Every one will admit that man is a social being. We see this in his dislike of solitude, and in his wish for society beyond that of his own family. Solitary confinement is one of the severest punishments which can be inflicted. . . . It is no argument against savage man being a social animal, that the tribes inhabiting adjacent districts are almost always at war with each other; for the social instincts never extend to all the individuals of the same species. Judging from the analogy of the majority of the Quadrumana, it is probable that the early ape-like progenitors of man were likewise social; but this is not of much importance for us. Although man, as he now exists, has few special instincts, having lost any which his early progenitors may have possessed, this is no reason why he should not have retained from an extremely remote period some degree of instinctive love and sympathy for his fellows. . . . As man is a social animal, it is almost certain that he would inherit a tendency to be faithful to his comrades, and obedient to the leader of his tribe; for these qualities are common to most social animals. He would consequently possess some capacity for self-command. He would from an inherited tendency be willing to defend, in concert with others, his fellow-men; and would be ready to aid them in any way, which did not too greatly interfere with his own welfare or his own strong desires.

THE YANOMAMO are a tribal people who dwell in remote forests on the border of Brazil and Venezuela. Until recent decades, they lived in a traditional manner, their practices unchanged by missionaries or other intruders from the civilized world. They dwell in settled villages and practice agriculture, deriving their staple food from their gardens of plantains, a kind of large cooking banana. The forest supplies many other prized foods, such as armadillos, and the delicious grubs, about the size of a mouse, that the Yanomamo harvest from the pith of palm trees and take home to roast.

The labor required to obtain food is a mere three hours a day. During their ample leisure time, the men snort hallucinogenic drugs prepared from a variety of forest trees while their shamans enter trances from which they communicate with the spirits and recite the myths of the Yanomamo world.

If life is so easy, why then do Yanomamo villages engage in almost continuous warfare with their neighbors? Villages entice others into alliances, bolstered with trade and ritual feasting, for the purpose of defending against or attacking rival coalitions. Not so rarely, the feasts are set-ups for a deadly massacre of the invited guests. The constant warfare carries a serious price. About 30% of all deaths among adult males are due to violence, according to Napoleon Chagnon, the anthropologist who has studied the Yanomamo over many decades.¹⁶⁹ Chagnon found that 57% of people over the age of 40 had lost two or more close relatives—a parent, sibling or child—to a violent death.

The Yanomamo way of life is entirely different from the daily experience of most people in developed economies. Yet it embodies all the institutions that are distinctive of human sociality, including warfare, trade, religion and a defined division of roles between the sexes. Where did these social institutions come from? Do they have biological roots or are they purely cultural? What is it that knits human societies together in the first place?

A possible answer to all these questions, though one for which there is at present no direct evidence, is that human social behavior is rooted in various ways in a genetic template that people have inherited from their primate forebears and that has been adapted throughout evolution to prevailing circumstances.

Those adaptations would seem to include a vigorous expansion of the chimpanzee propensity for territorial defense and aggression against fellow members of the same species. But they must also include a special array of quite different behaviors, ones that enable people to work effectively with others in large, complex societies. In chimpanzee groups, most of the males are related to each other; their common genetic interest is the glue that holds the group together. Humans have developed behaviors that enable even strangers to be treated as kin, a compact basic to all city life. These softer behaviors, which are as much a part of human nature as the propensity to kill and punish, provide the cohesion at the root of civilized societies.

The Dynamics of Primate Societies

The genes that influence human social behavior are inscribed somewhere in the genome but have not yet been recognized. Until they are, the best available guide to them has emerged from the new understanding of how chimpanzee and bonobo societies work. The two ape societies are quite different in character. That of chimps is male dominated and aggressive, whereas bonobos are female dominated and highly conciliatory. Presumably the elements of both kinds of behavior must have existed in the joint human chimp ancestor from which chimps and bonobos are descended. The social behaviors of the two apes therefore provide invaluable insights into the set of social behaviors that humans too may have inherited from the common primate ancestor.

Chimp society evolved, as might be expected, for the purpose of maximizing the reproductive success of its members. The society’s structure seems to be carefully attuned to chimps’ general environment, just as the very different social structure of bonobos is appropriate to their environment. Human societies too have a range of different structures, each of which can be seen as a solution to a particular environmental problem. The egalitarian structure of hunter-gatherer societies is well suited to managing the risk of uneven hunting success. The hierarchical structure of settled societies may be a more efficient way of administering surpluses and trade.

The templates for chimp and human social behavior are very similar in a central feature, that of territorial defense and the willingness to solve the problem of a hostile neighboring society by seeking its extermination. But they differ in other critical aspects. Humans have evolved a different relationship between the sexes, based on family units instead of separate male and female hierarchies. These family units require a considerably higher level of trust among males, enabling them to band together for social purposes like warfare with a reasonable degree of confidence that others will not steal their wives. Second, all human societies support institutions not found in the chimp repertoire. These include property rights, a propensity for ceremony, ritual and religion, and elaborate systems of trade and exchange, based on a universal expectation of reciprocity.

Chimpanzee groups, like primitive human societies, are held together by bonds of kin relationships, the evolutionary basis of which is well understood. But kin-bonded societies cannot grow beyond a certain size. Humans, with their special gift of language, have developed ways to knit together large groups of unrelated individuals. One of these binding forces is religion, which may have emerged almost as early as language.

Because of the richness of human culture, it is hard to define the genetic underpinnings of human social behavior. It is much easier to see a set of social behaviors, presumably genetically defined, among our primate cousins. Chimpanzees have been studied in the wild for some 45 years by two pioneers, Jane Goodall at Gombe and Toshisada Nishida at Mahale, both in Tanzania, and by their successors at these and several other sites in Africa. Only in recent years, as the fruit of much arduous research, has the big picture come together. Biologists can now explain many deep features of chimp society and how its components work. The dynamics of chimp society bear directly on the better-concealed game plan of human societies.

Though Goodall at first believed the chimpanzees at Gombe lived in one big happy group, it later became clear, through the stimulus of Nishida’s research, that the opposite is the case. Chimps are divided into communities of up to 120 members, which occupy and aggressively defend specific territories.

A chimp community never assembles as a whole. Chimps move around in bands of 20 or so, with shifting membership, in what chimp watchers call a fission-fusion society. The females often feed alone with their offspring or in small nursery parties. A striking parallel with human societies is that these communities are patrilocal, meaning the males stay in their home territory and females move to find mates in neighboring territories. Female chimps generally leave their home communities at the age of puberty and join other communities, whose males tend to find them more attractive than their own.

Most hunter-gatherer societies are patrilocal, in the sense that the wife goes to live with the husband’s family. The biological reason is to avoid inbreeding, a problem faced by all social animals. But the almost universal solution in the primate world is matrilocality: the females stay put and the males disperse at puberty. Patrilocality is the exception to the rule, and has probably evolved in only four other primate species besides humans and chimpanzees.¹⁷⁰

A second unusual feature of a chimp community, but one that chimps also share with people, is a propensity to conduct murderous raids on neighbors. Male chimps not only defend their territory but conduct regular, often lethal, attacks on neighboring communities. This discovery came as a considerable surprise to many biologists and sociologists who had assumed that warfare was a uniquely human phenomenon.

Why do chimps hold and defend territories in the first place? Why do they kill each other? Chimp researchers believe they have been able to unlock the basic logic of chimp social structure, at least in general outline. Chimp society turns out to be matched to the nature of the food supply, which is principally fruit. The trees come into fruit sporadically. They tend to be scattered and do not supply enough fruit for large parties of chimpanzees. Female chimps, needing to sustain themselves and their young, find it more efficient to feed by themselves. They live in home areas, a few square kilometers in size, which they rarely leave. The size of these areas is very important. Females have shorter intervals between births—in other words bear children faster—when their territory is larger, according to an analysis of Gombe chimps by Jennifer Williams and Anne Pusey.

Considering strategies for the male chimps, each could try to achieve reproductive success by guarding one female. But it seems to be more efficient for the males to band together and defend territory that includes a larger number of females. One reason that this makes sense is that the males tend to be related to each other, because of the patrilocal system, and therefore in defending a group of females they are assisting their male relatives’ reproductive efforts as well as their own. An individual’s kin carry many of the same genes as he does. As the evolutionary biologist William Hamilton pointed out in his doctrine of inclusive fitness, for a person to help get an equivalent number of his kin’s genes into the next generation is about as good as propagating his own. This is why genes favoring altruistic behavior have evolved in kin-based societies. The same logic underlies the cohesiveness of ant and bee societies, whose workers, by a special quirk of insect genetics, are more closely related to their sisters than to any daughters they might have. Because of this relationship, the workers have forsaken their own chance to raise children entirely and are content to live as sterile nurses for the queen’s children.

In chimpanzee society, males and females do not generally spend much time together except for the purpose of mating. The members of each sex are organized in separate hierarchies. Every adult male demands deference from every female, resorting to immediate violence if a submissive response is not forthcoming. Differences notwithstanding, chimp and human societies serve the same purpose, that of providing males and females appropriate ways of securing their individual reproductive advantage.

At the head of the male hierarchy is the alpha male, who maintains his position by physical strength and, just as importantly, by building alliances with other males. “A dominant male is constantly at risk from opportunistic coalitions formed by lower-ranking individuals and must continually assert his dominance through agonistic display,” write John Mitani and colleagues.¹⁷¹ These tests of leadership, which primatologists sometimes refer to ironically as elections, can occur at any time. Losing an election in chimp society is not a good idea. The loser’s defeat may take the form of having personal parts torn off of him and being left for dead. Long rule does not guarantee a peaceful retirement. Ntologi was alpha male at Mahale for 16 years before he was overthrown by a rival coalition and killed.

