Extraordinary claims, Carl Sagan liked to say, require extraordinary evidence. Here is an extraordinary claim: "Chimpanzeelike violence preceded and paved the way for human war, making modern humans the dazed survivors of a continuous, five-million-year habit of lethal aggression."

The anthropologist Richard Wrangham of Harvard University made this statement in his 1996 book Demonic Males: Apes and the Origins of Human Violence (Houghton Mifflin, co-written with journalist Dale Peterson) and has reiterated it ever since. He asserts that both male humans and chimpanzees, our closest genetic relatives, are "natural warriors" with an innate predisposition toward "coalitionary killing," which dates back to our common ancestor.

The theory has been touted by such influential intellectuals as the philosopher Francis Fukuyama of Johns Hopkins University and the psychologist Steven Pinker of Harvard. "Chimpicide," Pinker wrote in his 2002 bestseller The Blank Slate the Modern Denial of Human Nature, "raises the possibility that the forces of evolution, not just the idiosyncrasies of a particular culture, prepared us for violence."

A new study seems to corroborate Wrangham's demonic-males thesis. A group led by John Mitani of the University of Michigan at Ann Arbor observed a troop of chimpanzees in Uganda's Kibale National Park killing chimps from neighboring troops during a 10-year period of "territorial expansion". This "warfare," Nicholas Wade reported in The New York Times, suggests that "both humans and chimps inherited an instinct for aggressive territoriality from their joint ancestor."

I've been reading and talking to anthropologists about the demonic-males theory for years, and I've turned from a believer to a skeptic. Here are some reasons why:

Wrangham and other chimpanzee researchers often present the rate of "intercommunity killing" in terms of annual deaths per 100,000 population. Mitani, for example, estimates the mortality rate from coalitionary attacks in Kibale to be as high as "2,790 per 100,000 individuals per year." But the researchers witnessed only 18 coalitionary killings. All told, since Jane Goodall began observing chimpanzees in Tanzania's Gombe National Park in 1960, researchers have directly observed 31 intergroup killings, of which 17 were infants.

I obtained these figures by adding numbers from a 2006 paper by Wrangham and two colleagues and from the new report by Mitani's group. Researchers have "suspected" or "inferred" a few dozen more lethal attacks, in which a chimpanzee is found dead or simply disappears. All these violence statistics, according to an analysis published this year by the anthropologists Robert Sussman and Joshua Marshack of Washington University in Saint Louis (W.U.), are based on 215 total years of observations at nine different sites in Africa. In other words, researchers at a typical site directly observe one killing every seven years.

Wrangham conceded in a response to Sussman and Marshack, published in the same volume as their analysis, that chimpanzee coalitionary killings are "certainly rare." He also acknowledged that "there are various sites where scientists have studied chimpanzees without any record of coalitionary killing or other kinds of violence." He suggests that these nonviolent chimpanzees are not "habituated" to their human observers or are isolated from other communities. But that raises another question: Could unusual environmental conditions be triggering intergroup chimpanzee killing?

The first lethal gang attack was witnessed in 1974 at Gombe, after Goodall and her co-workers had spent 14 years closely observing chimpanzees. Goodall, who began supplying bananas to chimpanzees in 1965, once expressed concern that the feeding "was having a marked effect on the behavior of the chimps. They were beginning to move about in large groups more often than they had ever done in the old days. Worst of all, the adult males were becoming increasingly aggressive. When we first offered the chimps bananas the males seldom fought over their food; …now…there was a great deal more fighting than ever before." (This quote appears in Sussman and Marshack's paper.)

Chimpanzees throughout Africa are also increasingly threatened by poachers, farmers and other humans. Ian Tattersall, an anthropologist at the American Museum of Natural History, told me that chimpanzee violence is "plausibly related to population stress occasioned by human encroachment." In other words, outbreaks of lethal violence among chimpanzees may stem primarily from environmental and even cultural factors. Wrangham himself has emphasized that chimpanzees display "significant cultural variation" in tool use, courtship and other behaviors.

