Mothers and Others
Page 32
A prolonged childhood, in turn, may have relaxed the selection pressures opposing the evolution of bigger brains. Brains are enormously expensive organs, second only to hearts in how much energy they require. Only well-fed youngsters—such as those with fairly reliable allomaternal provisioning—could afford to grow and maintain such expensive tissue.5 Because evolutionary increases in brain size tend to be incremental, being modestly more cerebral than a competitor would scarcely be a sufficient advantage to compensate for the big handicap of taking nearly two decades to mature. A faster-maturing albeit dumber competitor could still outbreed her. But if apes in the hominin line were already living longer, and—because they were provisioned by others—already enjoying the luxury of growing up slowly, then incremental increases in brain size could evolve at a discount.6 Even without off-setting the full costs of waiting so late to breed, brains could gradually get bigger until they reached a size that actually enhanced the relative fitness of possessors—by allowing them to learn more, know more, become more efficient at food procurement, out-compete others for mates, and so forth.
Without a doubt, highly complex coevolutionary processes were involved in the evolution of extended lifespans, prolonged childhoods, and bigger brains.7 What I want to stress here, however, is that cooperative breeding was the pre-existing condition that permitted the evolution of these traits in the hominin line. Creatures may not need big brains to evolve cooperative breeding, but hominins needed shared care and provisioning to evolve big brains. Cooperative breeding had to come first.
But when? One place we might turn to answer this question is the fossil record. If fossil evidence allows us to attach a date to the emergence of longer childhoods or bigger brains or extended lifespans, we would be in a position to estimate the point by which cooperative breeding had already become established in the hominin line, and with it (I believe) the emergence of emotionally modern humans. So is there any fossil evidence to support my contention that modern emotional sensibilities emerged in the hominin line long before our late Pleistocene sprint into behavioral modernity?
WHEN DID COOPERATIVE BREEDING FIRST BEGIN?
We now know that by the beginning of the Pleistocene—a million and a half years before humans with fully sapient-sized brains were on the scene—the average African Homo erectus was twice as big as Australopithecus afarensis (believed to be a predecessor of the genus Homo). Males weighed around 130 pounds, with some individuals nearly six feet tall. Their 800–1,100 cc brains were nearly twice as large as those of australopithecines or chimpanzees—smaller than the brains of Homo sapiens but moving into their range (1,100–1,700 cc).8
Animals this big almost certainly took longer to grow, suggesting that African Homo erectus matured later than did either australopithecines or modern chimpanzees, although probably not nearly as late as modern humans. In all likelihood, these early Pleistocene hominins were already enjoying the prolonged postweaning dependence we would expect to find in a cooperative breeder. Evidence from Homo erectus leg bones and molar development (suggesting slightly later molar eruption) may also be consistent with slower development.9 While much controversy surrounds the interpretation of all the fossil evidence, one thing we can be certain of is that by 1.8 million years ago African Homo erectus were growing bigger brains than any of their known predecessors.
Another thing we know from the fossil record is that, while both sexes were taller and weighed more, this increase in body size was more pronounced in females than in males. Instead of being nearly twice as big as females, as is the case of australopithecines, Homo erectus males were only 18 percent larger than females.10 This degree of sexual dimorphism is only slightly more pronounced than that of modern humans. Why is increased growth among these females important? To grow bodies and especially brains this big, both sexes—but especially Homo erectus mothers and their children—needed high-quality diets. Yet as the behavioral ecologist and paleontologist James O’Connell and his colleagues emphasize, climatic conditions at the beginning of the Pleistocene would have decreased the availability of the palatable shoots and soft fruit that ape immatures in Africa fed on.11 Among bipedal apes in an arid habitat of mixed savanna and woodland, weanlings would have had to rely on foods obtained, processed, or premasticated by others before being shared. Their survival would have depended on food sharing by adults in their group.
In addition to eating starchy tubers when meat was in short supply, these hominins probably also obtained protein and lipids by efficient harvesting and processing of nuts, insect grubs, and perhaps shellfish. The omega-3 fatty acids in these foods would have been especially critical for pregnant women and nursing mothers, to fuel the fast-growing brains of late-stage fetuses and nursing young.12 Such a diet accentuates the interdependence between male hunters, who were eager for both meat and prestige, and female gatherers, who were likewise keen on meat but put an even higher priority on reliable meals. Such conditions would have greatly increased the survival value of food sharing and division of labor, as well as the flexible residence patterns that allowed pregnant females to remain or move near relatives—including their own mothers—who could be counted on to help with provisioning.
Rare as it is in other apes for new mothers to have their own mother in the same group, many other primates, including most of the cercopithecine Old World monkeys, opt for matrilocal residence, and with good reason.13 Social support from matrilineal kin means that a female can forage, breed, and rear young more nearly on her own terms. By contrast, patrilocal residence puts females at a disadvantage with respect to how much freedom of movement or control over their reproductive lives they have.14 Whether daughters stayed near their mothers or returned to be with them at childbirth, or whether mothers themselves moved, the added support from having matrilineal kin nearby would have made new mothers feel more secure and would have promoted the kind of interindividual trust essential for hominin mothers to be willing to share care of their young.