What is the upside of being alpha male if life is a daily gamble on retaining power, with violent death the only retirement plan on offer? Whether or not chimps ponder this question, evolution has provided the answer: high position in a chimp male hierarchy guarantees that a male will have more matings and more progeny.

This outcome was at first far from obvious to researchers. When females enter their fertile period, they advertise the fact with melon-sized pink swellings on their rear end. They become very gregarious and do their best to mate with every male in the community, with an average of 6 to 8 couplings a day. One female observed by Goodall achieved 50 copulations in one day.¹⁷² The females’ purpose, biologists believe, is to confuse paternity. If a male chimp believes there is a chance a baby is his, he is less likely to kill it.

Given this seemingly chaotic mating system, how do high ranking males in fact reap their due rewards of office? First, they do secure more matings, even though rarely exclusive ones. Second, there is the phenomenon of sperm competition. Because of the chimps’ multiple mating system, advantage will accrue to the male who can deliver the most sperm and flood out the competition. Hence evolution has favored male chimps with very large testes for their body size. But whether or not the senior males reaped the rewards of rank was unclear until the advent of DNA paternity testing. Julie Constable and colleagues recently reported the results from a 20-year study of chimps at the Kasakela community in Gombe. They found that despite appearances, the system works. The reigning alpha male accounted for 36% of all conceptions, and for 45% if one excludes his close female relatives, with whom conceptions would be avoided.¹⁷³ Another 50% of matings were scored by high ranking males. Usually at Gombe there is the alpha male and then two or more other males who count as high ranking.

Most of these conceptions studied by Constable occurred during general free-for-all sexual romps, or “opportunistic matings,” as the primatologists call them, suggesting that the alpha males owe of a lot of their fatherhoods to victory in the sperm competition wars.

Like males, female chimps have a hierarchy. It is less discernible, because females spend much of their time feeding alone in their core areas and are not in constant interaction as the males are, but it bears significantly on the females’ reproductive success.¹⁷⁴ Low ranking females lose more of their babies than do socially ascendant females. This is partly because socially superior females will sometimes kill the infants of lowly females. The high ranking Passion and her daughter Pom snatched and ate the babies of their neighbors at Gombe, perhaps to discourage trespass on their feeding areas.

What makes one female dominant over another is not yet clear, but in general terms rank in chimpanzee society seems to depend a lot on one’s mother’s status. Flo, a high ranking and sexually attractive female, was the mother of Figan, who was alpha male in his Gombe community for 10 years (his reign date was 1971-1981), as well as of Fifi, who became dominant female. Fifi helped her firstborn son Freud take his first steps to power by intervening on his side when, as an adolescent, he started to establish dominance over the females. Freud was alpha male from 1994 to 1998, when he fell sick with mange and was deposed by his younger brother Frodo.

Historians attribute dynastic wars among people to all kinds of complex motives, from glory to territorial gain to spread of religion. Chimpanzees’ intentions, unobscured by such rationalizations, can be judged strictly by their results. It’s all about reproductive advantage. Each player acts so as to get as many descendants as possible into the next generation. The males try to ascend the male dominance hierarchy so as to mate with as many females as possible. The females seek out the best feeding areas so as to bear as many surviving children as possible. The ultimate objective is simple, but in a complex society each individual must act in many intricate ways to achieve it.

Presumably chimps’ social behavior is genetically shaped, but like human societies they have culture too, in the sense of learned behavior that varies from one chimp community to another. In a recent survey of seven long term chimp studies, Andrew Whiten and colleagues identified 39 behaviors that differed from one community to another without obvious ecological explanation.¹⁷⁵ All chimpanzee communities habitually use tools, but the use of tools varies widely from one chimp community to another. Chimps in the Tai forest in the Ivory Coast use stones as hammers to open nuts; Gombe’s chimps have never learned or invented this useful art. Not a single case of habitual tool use has yet been observed among bonobos.¹⁷⁶ That suggests that chimps have a genetic propensity to use tools and bonobos do not.

If the variations between chimp communities are mostly due to culture, the constant features of chimp social behavior are probably framed by genes. And presumably a shift in that genetic framework for social behavior explains the difference between chimpanzees’ social arrangements and those of their cousins, the bonobos, from whom they have been separated for some two million years.

The Bonobo Alternative

Bonobos are so similar to chimpanzees in physical appearance that it took biologists many years to recognize that they are a separate species. Their behavior, however, is very different. Unlike in chimp societies, where males may violently coerce females to respect them, in bonobo land the females run the show. They manage this feat by forming close alliances with each other and facing down any male who tries to interfere in their affairs. Because of their dominance, they have managed to banish infanticide, the worst fear of female chimpanzees.

Bonobos have captured the attention of their human observers because they use sex not just for reproduction but also as a social greeting and general reconciliation technique. Bonobo sexual physiology has a small but socially critical difference from that of chimpanzees. Male chimpanzees seem to be able to tell, probably by smell, the almost exact time when a female is ovulating, setting off fierce competition for her favors. But bonobo ovulation, as with humans, is concealed. The males, who get to have sex with the females almost all the time anyway, do not enter into ferocious competition with each other because the goalpost, as it were, is no longer in sight. Bonobo social arrangements do a superb job, from the female point of view, of making paternity utterly obscure.

Bonobo communities are considerably less aggressive to each other than are those of chimpanzees. There are no border patrols by groups of males looking for trouble. Groups from two communities have even been observed mingling peacefully, to the astonishment of chimpanzee biologists.

Why is bonobo behavior so different from that of chimpanzees? The answer seems to be that bonobo society has evolved in adaptation to a subtle but profound difference in the bonobo environment. Following is the analysis offered by Richard Wrangham, a chimpanzee expert at Harvard University, based in part on the observations of bonobos by the Japanese primatologist Takayoshi Kano and his colleagues.

At first sight, there is no obvious ecological difference between chimp habitat and bonobo habitat. They both live in tropical rain forests, although the chimps inhabit some more open woodland as well. Chimps are found all across tropical Africa, from the west coast to the east, but bonobos live south of the Zaire river, and chimps live north of it.

The river is a barrier, and south of the river there are no gorillas. Gorillas are voracious eaters of herbaceous plants. The chimps north of the river, who share their territory with gorillas, eat only fruit, leaving the herbs for the gorillas. But the bonobos south of the river eat both, and their teeth are specially adapted for shearing herbs.

This difference in diet has far reaching consequences. Female chimpanzees forage alone, in their core feeding areas, because that is the most efficient way to get enough to eat. But since bonobo forests have more sources of food, bonobos can travel in larger parties with a more stable membership. This gives the females the opportunity to bond together, which they do with the usual bonobo social lubricant—plenty of sex. “Party stability, in other words, produced female power,” Wrangham says.¹⁷⁷

With both chimps and bonobos, social structure is designed so as let each species make best use of its environment. Considerable genetic change must have occurred for bonobos to evolve from a chimpanzee-like ancestor. ¹⁷⁸ Bonobo males had to become less aggressive, females more adept at forming coalitions powerful enough for their hierarchy to control that of the males.

Although the point cannot yet be proved, it seems more likely that bonobos are descended from chimpanzees, rather than the other way around. Still, both are descended from the joint ancestor of chimps and humans, and the joint ancestor presumably included both chimplike and bonobolike features in its behavioral template. That makes it easier to understand how humans came by their contradictory impulses of aggression and conciliation. “Being both systematically more brutal than chimps and more empathic than bonobos, we are by far the most bipolar ape,” writes the primatologist Frans de Waal. “Our societies are never completely peaceful, never completely competitive, never ruled by sheer selfishness, and never perfectly moral.”¹⁷⁹

The Costs and Benefits of Warfare

Besides being well adapted or designed for their environments, chimp and human societies possess another salient feature in common, that of a strong propensity to kill their own kind. A willingness to kill members of one’s own species is apparently correlated with high intelligence. It may be that chimps and people are the only species able to figure out that the extra effort required to exterminate an opponent will bring about a more permanent solution than letting him live to fight another day.

Military skills are probably underappreciated as a biological phenomenon, but in their own way are just as remarkable a human adaptation as is the artistic ability of the Upper Paleolithic cave painters. Warfare of the human kind has many levels of complexity and at its highest is an integral component of statecraft. At the lower end of the scale, however, it overlaps closely in both tactics and goals with the chimpanzee variety.

Chimp warfare takes the form of bands of males who patrol the borders of their territory, looking for an individual of the neighboring community who has been rash enough to feed alone. Occasionally they make raids deep into enemy territory. “Behavior during patrols is striking and unusual,” writes the primatologist John Mitani. “Males are silent, tense and wary. They move in tight file, often pause to look and listen, sometimes sniff the ground, and show great interest in chimpanzee nests, dung, and feeding remains.” Just like human raiders, they are tense and nervous.¹⁸⁰

Chimpanzees carefully calculate the odds and seek to minimize risk, a very necessary procedure if one fights on a regular basis. They prefer to attack an isolated individual and then retreat to their own territory. If they encounter an opposing patrol they will assess the size of their opponents’ party and retreat if outnumbered. Researchers have confirmed this behavior by playing the call of a single male through a loudspeaker to chimp parties of various sizes. They find that the chimps will approach as long as they number three or more; parties of two will slink away. Three against one is the preferred odds: two to hold the victim down and a third to batter him to death.

The raid is also the principal kind of warfare conducted by primitive human societies. Yanomamo raids too are carefully calculated to minimize risk. “The objective of the raid is to kill one or more of the enemy and flee without being discovered,” writes Napoleon Chagnon.¹⁸¹

Warfare is a bond that separates humans and chimps from all other species. “Very few animals live in patrilineal, male-bonded communities wherein females routinely reduce the risks of inbreeding by moving to neighboring groups to mate,” write Richard Wrangham and Dale Peterson. “And only two animal species are known to do so with a system of intense, male-initiated territorial aggression, including lethal raiding into neighboring communities in search of vulnerable enemies to attack and kill. Out of four thousand mammals and ten million or more other animal species, this suite of behaviors is known only among chimpanzees and humans.”

