It's tempting, then, to use the 10,000-year horizon as a boundary between two major eras of human existence: a hunter-gatherer era, in which we did most of our biological evolving and which may still be glimpsed in extant hunter-gatherers, and the era of civilization thereafter. That is the dividing line that figures in theories of the ecological niche to which humans are biologically adapted, which evolutionary psychologists call "the environment of evolutionary adaptedness." But that is not the cut that is most relevant to the Leviathan hypothesis.

The species we belong to, "anatomically modern Homo sapiens," is said to be 200,000 years old. But "behaviorally modern" humans, with art, ritual, clothing, complex tools, and the ability to live in different ecosystems, probably evolved closer to 75,000 years ago in Africa before setting out to people the rest of the world. When the species emerged, people lived in small, nomadic, egalitarian bands of kinsmen, subsisted by hunting and gathering, and had no written language or government. Today the vast majority of humans are settled in stratified societies numbering in the millions, eat foods cultivated by agriculture, and are governed by states. The transition, sometimes called the Neolithic (new stone age) Revolution, began around 10,000 years ago with the emergence of agriculture in the Fertile Crescent, China, India, West Africa, Mesoamerica, and the Andes.

For one thing, the 10,000-year milestone applies only to the first societies that farmed. Agriculture developed in other parts of the world later and spread outward from those cradles only gradually. Ireland, for example, was not lapped by the wave of farming that emanated from the Near East until around 6,000 years ago. Many parts of the Americas, Australia, Asia, and Africa were populated by hunter-gatherers until a few centuries ago, and of course a few still are.

Also, societies cannot be dichotomized into hunter-gatherer bands and agricultural civilizations. The nonstate peoples we are most familiar with are the hunters and gatherers living in small bands like the! Kung San of the Kalahari Desert and the Inuit of the Arctic. But these people have survived as hunter-gatherers only because they inhabit remote parts of the globe that no one else wants. As such they are not a representative sample of our anarchic ancestors, who may have enjoyed flusher environments. Until recently other foragers parked themselves in valleys and rivers that were teeming with fish and game and that supported a more affluent, complex, and sedentary lifestyle. The Indians of the Pacific Northwest, known for their totem poles and potlatches, are a familiar example. Also beyond the reach of states are hunter-horticulturalists, such as peoples in Amazonia and New Guinea who supplement their hunting and gathering by slashing and burning patches of forest and growing bananas or sweet potatoes in small gardens. Their lives are not as austere as those of pure hunter-gatherers, but they are far closer to them than they are to sedentary, full-time farmers.

It took around five thousand years after the origin of agriculture for true states to appear on the scene. That happened when the more powerful chiefdoms used their armed retinues to bring other chiefdoms and tribes under their control, further centralizing their power and supporting niches for specialized classes of artisans and soldiers. The emerging states built strongholds, cities, and other defensible settlements, and they developed writing systems that allowed them to keep records, exact taxes and tributes from their subjects, and codify laws to keep them in line. Petty states with designs on their neighbors' assets sometimes forced them to become states in defense, and bigger states often swallowed smaller states.

When the first farmers settled down to grow grains and legumes and keep domesticated animals, their numbers exploded and they began to divide their labors, so that some of them lived off the food grown by others. But they didn't develop complex states and governments right away. They first coalesced into tribes connected by kinship and culture, and the tribes sometimes merged into chiefdoms, which had a centralized leader and a permanent entourage supporting him. Some of the tribes took up pastoralism, wandering with their livestock and trading animal products with sedentary farmers. The Israelites of the Hebrew Bible were tribal pastoralists who developed into chiefdoms around the time of the judges.

For all these reasons, it makes no sense to test for historical changes in violence by plotting deaths against a time line from the calendar. If we discover that violence has declined in a given people, it is because their mode of social organization has changed, not because the historical clock has struck a certain hour, and that change can happen at different times, if it happens at all. Nor should we expect a smooth reduction in violence along the continuum from simple, nomadic hunter-gatherers to complex, sedentary hunter-gatherers to farming tribes and chiefdoms to petty states to large states. The major transition we should expect is at the appearance of the first form of social organization that shows signs of design for reducing violence within its borders. That would be the centralized state, the Leviathan.

