By NICHOLAS WADE, www.nytimes.com
Published: November 25, 2003
Fossil bones record the history of the human form but they say little about behavior. A richer source on the way human social behavior evolved may come from chimpanzees, with whom people shared a common ancestor as recently as five or six million years ago.
From knowledge of chimp behavior, biologists can plausibly infer the social behavior of the shared human-chimp ancestor, and from that reconstruct the evolutionary history of human social behavior.
Such reconstructions are subject to much uncertainty and debate, especially when they imply a genetic basis to human behaviors like living in communities based on male kinship, or conducting lethal campaigns against neighbors. But the goal is to shed light on the full sweep of human social behavior, tracing its evolution from an apelike community with separate male and female hierarchies five million years ago to the family-based societies of today.
A principal assumption is that chimpanzees, unlike people, have changed little and therefore their social behavior is a good guide to that of the common ancestor. One support for this idea is that the earliest fossils on the human side after the split are very chimplike. Another is that the chimps of western and eastern Africa are hard to tell apart, despite some 1.5 million of years of separate evolution.
After 40 years of arduous study, biologists have put together a coherent, if not yet complete, picture of chimpanzee societies. From observations at several different sites in Africa, there is an "emerging consensus regarding chimpanzee social structure, territory characteristics and intergroup interactions," say two primatologists, Dr. Michael L. Wilson of the University of Minnesota and Dr. Richard W. Wrangham of Harvard, in an article in The Annual Review of Anthropology.
Chimps are not easy to observe. It can take five years for them to get used to watchers. The animals must be followed, through hilly country, for 15 hours at a stretch. They can be dangerous. Humphrey, a male chimp at the Gombe National Park in Tanzania, nearly killed Dr. Wrangham by hurling a large rock at his head. "He was just showing off," Dr. Wrangham says indulgently. Recently a female chimpanzee at Mahale, another Tanzanian site, attacked and severely wounded two researchers whom she considered to be allies of males that had killed her infant.
A major surprise has been that chimps turn out to live in territories whose borders are aggressively defended by roving parties of males. Jane Goodall, who pioneered long-term studies of chimps at Gombe, at first believed she was watching a single peaceful community. But as researchers started to follow animals throughout the day and watch their interaction with others, they found that groups of male chimps went out on border patrols, ready to attack and kill the males of neighboring communities.
The males in each community are related to one another because they spend their lives where they were born, whereas the females usually migrate to neighboring communities soon after reaching puberty, a practice that avoids inbreeding. This patrilocal system, of a community based on male kin bonding, is unusual, but familiar to anthropologists because it is practiced by most hunter-gatherer societies.
The males' operational strategy seems to be to defend a territory as large as possible so as to improve the community's food supply, which is principally fruit, and thereby their reproductive success. Dr. Anne Pusey of the University of Minnesota has found that the larger the female chimp's home feeding area, the shorter the interval between births.
In two known cases, a chimp community has wiped out all of a neighbor's males. Though the females may be absorbed into the victors' community, the basic goal seems to be getting rid of a rival rather than capturing females, since male chimps often attack strange females.
Within a community, there is a male hierarchy that is subject to what primatologists euphemistically call elections. Alpha males can lose elections when other males form alliances against them. Losing an election is a bad idea. The deposed male sometimes ends up with personal pieces torn off him and is left to die of his wounds.
Very few other species live in male-kin-bonded communities with female dispersal. And only two practice lethal raids into neighbors' territory to kill off vulnerable enemies. "This suite of behaviors in known only among chimpanzees and humans," Dr. Wrangham and Dale Peterson write in their book "Demonic Males."
Many chimp behaviors are ones that people can recognize. Much of their body language — a mother holding her baby, the expression of an orphaned infant — is instantly readable. Some 19 varieties of tool use by chimps have been noted, though each community has its own cultural subset. Chimps dose themselves with medicinal plants when they are sick. But where they differ most seriously from human societies is in their sexual arrangements.
Males and females do not associate in families but in separate hierarchies. Males make females defer to them, with violence whenever necessary, and every female is subordinate to every male. A female chimp advertises her fertile period with a visible swelling and is then so pestered by males that she may get to eat only at night. But the great advantage of mating with every male in the community in a public orgy is that it confuses paternity, significant insurance given that males are liable to kill infants they know are not their own.
Males sometimes get females to leave the group and give them exclusive matings during a cycle. These consortships are sometimes forced on the female, who may recognize the risk to any infant conceived by a known father.
Chaotic as the mating system of chimpanzees may seem, it works, at least in terms of the way their society is structured. According to DNA paternity tests conducted by Dr. Julie Constable at the University of Minnesota and colleagues, over a 20-year period alpha males sired one-third of the conceptions at Gombe and high-ranking males many of the others. Some females at Gombe do not leave for other communities, as is the chimp custom, yet most managed somehow to avoid the risk of inbreeding.
