDED LIVES, LONGER CHILDHOODS, BIGGER BRAINS
Although shared care is not found in other Great Apes, it occurs in nearly half of all primates. Alloparental care is accompanied by at least minimal provisioning in approximately one fifth of primates, though only in humans and the subfamily Callitrichidae (marmosets and tamarins) is this provisioning both spontaneous and extensive (for details, see http://www.citrona.com/hrdy/documents/AppendixI.pdf). Outside of primates, alloparental care and provisioning have also evolved multiple times in a broad array of insects, birds, and other mammals. The remarkable thing about humans, then, is not so much cooperative breeding as it is cooperative breeding in an ape—and the highly unusual traits that emerged as a consequence of this unprecedented combination.
Consider the case of long postmenopausal lifespans. Provided they survive long enough, many female primates cease to menstruate before they die. Yet apart from women and some whales, no other mammals in the world go on living for decades after they are no longer able to reproduce. So what processes led to the extension of postmenopausal lifespans in humans? Well, if mothers with help are better nourished and safer from hazards, their chances of surviving long enough for genes favoring slightly longer lifespans to be expressed go up. And if help from surviving older females increases the survival of their kin, as Kristen Hawkes proposed was the case among Homo erectus, then genes conducive to even greater longevity would have been favored over evolutionary time.1 So cooperative breeding (which, based on its frequency in nature, evolves more readily than long postmenopausal lifespans do) would have set the stage for this highly unusual and derived life-history trait to evolve in humans.
As a general b
iological rule, the costs imposed by reproduction mean that individuals who breed tend to die sooner than those who have not bred. However, this rule is often reversed for females among cooperative breeders. In such extreme cases as the eusocial naked mole rat, ants, bees, or termites, breeding females are coddled and kept safe deep within tunnels or hives, thereby surviving longer than their nonbreeding helpers do. The lifespan of a honeybee queen is measured in years, while that of a worker is counted in weeks. The queen’s long lifespan is a derived life-history trait that could evolve only because other hive members expended the effort and took the risks to rear her young.2
Similarly to extended lifespans, the prolonged childhoods and bigger brains of humans also appear to be derived traits that evolved in the context of cooperative breeding. Whenever natural selection favors longer lifespans (which in the case of the genus Homo might be because of the help postmenopausal women provided to their relatives, or because of some other reason), longer childhoods follow as a matter of course.3 Once the likelihood of dying young is reduced, a later age of maturity becomes an evolutionary advantage. By waiting longer before diverting bodily resources to reproduction, animals can grow bigger bodies and possibly also develop more target-specific immune systems—an important investment for the long haul. Slower maturation also provides immatures the option to “pay as they grow,” opportunistically shifting to a slower pace of growth during times of food shortages and then catching up in times of plenty.4
Mothers and Others Page 31