In their resort to warfare, both chimps and human societies, at least those like the Yanomamo, have the same essential motivation. The chimps are defending fruit tree territory for the females, for their own reproductive advantage. The Yanomamo have the same idea in mind. Capture of women is seldom the prime reason for a raid but is an expected side benefit. A captured woman is raped by all members of the raiding party, then by everyone back home who wishes to do so, and is then given to one of the men as a wife.

But the real reproductive advantage of participating in a raid derives from the prestige of killing an enemy. When a man has killed someone he must perform a ritual purification called a unokaimou to avert retaliation by the soul of his victim. Those who have undergone this ritual are called unokai , and it is well known who they are. The unokais, Chagnon found, have on average 2.5 times as many wives as men who have not killed, and over three times as many children.

Chagnon’s study of the Yanomamo is unusual because he has studied them over such a long period of time. Despite the thoroughness of his fieldwork, some anthropologists have been reluctant to accept his conclusions, resisting the idea that violence could be reproductively rewarding. One critic, Marvin Harris, suggested that Yanomamo warfare was driven by a scarcity of protein. Chagnon describes the Yanomamo’s reception of this idea. “I explained Harris’s theory of their warfare to the Yanomamo: ‘He says you are fighting over game animals and meat, and insists that you are not fighting over women.’ They laughed at first, and then dismissed Harris’s view in the following way: ‘Yahi yamako buhii makuwi, suwa kaba yamako buhii barowo!’ (‘Even though we do like meat, we like women a whole lot more!’)”¹⁸²

Why would the Yanomamo pursue a way of life with such a high risk of violent death? The greater reproductive advantage of being a unokai is the obvious answer, a motivation that of course need not be conscious. Chimpanzees provide the same answer as the unokais, and bear an almost identical cost. In Gombe, some 30% of adult males died from aggression, the same toll as among the Yanomamo. A man or chimp may die defending his territory, but he still has a chance of propagating his genes. The males who may profit from his sacrifice are his relatives and carry many of the same genes. Raiders will be rewarded and have sons of similar character. That is the logic of patrilocality.

The Efficacy of Primitive Warfare

A propensity for warfare is prominent among the suite of behaviors that people and chimpanzees have inherited from their joint ancestor. The savagery of wars between modern states has produced unparalleled carnage. Yet the common impression that primitive peoples, by comparison, were peaceful and their occasional fighting of no serious consequence is incorrect. Warfare between pre-state societies was incessant, merciless, and conducted with the general purpose, often achieved, of annihilating the opponent. As far as human nature is concerned, people of early societies seem to have been considerably more warlike than are people today. In fact, over the course of the last 50,000 years, the human propensity for warfare has probably been considerably attenuated.

“Peaceful pre-state societies were very rare; warfare between them was very frequent, and most adult men in such groups saw combat repeatedly in a lifetime,” writes Lawrence H. Keeley, an archaeologist at the University of Illinois at Chicago. Primitive warfare was conducted not by arrays of troops on a formal battlefield, in the western style, but by raids, ambushes and surprise attacks. The numbers killed in each raid might be small, but because warfare was incessant, the casualties far exceeded the losses of state societies when measured as a percentage of population. “In fact, primitive warfare was much more deadly than that conducted between civilized states because of the greater frequency of combat and the more merciless way it was conducted. Primitive war was very efficient at inflicting damage through the destruction of property, especially means of production and shelter, and inducing terror by frequently visiting sudden death and mutilating its victims.” ¹⁸³

Keeley’s conclusions are drawn from the archaeological evidence of the past, including the Upper Paleolithic period, and from anthropological studies of primitive peoples. These include three groups of foragers that survived until recent times—the !Kung San, Eskimos and Australian aborigines—as well as tribal farmers such the Yanomamo of Brazil and the pig and yam cultivating societies of New Guinea.

To minimize risk, primitive societies chose tactics like the ambush and the dawn raid. Even so, their casualty rates were enormous, not least because they did not take prisoners. That policy was compatible with their usual strategic goal: to exterminate the opponent’s society. Captured warriors were killed on the spot, except in the case of the Iroquois, who took captives home to torture them before death, and certain tribes in Colombia, who liked to fatten prisoners before eating them.

Warfare was a routine occupation of primitive societies. Some 65% were at war continuously, according to Keeley’s estimate, and 87% fought more than once a year.¹⁸⁴ A typical tribal society lost about 0.5% of its population in combat each year, Keeley found. Had the same casualty rate been suffered by the population of the twentieth century, its war deaths would have totaled two billion people.

On the infrequent occasions when primitive societies fought pitched battles, casualty rates of 30% or so seem to have been the rule. A Mojave Indian war party was expected to lose 30% of its warriors in an average battle. In a battle in New Guinea, the Mae Enga tribe took a 40% loss. At Gettys burg, by comparison, the Union side lost 21%, the Confederates 30%.

An archaeologist, Steven LeBlanc of Harvard University, recently reached similar conclusions to Keeley after an independent study. “We need to recognize and accept the idea of nonpeaceful past for the entire time of human existence,” he writes. “Though there were certainly times and places during which peace prevailed, overall, such interludes seem to have been short-lived and infrequent. . . . To understand much of today’s war, we must see it as a common and almost universal human behavior that has been with us as we went from ape to human.”¹⁸⁵

Primitive warriors were highly proficient soldiers, Keeley notes. When they met the troops of civilized societies in open battle, they regularly defeated them despite the vast disparity in weaponry. In the Indian wars, the U.S. Army “usually suffered severe defeats” when caught in the open, such as by the Seminoles in 1834, and at the battle of Little Bighorn. In 1879 the British army in South Africa, equipped with artillery and Gatling guns, was convincingly defeated by Zulus armed mostly with spears and ox-hide shields at the battles of Isandlwana, Myer’s Drift and Hlobane. The French were seen off by the Tuareg of the Sahara in the 1890s. The state armies prevailed in the end only through larger manpower and attritional campaigns, not by any superior fighting skill.

How did the warriors of primitive societies get to be so extraordinarily good at their craft? By constant practice during some 50,000 years of unrestrained campaigning. Even in the harshest possible environments, where it was struggle enough just to keep alive, primitive societies still pursued the more overriding goal of killing one another. The anthropologist Ernest Burch made a careful study of warfare among the Eskimos of northwest Alaska. He learned, LeBlanc reports, “that coastal and inland villages were often located with defense in mind—on a spit of land, or adjacent to thick willows, which provided a barrier to attackers. Tunnels were sometimes dug between houses so people could escape surprise raids. Dogs played an important role as sentinels. The goal in all warfare among these Eskimos was annihilation, Burch reported, and women and children were normally not spared, nor were prisoners taken, except to be killed later. Burning logs and bark were thrown into houses to set them on fire and to force the inhabitants out, where they could be killed. Burch’s study reveals that the surprise dawn raid was the typical and preferred war tactic, but open battles did occur.”

Both Keeley and LeBlanc believe that for a variety of reasons anthropologists and their fellow archaeologists have seriously underreported the prevalence of warfare among primitive societies. “While my purpose here is not to rail against my colleagues, it is impossible to ignore the fact that academia has missed what I consider to be some of the essence of human history,” writes LeBlanc. “I realized that archaeologists of the postwar period had artificially ‘pacified the past’ and shared a pervasive bias against the possibility of prehistoric warfare,” says Keeley.

Keeley suggests that warfare and conquest fell out of favor as subjects of academic study after Europeans’ experiences of the Nazis, who treated them, also in the name of might makes right, as badly as they were accustomed to treating their colonial subjects. Be that as it may, there does seem a certain reluctance among archaeologists to recognize the full extent of ancient warfare. Keeley reports that his grant application to study a nine-foot-deep Neolithic ditch and palisade was rejected until he changed his description of the structure from “fortification” to “enclosure.” Most archaeologists, says LeBlanc, ignored the fortifications around Mayan cities and viewed the Mayan elite as peaceful priests. But over the last 20 years Mayan records have been deciphered. Contrary to archaeologists’ wishful thinking, they show the allegedly peaceful elite was heavily into war, conquest and the sanguinary sacrifice of beaten opponents.

Archaeologists have described caches of large round stones as being designed for use in boiling water, ignoring the commonsense possibility that they were slingshots. When spears, swords, shields, parts of a chariot and a male corpse dressed in armor emerged from a burial, archaeologists asserted that these were status symbols and not, heaven forbid, weapons for actual military use. The large number of copper and bronze axes found in Late Neolithic and Bronze Age burials were held to be not battle axes but a form of money. The spectacularly intact 5,000-year-old man discovered in a melting glacier in 1991, named Ötzi by researchers, carried just such a copper axe. He was found, Keeley writes dryly, “with one of these moneys mischievously hafted as an ax. He also had with him a dagger, a bow, and some arrows; presumably these were his small change.”

Despite the fact that the deceased was armed to the teeth, archaeologists and anthropologists speculated that he was a shepherd who had fallen asleep and frozen peacefully to death in a sudden snowstorm, or maybe that he was a trader crossing the Alps on business. Such ideas were laid to rest when an X-ray eventually revealed an arrowhead in the armed man’s chest. “In spite of a growing willingness among many anthropologists in recent years to accept the idea that the past was not peaceful,” LeBlanc comments, “a lingering desire to sanitize and ignore warfare still exists within the field, Naturally the public absorbs this scholarly bias, and the myth of a peaceful past continues.”

If primitive societies of the historic past were heavily engaged in warfare, it seems quite possible that their distant ancestors were even more aggressive. A genetic discovery made as part of a study of mad cow disease lends some credence to this idea.