But Fifi, a Gombe-born female, was not so lucky. On her seventh conception her oldest son, Freud, was alpha male and, despite his namesake's suspicions, took no interest in her. But her second son, Frodo, a very large and aggressive male, showed no such inhibitions. He turned out to be the father of her son Fred. Perhaps in illustration of the dangers of in-breeding, Fred died as an infant during an epidemic of mange, Dr. Constable reports.
An intriguing variation on the chimpanzee social system is that of bonobos, which split from chimps some 1.8 million years ago. With bonobos, who live in Congo south of the Congo River, the female hierarchy is dominant to that of males, and males do not patrol the borders to kill neighbors. Though bonobos are almost as aggressive as chimps, they have developed a potent reconciliation technique — the use of sex on any and all occasions, between all ages and sexes, to abate tension and make nice.
Assuming the common ancestor of people and chimps had social behavior that was essentially chimplike, how much of that behavior has been inherited by people? The unusual behavioral suite of male kin bonding and lethal territorial aggression may look as if it has been inherited with little change. Among the Yanomamo, a South American tribe, the number of males who die from aggression is about 30 percent, the identical rate found among Gombe chimps.
Dr. Wrangham said the consistent pattern of aggression seen at all the chimp sites suggests that male chimps have "a strong emotional disposition" to be aroused by the sight of strange males, to form coalitions against enemies, to be sensitive to balances of power and to be attracted to hunting. The same disposition could have been inherited down the human lineage.
"But it's equally possible," he said, that territorial aggression becomes less advantageous at certain stages of human evolution, so that it died out and re-emerged in humans later. "People hate the thought that we've had millions of years of a disposition to attack each other because that makes it seem harder to get away from," he said.
Not everyone believes that chimp social behavior is a good guide to human evolution. "All these things are suggestive and point tantalizingly to things we want to know," said Dr. Ian Tattersall, a paleoanthropologist at the American Museum of Natural History. "We just have to bear in mind that none of this is demonstrable in any highly convincing way."
But Dr. Robert Foley, an evolutionary anthropologist at the University of Cambridge in England, thinks a lot can be learned from ape sociality about the evolution of human social institutions. A community size of 80 to 100 people, typical among chimps and hunter-gatherers, is one feature inherited from the common ancestor. Another is a society formed on the basis of male kin bonding. "Out of that flows the notion of the way males relate to each other, they way they form alliances, the levels of cooperation," he said.
During the hunter-gatherer phase of human existence, girls would have always grown up knowing they would leave home and spend most of their lives among strangers, whereas boys stayed in their natal communities. "So from that you would evolve different ways of dealing with people: women have to have had a flexibility and adaptability," he said.
Dr. Robin Dunbar, a human behavioral ecologist at the University of Liverpool, said he agreed that women "do seem better adapted" to moving between groups, but he was not convinced that human societies had always practiced patrilocality, the system of women leaving home. They may have switched between patrilocality and matrilocality, depending on local resource needs, he said.
A critical event in human social evolution must have been the transition from the male and female hierarchies of chimp society to the conjugal bonds between men and women. Both Dr. Wrangham and Dr. Foley believe the new mating system is likely to have occurred about 1.9 million years ago with the evolution of Homo erectus. The size difference between the sexes shrank sharply at that time. Dr. Wrangham sees the invention of cooking, which opened up a wide new range of more nutritional foods, as the spur to the females' larger size. Dr. Foley favors meat eating, which helped mothers bear children with larger brains.
The two explanations, which are not incompatible, both envisage human institutions as adapting to some important improvement in available resources. The upshot would have been that men found they attained greater reproductive success by spending more time with the mother of their children. The larger-brained offspring would have been harder to raise. That required a more dependable food supply, leading to a "much tighter bond between males and females," Dr. Foley said.
The movement away from the male hierarchy evidently continued to evolve because hunter-gatherer societies, in which modern humans have spent four-fifths of their existence, are notably egalitarian, with not an alpha male in sight. Then, with the first sedentary societies and the rise of agriculture some 10,000 years ago, the first signs of social ranking appear in the archaeological record, followed by the chieftains, priests, property rules and the coercive authority of modern states.
What mediates the switch from one type of society to another? The change from a chimp-style society to bonobo mode seems to involve genetic elements — the change in male and female behavior — as well as resources: bonobos can congregate in larger numbers because they live in areas with richer food supply.
Does a changing genetic template accompany the evolution of human social institutions, from male hierarchies to conjugal bonding, to egalitarianism, to the hierarchies of modern societies? Dr. Foley said that in explaining the evolution of human social institutions, "there must be both a set of human behavioral propensities, shaped by selection, and a pattern of historically specific developments" like population density and available resources.
Male kin bonding seems an ancient inheritance, but many features of modern society may be responses to the increasing population densities made possible by the agricultural revolution.
"Different things evolve and come on stream at different times," Dr. Foley said. "Kin bonding goes back to the common ancestor, but falling in love is recent."
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