For my money, such a living arrangement where mothers had nearby kin and came to depend upon assistance from others in rearing their young provides the most promising answer to the question “Why us and not them?” Other apes in the early Pleistocene would have benefited every bit as much as humans from being better able to read the intentions of competitors or from evolving an even more Machiavellian intelligence than they did. One need only recall the fierce and highly strategic intra-and intergroup competition that researchers like Goodall, de Waal, Mitani, Nishida, and Wrangham all document for common chimpanzees to wonder why enhanced capacities for mind reading and cooperation did not evolve in them as well. The most compelling solution to the puzzle, in my view, has to do with the cognitive and emotional implications of cooperative breeding.
I don’t think humans ended up with greater inter-individual tolerance, aptitudes for mind reading and learning, and with them greater capacities for cooperation than other apes because they already possessed sapient-sized brains, symbolic thinking, and sophisticated language. Rather, I am convinced that our line of hominins ended up with these attributes because of an unprecedented convergence—the evolution of cooperative breeding in a primate already possessing the cognitive capacities, Machiavellian intelligences, and incipient “theory of mind” typical of all Great Apes. The ancestors of humans started from a different place than chimpanzees did.
From the outset, I have stressed that no one knows for sure when hominins began to share care. Nor do we know when hominins began to undergo the cognitive and emotional transformations that laid the groundwork for higher levels of cooperation—transformations that would eventually became hallmarks of the human species. Perhaps one day new methods for analyzing fossil bones and teeth will yield new insights into when hominin mothers started to wean babies earlier than other apes did, or began surviving longer. Perhaps comparisons between different ape genomes (including ancient DNA from fossil hominins) will shed light on when postpartum mothers became more tolerant of others. So far, how
ever, the fossil record has not yielded a definitive answer.
WHEN DID HUMANS BECOME EMOTIONALLY MODERN?
Almost all of those who study child development now accept the primate origins of human infants’ need to feel secure (the basis of attachment theory). But few speculate on the evolutionary origins of humankind’s unusual capacities for intuiting intentions, learning from others, sharing resources, and communicating ideas. Of the select few who have published on this topic, most assume that the quest for intersubjective engagement with others emerged later in the Pleistocene than suggested above, within the last 200,000 years, more or less concurrently with anatomically and behaviorally modern humans—big-brained animals complete with language and symbolic culture.
“After they understand others as intentional agents like themselves,” writes Michael Tomasello, “a whole new world of intersubjectively shared reality begins to open up. It is a world populated by material and symbolic artifacts that members of their culture, both past and present, have created for the use of others.”15 Karlen Lyons-Ruth of Harvard Medical School, another developmental psychologist whose ideas about the origins of intersubjective engagement have profoundly influenced me, likewise tilts toward a late Pleistocene origin. As she puts it, humans used their new awareness about mental states “to learn from and transmit knowledge to others and [this] capacity for conceiving of other minds accounts for the explosive rate of cultural evolution over the last 200,000 years.”16
None of these researchers claims to know for sure when humans became emotionally modern. But there has been this tendency to assume that new aptitudes for mind reading coevolved with language, symbolic thinking, new modes of cultural transmission, and art—in other words, to assume that emotional modernity emerged among humans who already possessed sapient-sized brains as well as language and symbol manipulation and who combined quests for intersubjective engagement with modern capacities for learning and cultural evolution. That is, in the late Paleolithic. Although there are hints that hominins might have been molding clay into anthropomorphic forms as far back as 250,000 years ago, the first unambiguous evidence that people were looking about their world and selecting materials in order to “make them special” come from the Middle Paleolithic, some 150,000 years later. This evidence includes barbed points made of bone (for a spear, perhaps, or a harpoon?) and perforated shell beads (for jewelry?). From about 30,000 years onward, cave paintings and carvings testify unequivocally to humankind’s signature creativity.17
Without question, the creators of the carefully observed and beautifully rendered lions, bison, and rhinoceroses on the rock walls of places like Chauvet Cave in France held complicated belief systems and were interested in sharing their personalized symbolic worlds with others. These artists were capable of new modes of learning and cultural transmission, and they built on one another’s inspiration and techniques. Indeed, at least some of the paintings were probably joint products created by many hands over time. It is not much of a stretch to assume that individuals who punctuated their paintings of animals with shamanlike half-men/half-animal bipeds would judge others according to whether they conformed to the same belief system. This represents an enormous divide between humans and other animals.18
Other primates possess social conventions as well, and individuals that fail to conform may be at a disadvantage. For example, sick animals who behave strangely may be shunned or ostracized, while those who ignore dominance protocols may be attacked. But we have no evidence that other animals monitor anything other than overt phenotypic signals such as physical appearance, smells, or behavior.19 In the case of behaviorally modern humans, however, socially transmitted knowledge is cumulative, resulting in increasingly elaborate conventions to which group members may be expected to conform, often in very detailed and seemingly arbitrary ways, as in ritual or public ceremonies. As the ethnographer and evolutionary ecologist Kim Hill points out, people everywhere become inordinately concerned not only with how others perform but with how they feel, think, and believe, and they monitor such conformity both in ceremonial contexts and in everyday life. Failures to conform may generate deep feelings of guilt or cause others to be angry. Given the universality of such emotions, they presumably predated the time within the past 200,000 years when the common ancestors of all modern humans migrated out of Africa—but by how much?20
To answer this question, it is important, first, to distinguish between tangible manifestations of behavioral modernity expressed in art and language, and intangible manifestations of emotional modernity expressed in the attention individuals pay to what others are thinking and feeling. Although the linguistic and symbolic gifts of behaviorally modern humans allow them to take intersubjectivity to new heights, in and of itself intersubjective engagement does not require language or symbol manipulation. Indeed, the former almost certainly evolved before the latter. The ability to intuit the needs and desires of others and respond appropriately doesn’t even require much of a brain—recall the tiny-brained meerkat allomothers who take developmental stage into account when teaching youngsters how to eat dangerous scorpions. There is no reason why emotional modernity could not have evolved long before humans became behaviorally or even physically modern.