The Skeleton in the Human Past

Among the least appetizing aspects of primitive warfare is cannibalism. Cannibalism implies the existence of warfare since the victims do not voluntarily place themselves on the menu. Anthropologists and archaeologists have long resisted the idea that cannibalism took place in the peaceful past. In his 1979 book Man-Eating Myth, William Arens, an anthropologist at the State University of New York at Stony Brook, argued that there was no well attested case of cannibalism and that most reports of it were propaganda made by one society to establish its moral superiority to another. Christy G. Turner, an archaeologist at Arizona State University, met only disbelief when he first proposed that the cut, burned, and defleshed bones of 30 individuals at a site occupied by Anasazi Indians were the remains of an ancient cannibal feast. His critics attributed the cuts on such bones to scavenging animals, funerary practices, the roof falling in—anything but anthropophagy.

Though some accounts of cannibalism may well have been fictive, Turner and Tim White of the University of California at Berkeley have now found cannibalized human remains at 25 sites in the American southwest. Turner believes these are the work of Anasazi Indians who dominated the area between AD 900 and 1700 and used cannibalism as an instrument of social control. Cannibalism has been reported from Central and South America, Fiji, New Zealand and Africa. The Aztecs made a state practice of sacrificing captives and their civilization has furnished a recipe for human stew. A common belief that accompanies ritual cannibalism is the notion that by eating particular parts of the victim, often a slain warrior, the consumer absorbs his strength or courage. The frequency of reports of cannibalism by societies in all regions of the world suggests, Keeley concludes, “that, while hardly the norm, ritual consumption of some part of enemy corpses was by no means rare in prestate warfare.”

Could cannibalism in fact have been so widespread and so deeply embedded in human practice as to have left its signature in the human genome? This gruesome possibility has emerged from the work of English researchers trying to assess the likely extent of the outbreak of mad cow disease among Britons who had eaten tainted beef. Mad cow disease belongs to a group of brain-eroding pathologies caused by misshapen brain proteins known as prions. Contrary to the expectations of British agricultural officials, prions can cross species barriers; cow prions, which rot cows’ brains, can also rot human brains if the cow’s neural tissue is eaten.

Even more effective at rotting the human brain are human prions. People are at risk of exposure to human prions when they eat other people’s brains. This was a regular practice among the Fore of New Guinea who, sometime around the year 1900, adopted the novel funerary practice of having women and children eat the brains of the dead. By about 1920, the first case of a brain-wasting disease they called kuru appeared.

Once kuru got a foothold in the Fore population, the disease progressed relentlessly until some villages became almost devoid of young adult women. The epidemic quickly subsided after Australian administrative authorities banned the Fore’s mortuary feasts in the 1950s.

A research team led by Simon Mead of University College, London, recently looked at the genetics of Fore women aged over 50. All these survivors had attended many funeral feasts and presumably must have possessed some genetic protection against the disease. Mead’s team analyzed the DNA of their prion protein gene and found that more than 75% had a distinctive genetic signature.¹⁸⁶ Every person in Britain infected with mad cow disease, on the other hand, had the opposite genetic signature.^g

Having identified this protective signature, Mead’s team then analyzed other populations around the world. They found that every ethnic group they looked at possessed the signature with the exception of the Japanese, who had a protective signature of their own at a different site in the gene.

Various genetic tests showed that the protective signature was too common to have arisen by chance, and must have been amplified through natural selection. Other tests suggested the signature was very ancient and was probably present in the human population before it dispersed from Africa. Under this scenario the Japanese presumably lost the signature through the process known as genetic drift, but developed a new one instead because it was so necessary.

So why has the British epidemic of mad cow disease proved not nearly so deadly in that nation of beef-eaters as was initially feared? It seems that Britons have been in part protected by their ancient cannibal heritage. That the British and other world populations have maintained the protective signature many generations after their last cannibal feast is an indication of how widespread cannibalism may have been in the ancestral human population and its worldwide descendants. The frequency of cannibalism in turn attests to the prevalence of warfare among the earliest human populations.

“There is an innate predisposition to manufacture the cultural apparatus of aggression, in a way that separates the conscious mind from the raw biological processes that the genes encode,” writes the biologist Edward O. Wilson. “Culture gives a particular form to the aggression and sanctifies the uniformity of its practice by all members of the tribe.”¹⁸⁷ The genes supply the motivation for warfare, Wilson is saying, in humans as they do in chimps, but people, blessed with the power of language, look for some objective cause of war. A society psychs itself up to go to war by agreeing that their neighbors have wronged them, whether by seizing property or failing to deliver on some promise. Religious leaders confirm that the local deity favors their cause and off go the troops.

The human predisposition for socially approved aggression falls into a quite different category from that of individual aggression. Bellicose individuals usually get themselves locked up in jail for long periods or, in primitive societies, social sanction is given to having them killed. Individual aggression is seldom a good strategy for propagating one’s genes. But socially approved aggression—that is, warfare—can be. A predisposition to warfare does not mean war is inevitable since the predisposition is only executed in certain contexts. The warlike Vikings of the tenth century became the peaceful Scandinavians of the twentieth.

Among forager societies, warfare can benefit the victor, by expanding territory and increasing reproductive success. That is the conclusion that archaeologists and anthropologists have been so anxious to avoid endorsing, because it seems to offer a justification for war, even a glorification of it. But by playing down the prevalence of warfare in the past they have obscured the important and surprising fact adduced by Keeley, that modern societies have succeeded in greatly reducing the frequency of warfare.

On the assumption that warfare was an incessant preoccupation of early human existence, the picture of the Upper Paleolithic era that specialists have so far constructed seems strangely incomplete. What does it mean to say that the Aurignacian culture was succeeded by the Gravettian? That the makers of the Aurignacian tool kit woke up one morning and decided thenceforward they would all do things the Gravettian way? Or that after many sanguinary battles people bearing the Gravettian culture ousted those following the Aurignacian? When the Last Glacial Maximum made northern latitudes uninhabitable and the glaciers pushed their populations south, is it likely they were welcomed with open arms by the southerners whose territory they invaded? If warfare was the normal state of affairs, it would have shaped almost every aspect of early human societies.

Warfare is a dramatic and distinctive feature of history, and it thoroughly overshadows an even more remarkable feature of human societies. This feature, the polar opposite of war, is the unique human ability to cooperate with others, and specifically with unrelated individuals. Social organisms like bees and ants form groups centered around members who are related to each other and have a common genetic interest. So do people to some extent when organized in tribal societies. But humans have extended sociality far beyond the extended family or tribe and have developed ways for many unrelated individuals to cooperate in large, complex, cohesive societies.

The uniquely human blend of sociality was not easily attained. Its various elements evolved over many years. The most fundamental, a major shift from the ape brand of sociality, was the human nuclear family, which gave all males a chance at procreation along with incentives to cooperate with others in foraging and defense. A second element, developed from an instinct shared with other primates, was a sense of fairness and reciprocity, extended in human societies to a propensity for exchange and trade with other groups. A third element was language. And the fourth, a defense against the snares of language, was religion. All these behaviors are built on the basic calculus of social animals, that cooperation holds more advantages than competition.

The Evolutionary Basis of Social Behavior

Though we take the necessities of social behavior for granted, group living in the animal world is highly unusual. In fact even the most rudimentary forms of sociality have long been a puzzle for biologists to explain in terms of evolutionary theory.

The reason is that a society serves no purpose unless members help one another, yet any effort an individual makes assisting others takes away from investment in his own offspring and reproductive success. If altruists have fewer children, altruistic behavior will be eliminated by natural selection. Yet without altruism there is no benefit to living in a society. How therefore can social behavior ever have evolved?

Evolutionary biologists have developed a reasonably good account of how social behavior may have emerged in groups of closely related individuals, in a theory about what is known as inclusive fitness. Another theory, that of reciprocal altruism, explains how behavior could have evolved for helping even unrelated people, or at least those who can be expected to reciprocate the favor at a later time.

Why will a bee sacrifice its life in the hive’s defense? Why should a worker ant embrace sterility and devote her life to raising the queen’s offspring? The late William Hamilton made a major addition to Darwin’s theory in showing how altruism, at least toward one’s own kin, makes evolutionary sense. Darwinian fitness, defined as reproductive success, is all about getting as many of one’s own genes as possible into the next generation. Hamilton’s insight was that the notion of Darwinian fitness should properly be expanded to include the genes one shares with one’s kin. Since these shared genes are the same, being inherited from the same parent, grandparent or great-grandparent, then helping get those into the next generation is as good as transmitting one’s own.

This notion of expanded fitness, or inclusive fitness as Hamilton called it, predicts that individuals will have a special interest in promoting the survival of children, full siblings and parents, with all of whom they have about 50% of their genes in common, and a substantial though lesser interest in the survival of grandchildren, nephews and nieces, half siblings, grandparents, and aunts and uncles, with whom they share 25% of their genes.

To maximize their inclusive fitness, individuals must restrain their own competitive behavior and make some degree of self-sacrifice on behalf of kin—in other words, develop social behavior. Thus altruists can be inclusively fitter than non-altruists and their genes, under certain conditions, will spread. Hamilton’s theory of inclusive fitness explains many otherwise puzzling features of social organisms, such as the self-sacrificing behavior of social insects like bees, ants and termites. It also helps explain why chimp communities and human tribal societies are organized along kinship lines.