Anthropologists have proposed many subtypes and intermediate cases among these kinds of societies, and have noted that there is no cultural escalator that inevitably turns simpler societies into more complex ones. Tribes and chiefdoms can maintain their ways indefinitely, such as the Montenegrin tribes in Europe that lasted into the 20th century. And when a state breaks down, it can be taken over by tribes, as in the Greek dark ages (which followed the collapse of the Mycenaean civilization and in which the Homeric epics were set) and the European dark ages (which came after the fall of the Roman Empire). Even today, many parts of failed states, such as Somalia, Sudan, Afghanistan, and the Democratic Republic of the Congo, are essentially chiefdoms; we call the chiefs warlords.

The topic of violence in nonstate societies has a long and politicized history. For centuries it was conventional wisdom that native peoples were ferocious barbarians. The Declaration of Independence, for instance, complained that the king of England "endeavoured to bring on the inhabitants of our frontiers, the merciless Indian Savages whose known rule of warfare, is an undistinguished destruction of all ages, sexes and conditions."

It's not that any early state was (as Hobbes theorized) a commonwealth vested with power by a social contract that had been negotiated by its citizens. Early states were more like protection rackets, in which powerful Mafiosi extorted resources from the locals and offered them safety from hostile neighbors and from each other. Any ensuing reduction in violence benefited the overlords as much as the protectees. Just as a farmer tries to prevent his animals from killing one another, so a ruler will try to keep his subjects from cycles of raiding and feuding that just shuffle resources or settle scores among them but from his point of view are a dead loss.

Today the passage seems archaic, indeed offensive. Dictionaries warn against using the word savage (related to sylvan, "of the forest") to refer to native peoples, and our awareness of the genocides of Native Americans perpetrated by European colonists makes the signatories seem like a black pot in a glass house casting the first stone. A modern concern with the dignity and rights of all peoples inhibits us from speaking too frankly about rates of violence in preliterate peoples, and the "anthropologists of peace" have worked to give them a Rousseauian image makeover. Margaret Mead, for example, described the Chambri of New Guinea as a sex-reversed culture because the men were adorned with makeup and curls, omitting the fact that they had to earn the right to these supposedly effeminate decorations by killing a member of an enemy tribe. Anthropologists who did not get with the program found themselves barred from the territories in which they had worked, denounced in manifestoes by their professional societies, slapped with libel lawsuits, and even accused of genocide.

To be sure, it is easy to come away from tribal battles with the impression that they are fairly harmless in comparison with modern warfare. Men with a grievance against a neighboring village challenge its men to appear at a given time and place. The two sides face off at a distance at which their missiles can barely reach each other. They talk trash, cursing and insulting and boasting, and fire arrows or chuck spears while dodging those from the other side. When a warrior or two are injured or killed, they call it a day. These noisy spectacles led observers to conclude that warfare among primitive peoples was ritualistic and symbolic, very different from the glorious carnage of more advanced peoples. The historian William Eckhardt, who is often cited for his claim that violence has vastly increased over the course of history, wrote, "Bands of gathering-hunters, numbering about 25 to 50 people each, could hardly have made much of a war. There would not have been enough people to fight, few weapons with which to fight, little to fight about, and no surplus to pay for the fighting."

But the main distortion comes from a failure to distinguish the two kinds of violence that turned out to be so important in studies of chimpanzees: battles and raids. It is the sneaky raids, not the noisy battles, that kill in large numbers. A party of men will slink into an enemy village before dawn, fire arrows into the first men who emerge from their huts in the morning to pee, and then shoot the others as they rush out of their huts to see what the commotion is about. They may thrust their spears through walls, shoot arrows through doorways or chimneys, and set the huts on fire. They can kill a lot of drowsy people before the villagers organize themselves in defense, by which time the attackers have melted back into the forest.