Infants whose brains are immature and who cannot yet talk or draw pictures are nevertheless attuned to the expectations and emotional reactions of caregivers. As the psychologist Vasudevi Reddy and others have shown, children less than a year old exhibit embarrassment and what looks very much like shame, as if they are acutely aware of how they might have failed to meet the expectations of someone else. These infants are not just afraid of punishment (other animals—dogs, for example—when caught doing something they were trained not to do, can act “embarrassed”). Rather, at a much earlier age than previously realized or even considered possible, and long before they acquire language, human children appear to monitor what others think of them and care deeply about what others feel and intend. By age four, around the time a child in a foraging society would be weaned, modern children begin to use their intersubjective gifts and growing language skills in quite sophisticated ways, not only to intuit what others want but also to intuit what they want to hear. Four-year-olds are already able to use such knowledge to flatter others and to ingratiate themselves with the sort of people upon whom children’s survival once depended.21
At some point in the course of hominin evolution, then, our ancestors adopted modes of food sharing and childcare that were very different from those observed in other apes, with profound implications for the nature of maternal commitment, the intention-reading aptitudes of their young, and the prosocial impulses of other group members. That is why in a book about the origins of emotionally modern humans I focused on the early hominin prequel rather than on the main human feature film, the great cultural leaps forward that cooperative tendencies and language eventually made possible. For in such modest beginnings—perhaps as long as two million years ago—I believe we can identify the groundwork for spectacular later developments.
If communal childcare goes back in the human lineage as far as I believe it does, then quests for intersubjective engagement emerged among creatures who looked quite different from us, who could not talk nor transmit knowledge the same way we do, but who were already attributing mental states to others and empathizing with them more than living Great Apes do. But regardless of whether emotional modernity originated among Homo sapiens or Homo erectus—that is, among people who either did or did not look like us and behave like us, and who did or did not use language the way we do—at some point human mothers began to bear offspring too costly to rear by themselves. This made a mother’s commitment to any given child contingent on her perception of social support. Indeed, I have wondered whether this might not be one reason for the correlation researchers still find between a new mother’s perception of low social support and postpartum depression.22 My focus here, however, is not on the psychological risks to mothers but on the psychological
risks that prolonged dependency and highly contingent commitment set up in infants, who, unlike other apes, lack the same guarantee of maternal succor.
This reconstruction is based on fossils from Dmanisi, the Republic of Georgia (sometimes designated Homo georgicus), and shows how Homo erectus might have looked 1.8 million years ago. Quite possibly these distant ancestors were already beginning to reflect on the subjective mental states and intentions of others, long before anatomically modern, large-brained Homo sapiens emerged. (Sawyer and Deak 2007:155, Nèvraumont Publishing)
NEW DIMENSIONS TO THE TIES THAT BIND
As novel contexts for development produced novel phenotypes and generated new selection pressures to act on those phenotypes, the outcome was little apes who were every bit as manipulative and socially astute as other apes but were in addition emotionally more sensitized to cues of commitment than any ape had ever needed to be before. The rest of course was history—literally. But there was a downside to such sensibilities.
All babies have different needs and priorities than their mothers do, and very different notions about when their mothers should carry them versus delegating their care to someone else or providing no immediate attention at all.23 Like all primates, human infants need to feel connected. Just as Bowlby pointed out more than half a century ago, dread of separation is the most powerful motivating force in the lives of infant primates. But intersubjectively gifted human infants seek more than the security that comes from tactile contact (though they certainly seek that as well). These special-needs primates want (in the words of poet Daphne Merkin) to see their “singular passions” reflected in some “larger pond” of emotional attachment. They feel a chronic need to factor perceived intentions into their quest for reassurance.