It can take extreme circumstances to make evident the survival value of human kinship ties. Some 51% of the 103 Mayflower pioneers in the Ply-mouth colony perished after their first winter in the New World. It turns out that the survivors had significantly more relatives among other members of the colony than did those who died. Among the Donner party, a group of 87 people stranded in the Sierra Nevada in the winter of 1846, only 3 of 15 single young men survived, whereas men who survived had an average of 8.4 family members with them.¹⁸⁸

But kinship alone seems to have limited power as a cohesive social force. Napoleon Chagnon, in his study of the Yanomamo, noticed that as village populations grew larger, the average degree of relatedness would decrease. The population would then split, usually along kinship lines, with the result that people within the two smaller groups would be more highly related to each other. “Kinship-organized groups can only get so large before they begin falling apart,” Chagnon writes. Disputes break out over the usual things—sexual trysts, infidelity, snide comments or veiled insults. “As villages grow larger, internal order and cooperation become difficult, and eventually factions develop: Certain kin take sides with each other, and social life becomes strained. There appears to be an upper limit to the size of a group that can be cooperatively organized by the principles of kinship, descent and marriage, the ‘integrating’ mechanisms characteristically at the disposal of primitive peoples.”¹⁸⁹

In most Yanomamo villages, members are on average related to each other more closely than half-cousinship.¹⁹⁰ But nontribal societies are a lot larger, as if some new cohesive factor has to come into play if a community is to outgrow the organizational limits imposed by kinship. Recent human history, Chagnon writes, could be viewed as a struggle to overcome these limits: “Many general discussions of our social past as hunters and early cultivators allude to the ‘magic’ numbers of 50 to 100 as the general community size within which our recent cultural and biosocial evolution occurred, a maximal community size that was transcended only in the very recent past—within the last several thousand years.”¹⁹¹

One principle that biologists think may help explain larger societies, both human and otherwise, is that of reciprocal altruism, the practice of helping even a nonrelated member of society because they may return the favor in future. A tit-for-tat behavioral strategy, where you cooperate with a new acquaintance, and thereafter follow his strategy toward you (retaliate if he retaliates, cooperate if he cooperates), turns out to be superior to all others in many circumstances. Such a behavior could therefore evolve, providing that a mechanism to detect and punish freeloaders evolves in parallel; otherwise freeloaders will be more successful and drive the conditional altruists to extinction.

Conditional or tit-for-tat altruism cannot evolve in just any species. It requires members to recognize each other and have long memories, so as to be able to keep tally. A species that provides a shining example of reciprocal altruism is none other than the vampire bat. The bats, found in South America, hang out in colonies of a dozen or so adult females with their children. They feed by biting a small incision in the skin of sleeping animals, nowadays mostly cattle or horses, and injecting a special anticoagulant named, naturally enough, draculin. But their blood collection drives are not always successful. On any given night a third of the young bats and 7% of the adults are unsuccessful, according to a study by Gerald W. Wilkinson of the University of Maryland.

This could pose a serious problem because vampire bats must feed every three days, or they die. The colony’s solution, Wilkinson found, is that successful bats regurgitate blood to those who went hungry. Bats are particularly likely to donate blood to their friends, with whom they have grooming relationships, to those in dire need, and to those from whom they received help recently.¹⁹² The vampires’ reciprocal altruism must be particularly effective since the bats, despite the risk of death after three bloodless nights, can live for 15 years.

If social altruism has evolved among vampire bats, there is no reason why it could not also emerge among primates. And indeed it can be seen at work in the coalitionary politics of male chimpanzees, where the alpha male depends on allies to preserve his dominance of the male hierarchy. The biologist Robert L. Trivers, who first showed how reciprocal altruism could be favored by natural selection, suggested that in people a wide range of sophisticated behaviors grew up around it, including cheating (failure to return an altruistic favor to the giver), indignation at cheating, and methods to detect cheating.¹⁹³

Many common emotions can be understood as being built around the expectation of reciprocity and the negative reaction when it is made to fail. If we like a person, we are willing to exchange favors with them. We are angry at those who fail to return favors. We seek punishment for those who take advantage of us. We feel guilty if we fail to return a favor, and shame if publicly exposed. If we believe someone is genuinely sorry about a failure to reciprocate, we trust them. But if we detect they are simulating contrition, we mistrust them.¹⁹⁴

The instinct for reciprocity, and the cheater-detection apparatus that accompanies it, seem to be the basis for a fundamental human practice, that of trade with neighboring groups. Long distance trade is one of the characteristic behaviors of the human societies that emerged in the Upper Paleolithic age starting some 50,000 years ago. Tribal societies developed trading systems of considerable sophistication. The Yit Yoront, a foraging society of northern Australia, lived until recently in the Stone Age. One of their most necessary possessions, used in everything from hunting to wood-gathering, were hafted stone axes. But they lived on an alluvial coast and the nearest stone quarry was four hundred miles away. How did they acquire their polished stone axes? They made a product much in demand with their neighbors to the south, spears tipped with the barbs of stingrays. The spears were traded inland, through a long line of trading partners, being exchanged at each stage for a varying number of stone axes. The spear/axe exchange rate was sufficient at each trading post to push stone axes northward and pull barb-tipped spears southward.¹⁹⁵

Trade is a foundation of economic activity because it gives the parties to a transaction a strong incentive to specialize in making the items that the other finds valuable. But trade depends on trust, on the decision to treat a total stranger as if he were a member of the family. Humans are the only species to have developed such a degree of social trust that they are willing to let vital tasks be performed by individuals who are not part of the family. This set of behaviors, built around reciprocity, fair exchange and the detection of cheaters, has provided the foundation for the most sophisticated urban civilizations, including those of the present day.

Reciprocity, and an ability to calculate the costs and benefits of cooperation, underpin our social life, writes the economist Paul Seabright, “making it reasonable for us to treat strangers as though they were honorary relatives or friends.” It is remarkable that this behavior evolved at a time when primitive warfare was at its most intense and people had every reason to regard strangers with deep suspicion. Strangers can still be dangerous, yet in the right circumstances we habitually trust them. “The knowledge that most people can be trusted much of the time to play their part in the complex web of social cooperation has had dramatic effects on the psychology of our everyday life,” Seabright says, making it possible “to step nonchalantly out of the front door of a suburban house and disappear into a city of ten million strangers.”¹⁹⁶ Without this innate willingness to trust strangers, human societies would still consist of family units a few score strong, and cities and great economies would have had no foundation for existence.

How might this greater level of trust have arisen? Two hormones, known as oxytocin and vasopressin, are emerging as central players in modulating certain social behaviors in the mammalian brain. The hormones are generated in the pituitary gland at the base of the brain and have effects both on the body and in the brain. Oxytocin induces both labor in childbirth and the production of milk. Its effects on the mind, at least in experimental animals, have the general property of promoting affiliative or trusting behavior, lowering the natural resistance that animals have to the close proximity of others.

So what does oxytocin do in people? Researchers at the University of Zurich have found that it substantially increases the level of trust. Oxytocin, they say, “specifically affects an individual’s willingness to accept social risks arising through interpersonal interactions.” The findings emerged from giving subjects a sniff of oxytocin before playing a game that tested trusting behavior. ¹⁹⁷

If the biological basis of trusting behavior is mediated in this manner, the degree of trust could easily be ratcheted up or down in the course of human evolution by genetic changes that either increased individuals’ natural production of the hormone or enhanced the brain’s response to it. Thus hunter-gatherers might have a genetically lower response to oxytocin while city-dwellers would have evolved a greater sensitivity. Whatever the exact mechanism, it is easy to see how greater levels of trust might have evolved at various stages in human evolution, given that there is a biological basis for the behavior.

Trust is an essential part of the social glue that binds people together in cooperative associations. But it increases the vulnerability to which all social groups are exposed, that of being taken advantage of by freeloaders. Freeloaders seize the benefits of social living without contributing to the costs. They are immensely threatening to a social group because they diminish the benefits of sociality for others and, if their behavior goes unpunished, they may bring about the society’s dissolution.

Human societies long ago devised an antidote to the freeloader problem. This freeloader defense system, a major organizing principle of every society, has assumed so many other duties that its original role has been lost sight of. It is religion.

The Evolution of Religion

The essence of religion is communal: religious rituals are performed by assemblies of people. The word itself, probably derived from the Latin religare, meaning to bind, speaks to its role in social cohesion. Religious ceremonies involve emotive communal actions, such as singing or dancing, and this commonality of physical action reinforces the participants’ commitment to the shared religious views.

The propensity for religious belief may be innate since it is found in societies around the world. Innate behaviors are shaped by natural selection because they confer some advantage in the struggle for survival. But if religion is innate, what could that advantage have been?

No one can describe with certainty the specific needs of hunter-gatherer societies that religion evolved to satisfy. But a strong possibility is that religion coevolved with language, because language can be used to deceive, and religion is a safeguard against deception. Religion began as a mechanism for a community to exclude those who could not be trusted. Later, it grew into a means of encouraging communal action, a necessary role in hunter-gatherer societies that have no chiefs or central authority. It was then co-opted by the rulers of settled societies as a way of solidifying their authority and justifying their privileged position. Modern states now accomplish by other means many of the early roles performed by religion, which is why religion has become of less relevance in some societies. But because the propensity for religious belief is still wired into the human mind, religion continues to be a potent force in societies that still struggle for cohesion.

A distinctive feature of religion is that it appeals to something deeper than reason: religious truths are accepted not as mere statements of fact but as sacred truths, something that it would be morally wrong to doubt. This emotive quality suggests that religion has deep roots in human nature, and that just as people are born with a propensity to learn the language they hear spoken around them, so too they may be primed to embrace their community’s religious beliefs.

Can the origin of religion be dated? A surprising answer is yes, if the following argument is accepted. Like most behaviors that are found in societies throughout the world, religion must have been present in the ancestral human population before the dispersal from Africa 50,000 years ago. Although religious rituals usually involve dance and music, they are also very verbal, since the sacred truths have to be stated. If so, religion, at least in its modern form, cannot pre-date the emergence of language. It has been argued earlier that language attained its modern state shortly before the exodus from Africa. If religion had to await the evolution of modern, articulate language, then it too would have emerged shortly before 50,000 years ago.