Only in the past fifteen years have scholars with no political ax to grind, such as Lawrence Keeley, Steven LeBlanc, Azar Gat, and Johan van der Dennen, begun to compile systematic reviews of the frequency and damage of fighting in large samples of nonstate peoples. The actual death counts from primitive warfare show that the apparent harmlessness of a single battle is deceptive. For one thing, a skirmish may escalate into all-out combat that leaves the battlefield strewn with bodies. Also, when bands of a few dozen men confront each other on a regular basis, even one or two deaths per battle can add up to a rate of casualties that is high by any standard.

Men in nonstate societies (and they are almost always men) are deadly serious about war, not just in their tactics but in their armaments, which include chemical, biological, and antipersonnel weapons. Arrowheads may be coated with toxins extracted from venomous animals, or with putrefied tissue that causes the wound to fester. The arrowhead may be designed to break away from its shaft, making it difficult for the victim to pull it out. Warriors often reward themselves with trophies, especially heads, scalps, and genitals. They literally take no prisoners, though occasionally they will drag one back to the village to be tortured to death. William Bradford of the Mayflower pilgrims observed of the natives of Massachusetts, "Not being content only to kill and take away life, [they] delight to torment men in the most bloody manner that may be, flaying some alive with the shells of fishes, cutting off members and joints of others by piecemeal and broiling on the coals, eat collops of their flesh in their sight while they live."

Sometimes enough attackers show up to massacre every last member of the village, or to kill all the men and abduct the women. Another stealthy but effective way to decimate an enemy is by ambuscade: a war party can hide in the forest along a hunting route and dispatch enemy men as they walk by. Still another tactic is treachery: the men can pretend to make peace with an enemy, invite them to a feast, and at a prearranged signal stab the unsuspecting guests. As for any solitary man who blunders into their territory, the policy is the same as it is with chimpanzees: shoot on sight.

In the early 19th century an English convict named William Buckley escaped from a penal colony in Australia and for three decades lived happily with the Wathaurung aborigines. He provided firsthand accounts of their way of life, including their ways of war:

Though we bristle when we read of European colonists calling native people savages, and justly fault them for their hypocrisy and racism, it's not as if they were making the atrocities up. Many eyewitnesses have brought back tales of horrific violence in tribal warfare. Helena Valero, a woman who had been abducted by the Yanomamö in the Venezuelan rain forest in the 1930s, recounted one of their raids:

Meanwhile from all sides the women continued to arrive with their children, whom the other Karawetari had captured… Then the men began to kill the children; little ones, bigger ones, they killed many of them. They tried to run away, but they caught them, and threw them on the ground, and stuck them with bows, which went through their bodies and rooted them to the ground. Taking the smallest by the feet, they beat them against the trees and rocks… All the women wept.

When the women saw them returning, they also raised great shouts, dancing about in savage ecstasy. The bodies were thrown upon the ground, and beaten about with sticks -- in fact, they all seemed to be perfectly mad with excitement.

On approaching the enemy's quarters, they laid themselves down in ambush until all was quiet, and finding most of them asleep, laying about in groups, our party rushed upon them, killing three on the spot and wounding several others. The enemy fled precipitately, leaving their war implements in the hands of their assailants and their wounded to be beaten to death by boomerangs, three loud shouts closing the victors' triumph. The bodies of the dead they mutilated in a shocking manner, cutting the arms and legs off, with flints, and shells, and tomahawks.

It was not just Europeans gone native who recounted such episodes but the natives themselves. Robert Nasruk Cleveland, an Iñupiaq Inuit, provided this reminiscence in 1965:

The next morning the raiders attacked the camp and killed all the women and children remaining there… After shoving sheefish into the vaginas of all the Indian women they had killed, the Noatakers took Kititigaagvaat and her baby, and retreated toward the upper Noatak River… Finally, when they had almost reached home, the Noatakers gang-raped Kititigaagvaat and left her with her baby to die…

Some weeks later, the Kobuk caribou hunters returned home to find the rotting remains of their wives and children and vowed revenge. A year or two after that, they headed north to the upper Noatak to seek it. They soon located a large body of Nuataagmiut and secretly followed them. One morning the men in the Nuataagmiut camp spotted a large band of caribou and went off in pursuit. While they were gone, the Kobuk raiders killed every woman in the camp. Then they cut off their vulvas, strung them on a line, and headed quickly toward home.

Cannibalism has long been treated as the quintessence of primitive savagery, and in reaction many anthropologists used to dismiss reports of cannibalism as blood libels by neighboring tribes. But forensic archaeology has recently shown that cannibalism was widespread in human prehistory. The evidence includes human bones that bear human teethmarks or that had been cracked and cooked like those of animals and thrown out in the kitchen trash.

Some of the butchered bones date back 800,000 years, to the time when Homo heidelbergensis, a common ancestor of modern humans and Neanderthals, first appears on the evolutionary stage. Traces of human blood proteins have also been found in cooking pots and in ancient human excrement. Cannibalism may have been so common in prehistory as to have affected our evolution: our genomes contain genes that appear to be defenses against the prion diseases transmitted by cannibalism. All this is consistent with eyewitness accounts, such as this transcription by a missionary of a Maori warrior taunting the preserved head of an enemy chief:

Foraging peoples can invade to gain territory, such as hunting grounds, watering holes, the banks or mouths of rivers, and sources of valued minerals like flint, obsidian, salt, or ochre. They may raid livestock or caches of stored food. And very often they fight over women. Men may raid a neighboring village for the express purpose of kidnapping women, whom they gang-rape and distribute as wives. They may raid for some other reason and take the women as a bonus. Or they may raid to claim women who had been promised to them in marriage but were not delivered at the agreed-upon time. And sometimes young men attack for trophies, coups, and other signs of aggressive prowess, especially in societies where they are a prerequisite to attaining adult status.

You wanted to run away, did you? But my war club overtook you. And after you were cooked, you made food for my mouth. And where is your father? He is cooked. And where is your brother? He is eaten. And where is your wife? There she sits, a wife for me. And where are your children? There they are, with loads on their backs, carrying food, as my slaves.

Many scholars have found the image of harmless foragers to be plausible because they had trouble imagining the means and motives that could drive them to war. Recall, for example, Eckhardt's claim that hunter-gatherers had "little to fight about." But organisms that have evolved by natural selection always have something to fight about (which doesn't, of course, mean that they will always fight). Hobbes noted that humans in particular have three reasons for quarrel: gain, safety, and credible deterrence. People in nonstate societies fight about all three.

People in nonstate societies also invade for safety. The security dilemma or Hobbesian trap is very much on their minds, and they may form an alliance with nearby villages if they fear they are too small, or launch a preemptive strike if they fear that an enemy alliance is getting too big. One Yanomamö man in Amazonia told an anthropologist, "We are tired of fighting. We don't want to kill anymore. But the others are treacherous and cannot be trusted."

But in most surveys the most commonly cited motive for warfare is vengeance, which serves as a crude deterrent to potential enemies by raising the anticipated long-term costs of an attack. In the Iliad, Achilles describes a feature of human psychology that can be found in cultures throughout the world: revenge "far sweeter than flowing honey wells up like smoke in the breasts of man." Foraging and tribal people avenge theft, adultery, vandalism, poaching, abduction of women, soured deals, alleged sorcery, and previous acts of violence. One cross-cultural survey found that in 95 percent of societies, people explicitly endorse the idea of taking a life for a life. Tribal people not only feel the smoke welling up in their breasts but know that their enemies feel it too. That is why they sometimes massacre every last member of a village they raid: they anticipate that any survivors would seek revenge for their slain kinsmen.