If both religion and language evolved at the same time, it is reasonable to assume that each emerged in interaction with the other. It is easy enough to see why religion needed language, as a vehicle for the sharing of religious ideas. But why should language have needed religion?

The answer may have to do with the instinct for reciprocal altruism that is a principal cohesive force in human society, and specifically with its principal vulnerability, the freeloaders who may take advantage of the system without returning favors to others. Unless freeloaders can be curbed, a society may disintegrate, since membership loses its advantages. With the advent of language, freeloaders gained a great weapon, the power to deceive. Religion could have evolved as a means of defense against freeloading. Those who committed themselves in public ritual to the sacred truth were armed against the lie by knowing that they could trust one another.

The anthropologist Roy Rappaport argued that sanctified statements were early societies’ antidote to the misuse of the newly emerged powers of language. “This implies that the idea of the sacred is as old as language,” he wrote, “and that the evolution of language and of the idea of the sacred were closely related, if not bound together in a single mutual causal process.” The emergence of the sacred, he suggested, “possibly helped to maintain the general features of some previously existing social organization in the face of new threats posed by an ever-increasing capacity for lying.”¹⁹⁸

For early societies making the first use of language, there had to be some context in which statements were reliably and indubitably true. That context, in Rappaport’s view, was sanctity. This feature has been retained to a considerable degree in modern religions, which are centered around sacred truths, such as “The Lord Our God the Lord is One,” or “There is no god but God.” These sacred truths are unverifiable, and unfalsifiable, but the faithful nevertheless accept them to be unquestionable. In doing so, like assemblies of the faithful since the dawn of language, they bind themselves together for protection or common action against the unbelievers and their lies.

From his study of the Maring, primitive agriculturalists of the New Guinea central highlands, Rappaport also recognized that ritual was an essential source of authority in an egalitarian society without headmen or ruling elites. It was by their attendance at ritual dances that the Maring would commit themselves to fight as their host’s allies in the next war cycle. “It is plausible to argue that religious ritual played an important role in social and ecological regulation during a time in human history when the arbitrariness of social conventions was increasing but it was not yet possible for authorities, if they existed at all, to enforce compliance,” he wrote.

Rappaport’s ideas about the role of religion in early societies have been buttressed by a remarkable series of excavations in the Oaxaca valley of Mexico. The archaeologists Joyce Marcus and Kent V. Flannery traced the development of religion over a 7,000-year period as the people of the valley went through four stages of social development, from hunters and gatherers, to a settled egalitarian society, to a society ruled by an elite, and finally to an archaic state known as the Zapotec state. As the Oaxacan people’s society evolved, so too did their form of religion.¹⁹⁹

At the hunting and gathering stage, Joyce and Flannery found signs of a plain dance floor, its sides marked by stones. The dance floor, assuming it was used like those of modern hunter-gatherers, would have been the site of ritual dancing on ad hoc occasions when many different groups came together for initiations and courtship.

By 1500 BC the Oaxacans had developed strains of maize that allowed them to settle down and practice agriculture (the reverse of the sequence in the Near East, where settlement long preceded agriculture). At first their society was egalitarian, as it had been in their hunter-gatherer days, but their rituals became more formal. Marcus and Flannery have excavated four men’s houses, all oriented in the same direction, which may have been determined by the sun’s path at spring equinox. The orientation suggests that religious ceremonies were now held at fixed times, determined by astronomical events. The men’s houses, to judge by practice in contemporary societies, may have been open only to men who had passed acceptability tests and been initiated into secret rituals.

By 1150 BC the third stage of society had began to emerge, with an elite who lived in large houses, wore jade-studded clothes and deformed their skulls in childhood as a sign of nobility. The men’s houses were replaced with temples, also oriented in the same direction. Religious practice had become more elaborate, the archaeologists found, with ritual bloodletting, a symbolic self-sacrifice, and the cooking and eating of sacrificial victims.

The fourth stage of society, the Zapotec state, which was founded in 500 BC, was accompanied by a more complex form of religion. The temples now had rooms for a special caste of religious officers, the priests.

The advent of the priests marked the culmination of a steady trend in the evolution of Oaxacan ritual, its growing exclusivity. At the hunter-gatherer stage, the ritual dances were open to everyone. By the time of the men’s houses, only initiated members of the public could participate in rituals, and by the stage of the Zapotec state, religion had come under the control of a special priestly caste.

What underlay this coevolution of religion with social structure? It seems that the important coordinating role of ritual in hunter-gatherer societies did not end when leaders and elites emerged in settled societies. Instead, the elites coopted the ritual practices as another mechanism of social control and as a means of justifying their privileged position. Making the religion more exclusionary gave the elites greater power to control the believers. To justify the ruler’s position, new truths, also unverifiable and unfalsifiable, were added as subtexts to the religion’s sacred postulates, such as “The chief has great mana,” “Pharaoh is the living Horus,” or “Henry is by the Grace of God King.”

Rappaport believed that the conditions that enabled authorities to exercise civil power emerged only recently, and that for much of human existence rulers invoked sanctity as a principal source of their authority. Even archaic states were theocratic, at least to begin with. Modern states too, despite the ample civil power at their disposal, have not entirely dispensed with appeals to religious cohesion and authority. Even in a society like that of the United States, political allegiance is sealed with the declaration of “One nation under God.”

Religion’s other ancient role, that of protecting the community from freeloaders, can also been seen still at work in contemporary societies. Among ultra-Orthodox Jews in New York’s diamond district, the level of trust is so high that multi-million-dollar deals can be sealed by a handshake. Islam is said to have spread through Africa as a facilitator of trade and trust.²⁰⁰

Trust and cohesiveness are nowhere more important than in wartime. Contemporary religions preach the virtues of peace in peacetime but in war the bishops are expected to bless the cannon, and official churches almost always support national military goals. “Religion is superbly serviceable to the purposes of warfare and economic exploitation,” writes the biologist Edward O. Wilson, noting that it is “above all the process by which individuals are persuaded to subordinate their immediate self-interest to the interests of the group.”²⁰¹

Why does religion persist when its primary role, that of providing social cohesion, is now supplied by many other cultural and political institutions? While religion may no longer be socially necessary, it nevertheless fills a strong need for many people, and this may reflect the presence of genetic predisposition. Wilson, for one, believes that religion has a genetic basis, that its sources “are in fact hereditary, urged into birth through biases in mental development encoded in the genes.”

Religion, language and reciprocity are three comparatively recent elements of the glue that holds human societies together. All seem to have emerged some 50,000 years ago. But a far more ancient adaptation for social cohesiveness, one that set human societies on a decisively different path from those of apes, was the formation of the pair bond. Much of human nature consists of the behaviors necessary to support the male-female bond and a man’s willingness to protect his family in return for a woman’s willingness to bear only his children.

The Privatization of Sex

Ape societies are driven by intrasexual competition, the rivalry between males for access to females. Although male chimpanzees form coalitions to seize power within the male hierarchy, these are shifting, ad hoc arrangements. A male chimp probably sees most other adult males as potential rivals, an attitude that limits the degree of cohesiveness in chimp society.

The human line of descent probably inherited the ape system of separate male and female hierarchies. But around 1.7 million years ago, the size difference between males and females started to diminish, according to the paleontological record. This shift in size is almost certainly a sign that competition between males had diminished because of the transition to the pair bond system.

The novel arrangement of pairing off males and females creates a whole new set of social calculations. Most males in the society now have a chance to reproduce since they possess socially endorsed mating access to at least one female. So each male has a much greater incentive to invest in cooperative activities, such as hunting or defense, that may benefit the society as a whole.

The pair bond takes much of the edge out of male-to-male aggression. It also requires that men trust one another more, and can have some confidence that those who go hunting won’t be cuckolded by those who stay to defend the women.

For the females there is a trade-off. They must give up mating with all the most desirable males in the community and limit their reproductive potential to the genes of just one male. On the other hand they gain an implied guarantee of physical protection for themselves and their children, as well as some provisioning. In some foraging societies the men bring back meat but the staple foods are plants and small animals that are mostly gathered by the women. But a man’s food gathering efforts were probably particularly helpful during the frequent periods that a woman was nursing and could find less food for herself.²⁰²

Like most of evolution’s behavioral arrangements, pair-bonding was not a rigid prescription, one that dictated a one man, one woman nuclear family. Many human societies are polygamous, allowing men to have more than one wife if they can support more. Societies in special ecological conditions allow women to have more than one husband, such as in the high altitude agriculture of Tibet where a set of brothers may marry one wife and raise the children as a single family.

With the institution of pair bonds, sex became something conducted within families. It was presumably at this time that human societies developed a taboo against public sex, a custom that would bring chimp or bonobo societies to an almost complete standstill. The privatization of sex would help considerably in removing sex as a provocation of male rivalry.

But though the pair bond system alleviated discord between males, it raised new tensions between men and women. The asymmetry between the male and female roles in the family unit—a woman looks after the children, a man protects and supports her and her children—sets up an inevitable difference of reproductive interests and strategies. Men have evolved traits like sexual jealousy, for the sound reason that complaisant husbands are likely to pass on fewer of their genes.

Sexual infidelity poses very different kinds of risk for men and for women. For a woman, the threat posed by her husband having a mistress is not so much the sexual dalliance in itself but rather the possibility that he may switch his support to his paramour and away from his family. The withdrawal of support would reduce her reproductive fitness, as measured by the chance of raising her children successfully to maturity.

Serious as that danger may be to a woman’s interests, the risk to a man of his wife’s infidelity is considerably graver. For him, the threat is that he may not be the father of his wife’s children. In evolutionary terms, a man who devotes his life to raising another man’s children has seen his Darwinian fitness reduced to zero.

Men’s fear of being deceived is not without basis, in a general sense, since a woman has a heavy incentive to seek another partner should her husband prove infertile, a not uncommon occurrence. Even if her husband is fertile, a woman might improve her reproductive success by having children with more than one partner. Ideally a woman will seek both support and good genes from her husband, but as long as that support is guaranteed her reproductive interests could in principle be improved by seeking better genes elsewhere. Many men may be willing to offer her this service, since they can greatly improve their reproductive success by having children with as many women as possible, especially if another man bears the cost of raising them.

It is no surprise, therefore, that men have gone to great lengths to secure exclusive access to women whose children they have undertaken to raise, with methods that range from foot-binding and genital mutilation to purdah, veils, chadors and an array of laws and customs restricting women’s activities. However abhorrent the means, the motivation stems from the inherent vulnerability of male reproductive strategy: mother’s baby, father’s maybe.

How often do women conceive children with men who are not their husbands? Ornithologists used to rhapsodize about the marital fidelity of bird species that stayed pair-bonded for life. That was until the advent of protein-based and later DNA tests for assessing paternity. Despite the appearance of fidelity, extra-pair liaisons in the bird world turned out to be routine. The preeminent adulteress is an Australian bird, the Superb Fairy-wren, 76% of whose offspring are fathered by extra-pair copulations.²⁰³

Human geneticists testing people for heritable diseases quite frequently stumble across cases where the father of record cannot be the biological parent. Genetic counselors have a rule of thumb that these discrepancies, known delicately as nonpaternity cases, will range from 5 to 10% in an average American or British population. For the U.S. population as a whole, “The generic number used by us is 10 percent,” said Bradley Popovich, vice president of the American College of Medical Genetics.²⁰⁴

The degree of nonpaternity that has come to light in the United States and Europe is particularly surprising in light of the control that women now exert over their reproductive behavior. Presumably many of the children involved in nonpaternity cases are not conceived by accident. The evident implication is that the woman’s conception with a man other than her husband is in some cases deliberate.

That women in modern societies sometimes choose to conceive with alternative partners is a matter that bears on an issue of considerable debate among primatologists, that of whether the phenomenon of sperm competition occurs to any significant degree in people. In many species the female is inseminated by more than one male at the same time, and direct competition takes place in the female’s reproductive tract between the sperm of rival males to fertilize the eggs. The female reaps the significant genetic benefit of having her eggs fertilized by the best of the competing sperm. Has evolution dispensed with this useful grading method in humans or does it apply in our species too? “Sperm competition is possible in Homo sapiens, though whether it has played a significant role during human evolution remains highly debatable,” says Alan Dixson, an authority on primate reproduction. ²⁰⁵

Geneticists have recently studied the DNA sequence of several genes involved in sperm production in three primate species, chimpanzees, gorillas and people. In chimpanzees, among whom sperm competition is fierce, the genes show signs of being under strong selective pressure. The pressure is much less fierce in the gorilla version of these genes, as would be expected given the silverback’s exclusive access to his harem. In humans the genes have evolved at a rapid clip, faster than that of gorillas and equal to that of chimps. The human sperm genes are clearly under some kind of fierce evolutionary pressure, and sperm competition may be the reason.²⁰⁶

Sperm competition requires not just that a woman has more than one lover but that she has two within a rather short time of each other.²⁰⁷ Some 4% of people in Britain are conceived under such competitive conditions, according to Robin Baker, a University of Manchester biologist.²⁰⁸ This estimate receives some support from data on heteropaternity, a phenomenon that occurs in fraternal twins. Unlike identical twins, who arise from splitting of the same egg, fraternal twins result when the mother releases more than one egg in an ovulatory cycle. Heteropaternity refers to the circumstance in which each of two eggs is fertilized by different fathers. In Greek mythology the twins Castor and Pollux were the sons respectively of Tyndareus, a king of Sparta, and Zeus, who in the guise of a swan seduced Tyndareus’s wife Leda. This may have been the first case of heteropaternity but it was by no means the last. Of fraternal twins born to white women in the United States, 1 in 400 pairs is estimated to have two fathers.²⁰⁹ Among cases where paternity is disputed, 2.4% of cases have been found to be heteropaternal.²¹⁰

For a woman to have a child extramaritally carries a serious risk—that the infant of an extramarital liaison may look suspiciously unlike the father of record, putting both its own life and its mother’s at risk. This hazard would have created a strong selective pressure in favor of genes that prevented infants from looking too much like their parents. And indeed babies tend to have chubby faces with indistinct features that give them a rather generic appearance, sharply mitigating the risk that they will look like the wrong father.

To the extent they resemble anyone, babies would be expected to look as much like their mother as their father. But researchers have found that grandparents and others comment far more frequently on a baby’s similarities to its father. Mothers tend to state that a baby resembles its father, and do so more often when the father is present, as if trying to assure him of his paternity. “Whether mothers do this consciously, knowing full well that the baby looks nothing like its dad, or whether they deceive themselves into thinking that the baby really does look like the father is unclear,” say the authors of a textbook on evolutionary psychology.²¹¹

Because of the central significance of reproductive success, evolutionary psychologists have paid particular attention to human mating habits, exploring the signals that govern male and female choice of a mate, and the strategies that each pursues to accomplish its reproductive goals.

In studying the mating signals that the human psyche is genetically primed to assess, evolutionary psychologists have found that men in many different cultures of the world prefer women with a waist to hip ratio of 7 to 10. The male eye is probably attuned to these proportions because they signal a woman’s fertility. Young women tend to put fat on their hips, breasts and buttocks whereas older women, and those who are pregnant, get thicker at the waist. “A relatively narrow waist means ‘I’m female, I’m young, and I’m not pregnant,’” writes the evolutionary biologist Bobbi Low.²¹² Symmetry of features, especially of the face, is another indicator of good genes; it requires a normal development in the womb and is thus a marker for general health. There are of course variations on the general theme. Among the !Kung, men are driven wild by a sizable protuberance of fat on a woman’s buttocks, presumably a signal of being able to nourish a child in difficult environments.

Because reproduction is a greater risk and investment for women than for men, according to the biologist Robert Trivers, it follows that women will be more choosy about their partners than men are; and because women are more selective, men will find themselves being more competitive with each other for women’s favors. A woman looks for indicators not just of good health in a man but also of commitment to look after her and her family. This is a matter partly of emotional commitment, which women assess with care, and also of wealth or the ability to acquire it, as may be indicated by a man’s social status.

Surveys conducted over many years have consistently indicated that American women care more about a partner’s wealth than men do. The evolutionary psychologist David Buss expanded this survey to 10,000 individuals in 37 world cultures and found the same pattern—that women placed more value than did men on a partner’s financial prospects.²¹³ Women in almost all cultures prefer men of high status, presumably because this is likely to be correlated with wealth. Women consistently prefer men who are slightly older, for reasons that are not obvious. The preference could be a holdover, Buss suggests, from hunter-gatherer days when older men, at least through their twenties and thirties, were stronger and better able to offer physical protection to their family.²¹⁴ Perhaps for the same reason, women consistently prefer tall men to short.

If fitness indicators for health and fertility are useful, wouldn’t indicators for mental ability be even more useful in choosing a partner for a long-term relationship? The evolutionary psychologist Geoffrey Miller has advanced the striking theory that such indicators do exist, but they are familiar under names that give no clue to their biological function. The indicators of mental fitness, in his view, include both cultural activities such as art, music, dance and literature and moral qualities such as kindness.²¹⁵

Evolutionary biologists have gained considerable insight into what makes fitness indicators true signals, and why they must be qualities that vary in a population. Fitness indicators, and the behavioral preferences for them, are brought about by sexual selection, a form of natural selection but one that works through mating success rather than physical survival. The mechanism of sexual selection was first recognized by Darwin, who had long been puzzled why the males of many species are heavily ornamented, with conspicuous horns or antlers or feathers. These baroque decorations seemed to contribute nothing to survival, posing an apparent challenge to Darwin’s idea that the fittest survive. The solution he proposed in The Descent of Man was female choice: peahens for some reason preferred peacocks with gaudy tails, who got to sire more offspring, including sons with gaudy tails and daughters with a taste for them. These male adornments were therefore a worthwhile handicap to their owners because they assisted toward evolution’s bottom line of getting more genes into the next generation.

That still left the question of how these male embellishments evolved in the first place. Darwin’s theory of sexual selection was largely ignored for a century—his contemporaries placed no credence in the idea that female choice could be a major evolutionary force—and it was not until the 1970s that biologists started to develop the theory. One insight was that male ornaments like long plumage were hard to grow and therefore served as an overall indicator of good genes. But if long red tail feathers, say, were the key to male reproductive success, soon every male would be wearing them and they would lose their utility in helping females choose between males of different quality.

The evolutionary biologist Amotz Zahavi realized that sexual signals of one’s health and fitness, if they were to be true and reliable, had to be so costly as to constitute a serious handicap for the displayer. Weak peacocks grow unappetizing tails and only the strongest can grow really beautiful ones. It’s that spectrum of ability that provides peahens with a basis for choice. Biologists call such a trait heritable—the quality of the tail varies from one individual to another and part of the variation is caused by the genes.

The physical features that have evolved as fitness indicators in the human mating dance—symmetry of features, fine skin, a shapely body—are known by another name: beauty. People find these features attractive not through some arbitrary criterion or dictate of fashion but because the male and female minds have evolved to look for and appreciate such qualities in a potential mate.

Zahavi’s costly signaling theory explains, sad to say, why it is impossible for everyone to be equally beautiful or handsome. Since beauty serves as a fitness indicator, it needs to vary from one individual to another. If everyone were equally beautiful, beauty would have no value as a criterion for sexual selection.

The privatization of sex that began 1.7 million years ago did not bring an end to all competition between males for females. But it was a major step in reducing human aggressiveness within societies. And it was followed, many thousands of years later, by a serious evolutionary reduction in the level of aggression between societies.

The Domestication of People

The evidence that human tribes have become less passionately hostile to each other lies in a worldwide thinning, or gracilization, of the human skull that took place during the Upper Paleolithic era. The fossils of early modern humans are both large and very robust, or thick boned. But these generic early modern skulls started to change around 40,000 years ago. In each region of the world they follow an independent, but largely parallel course, as if similar genetic changes are occurring independently in each population. “Cranial size reduction and gracilization may have been homoplasic [arising by independent evolution] in most populations,” writes the physical anthropologist Marta Mirazón Lahr.²¹⁶

The gracilization occurred at different rates in different regions but all followed a common trend, except for two populations at the extremities of the human diaspora, Australian aborigines and Fuegians at the tip of South America. The Australian skulls became smaller like the rest, but retained their robusticity, presumably as a result of independent evolution. The Fuegians seem to be a case of genetic drift—a small isolated population developing its own special characteristics.²¹⁷ Gracilization is farthest advanced in sub-Saharan Africans and Asians, with Europeans still in some instances showing large size and robusticity.²¹⁸

What caused the gracilization of human skulls and the shrinking of human skulls and teeth all over the world? This is a large and complex issue, not least because a very similar downsizing affected the sheep, goats and other animals that were domesticated in the Neolithic era after the advent of agriculture. Researchers have attributed the smaller size of domesticated animals (compared with their wild forebears) to such facts as different diet, less physical activity, and a relaxation of the selective pressure favoring larger males under the conditions of captive breeding.²¹⁹

Lesser physical activity and the dietary changes brought about by agriculture have also been suggested as the reason why humans became lighter-boned and smaller. But the explanation must be sought elsewhere because it is clear that the gracilization of humans started well before the beginning of agriculture, and around the time of the earliest settlements some 15,000 years ago. The Natufians, the first settlers of the Near East, already had more gracile features, shorter stature and smaller teeth.²²⁰

Lahr believes that the more gracile features appearing in human skulls of the Upper Paleolithic “have a strong genetic basis,” but her study is purely descriptive and she offers no explanation for the forces that might have driven the genetic change. The primatologist Richard Wrangham, however, has provided an intriguing insight into gracilization.

His argument goes as follows. Consider first the bonobos, who are much more peaceful and playful than chimpanzees. Their skulls look like those of juvenile chimpanzees, just as their behavior is more juvenile than that of chimpanzees. This kind of change is called pedomorphic—meaning a trend toward the juvenile form—in reference to the evolutionary process of developing a new species by truncating the fully mature development of the ancestral species. Bonobos presumably found themselves in an environment where aggression was less beneficial, and so evolution kept selecting individuals whose development was completed before the arrival of the aggressive traits typical of adult males.

Pedomorphic evolution is familiar to biologists in another context, that of domestication. Comparing dogs with wolves, the dog’s skull and teeth are smaller and its skull looks like that of a juvenile wolf. The same process occurred when Dmitri Belyaev, in the experiment already discussed, set out to domesticate silver foxes. Belyaev selected foxes solely for tameness, but a whole set of other traits appeared in his animals along with the tolerance of people, including the white marks on the coat, curly hair, and smaller skulls and brains.

Viewed in this context, the gracilization of the human skull looks very much like one of those changes that come along for the ride when a species is undergoing pedomorphosis or domestication. Gracilization, Wrangham believes, occurred because early modern humans were becoming tamer.

And who, exactly, was domesticating them? The answer is obvious: people were domesticating themselves. In each society the violent and aggressive males somehow ended up with a lesser chance of breeding. This process started some 50,000 years ago, and, in Wrangham’s view, it is still in full spate. “I think that current evidence is that we’re in the middle of an evolutionary event in which tooth size is falling, jaw size is falling, and it’s quite reasonable to imagine that we’re continuing to tame ourselves. . . . This puts humans in a picture of now undergoing a process of becoming increasingly a peaceful form of a more aggressive ancestor.”²²¹

With tamer people, the path was now set for larger and more complex societies, ones that would transcend the limited horizons of the hunter-gatherer band.

The Progression of Human Society

The vocabulary of evolutionary biology does not include the word progress, for evolution has no goal toward which progress might be made. But in the case of human evolution, this exclusion may not be entirely justified. People, after all, make choices. If those choices shape a society for generation after generation, and if they permit individuals of a certain character to have more children and propagate their genes, then the overall nature of society may come to be shaped, in part, by human choice. If the character in question is a tendency to cooperate with others, then such a society would become more cohesive internally and more conciliatory in its relations with neighbors. Other societies might become more aggressive in character, or more paranoid, or more adventurous. Yanomamo society, given that the unokais have more children, has surely been positioned to become more aggressive. But overall, despite many setbacks and reversions, human societies have made vast gains in peacefulness, complexity and cohesion in the last 15,000 years.

It is often assumed that evolution works too slowly for any significant change in human nature to have occurred within the last 10,000 or even 50,000 years. But this assumption is incorrect. The development of new brain gene alleles 37,000 and 6,000 years ago, and of lactose tolerance 5,000 years ago, have already been mentioned; several other instances of recent human evolution are cited in chapter 12. There is no reason to suppose that human nature ceased to evolve at some finishing post in the distant past or to assume, as do some evolutionary psychologists, that people are struggling to function in modern societies with Stone Age minds. Genomes adapt to current circumstances or perish; the human genome is unlikely to be an exception.

Human societies have progressed through several major transitions in the last 15,000 years, and it may well be that these transformations were accompanied by evolutionary as well as cultural changes. It was only after people had become less violent that they were able to abandon the nomadic life of hunting and gathering that they had followed for the last 5 million years, and began to settle down. The first settled societies appeared in the Near East some 15,000 years ago. Though they were probably egalitarian at first, they soon developed a hierarchical form, with elites, leaders and specialization of roles.

Once settlement began, human societies became larger and more complex, presenting a new set of environments for people to adjust to. Societies come in many forms, and each may have punished or rewarded different character traits. The anthropologists Allen Johnson and Timothy Earle have traced the emergence of human societies of various levels of complexity, arguing that each is a response to the environmental problems it had to tackle, notably those of food production, surpluses, defense and trade. They distinguish three broad levels of complexity—family-based societies, local groups and regional polities.²²² Each of these major cultural transitions could well have prompted changes in social behavior and these, though Johnson and Earle make no such suggestion, could have become genetically embedded as the individuals who best adapted to each new social stage left more children.

Hunter-gatherer societies, Johnson and Earle say, were based on fairly autonomous family groups, though with a degree of organization that extends beyond the family. To spread the risk of catching nothing, hunters like the !Kung have firm rules for distributing the meat from a kill beyond the hunter’s immediate family. A large animal may have more meat than a single family can consume, so sharing it buys entitlement to a reciprocal gift in future.

Two themes already apparent in foraging societies—reciprocity and leadership—emerged more strongly in settled societies. Settled societies, in the Johnson-Earle analysis, needed assurance of food supply. But instead of sharing on an ad hoc basis, as foragers do, they had another option, that of generating and storing surpluses.

Surpluses, largely unknown to hunter-gatherers, were of critical importance to settled societies. The surpluses had to be stored, protected and distributed, activities that required a greater level of social organization than the loose associations of a family-based foraging group. Local groups emerged, like a Yanomamo village, in which there was a headman, though with few powers beyond those of personal persuasion. Religious ceremonies played a leading role in integrating group activities.

Surpluses also generated items that could be traded. The increasing complexity of managing a local group’s trade, defense and investment (such as in fishing weirs or irrigation) required stronger leadership. Eventually chiefs emerged, along with specialists and elites. These leaders integrated village-size communities into a regional economy by managing long distance trade and spreading the risks of food production beyond the family level.

The ground had then been laid, Johnson and Earle suggest, for the association of local groups into a larger society. Continuing intensity of economic activity led to the emergence of the first states, known as archaic states. In Japan, for example, people lived as hunter-gatherers until around 250 BC when the cultivation of dry rice was introduced. Foraging and dry rice farming existed side by side until AD 300 when wet rice began to be cultivated. This required large scale irrigation, and at the same period the first chiefdoms and archaic states emerged.

Archaic states have existed only in the last 5,000 years. During Neolithic times, Johnson and Earle estimate, there were probably more than 100,000 independent political units of the family-based or local group level of organization. But at all levels of the social organization, from hunter-gatherers to archaic states, the goal was the same, that of organizing resources in a way that benefited the reproductive strategies of its members.

In the emergence of these early human states, two strong forces were at work, and still shape relations between states in the contemporary world. One is the need for defense, the other the dependence on trade. Both of these state behaviors spring from the deepest wells of human nature, the contrary instincts for aggression and reciprocity. Though war gets more space in the history books, it is the conciliatory arts of trade and exchange that have prevailed in the long run. According to the World Health Organization, only 0.3% of deaths in 2002 were caused by war.²²³

Our bones are more gracile than those of our Upper Paleolithic ancestors, our personalities less aggressive, our societies more trusting and cohesive. An element of human choice, a preference for negotiation over annihilation, has perhaps been injected into the genome. And that might explain why there is an inescapable sense of progress about human evolution over the last 50,000 years: human choice has imposed a direction on the blind forces that hitherto have shaped evolution’s random walk.

In parallel with human social evolution, the human physical form continued to evolve. Because the human population was dispersed across different continents, between which distance and hostility allowed little gene flow, the people on each continent followed independent evolutionary paths. It was these independent trajectories that led over the generations to the emergence of a variety of human races.