Another challenge to Wrangham's theory is Pan paniscus, otherwise known as the pygmy chimp, or bonobo. Bonobos are darker-skinned and slimmer than Pan troglodytes, the more common chimpanzee species, and much less aggressive. Researchers have never observed coalitionary killing among bonobos. Noting that bonobos are just as genetically related to us as chimpanzees, Frans de Waal, a primatologist at Emory University, suggested last year in The Wall Street Journal that bonobos may be "more representative of our primate background" than are chimpanzees.

As evidence, de Waal cited new studies of Ardipithecus ramidus, or "Ardi," who roamed Ethiopia 4.4 million years ago and is the oldest known human ancestor. Although the species was first discovered in the early 1990s, its evolutionary significance was only spelled out in detail last fall in a multipaper report in Science. One author, anthropologist Owen Lovejoy of Kent State University, told me that Ardi has triggered a "tectonic shift" in views of human evolution. "We now know, especially in light of Ardipithecus, that hominids have always been a far less aggressive clade than are chimpanzees or even bonobos."

Male and female Ardipithecenes were closer in size than male and female chimpanzees and hence more likely to engage in pair-bonding. Ardi also lacked the fanglike canines that chimpanzees employ as weapons. These traits of minimal sexual dimorphism and small canines persisted in later hominid species such as Australopithecus and Homo erectus, which emerged four million and two million years ago, respectively.

There is also no fossil or archaeological evidence that our ancestors fought millions or even hundreds of thousands of years ago. Yes, archaeological digs and modern ethnography have established that warfare was common among pre-state societies, notably hunter–gatherers; our ancestors are thought to have lived as hunter–gatherers since the emergence of the Homo genus two million years ago. Advocates of the demonic-males thesis suggest that if hunter–gatherers ever engaged in warfare, they must have always done so.

But as the anthropologist Douglas Fry of Åbo Akademi University in Finland pointed out in his book Beyond War: The Human Potential for Peace (Oxford University Press, 2009), the oldest clear-cut relic of group violence is a 13,000-year-old grave along the Nile River in the Jebel Sahaba region of Sudan. The grave contains 59 skeletons, 24 of which bear marks of violence, such as embedded projectile points. The oldest known murder victim was a young man who lived 20,000 years ago near the Nile about 200 miles downstream of the Jebel Sahaba site; two stone projectile points were embedded in his pelvic bone.

Evidence of earlier lethal human violence is ambiguous, at best. One example is a 50,000-year-old male Neandertal, found in Iraq's Shanidar Cave, whose ribs were pierced by a sharp object; he lived for several weeks after being wounded. The wound may have resulted from a fight with another Neandertal, according to the anthropologist Erik Trinkaus of W.U. But most Paleolithic injuries probably resulted from "hunting large animals who object to being speared," Trinkaus told me. "You find a lot of evidence of bumps and bruises and broken bones" among Neandertals and other early humans. "There is absolutely no evidence," Trinkaus says, that "war is continuous back to the common ancestor with chimps."

Advocates of the demonic-males thesis insist that absence of evidence of warfare does not equal evidence of absence, especially given the paucity of ancient human and prehuman remains. The absence-of-evidence argument, anthropologist Brian Ferguson of Rutgers University told me, "would be valid if the earlier skeletal and settlement remains were so limited that they would not reliably reveal war." In fact, many regions around the world yield evidence of human habitation "for centuries, even millennia, with no indications of war." For example, excavations have revealed that people settled in Abu Hureya, near the Euphrates River, 11,500 years ago, and lived there for more than 4,000 years while leaving no signs of violence.

Meanwhile, Ferguson notes, "unmistakable" signs of group violence emerged in other regions in northern Africa, the Middle East, Europe, Asia, Australia and the Americas. The evidence consists of skeletons with crushed skulls, hack marks and projectile points embedded in them; rock art depicting battles with spears, clubs, and bows and arrows; and fortifications for protection against attacks. These relics indicate that warfare arose as humans began shifting from "a nomadic existence to a sedentary one, commonly although not necessarily tied to agriculture," Ferguson says.

Archaeologist Jonathan Haas of the Field Museum in Chicago concurs: "There is a very tiny handful of incidences of conflict and possible warfare before 10,000 years ago. And those are very much the exception." In an interview with me he attributed the emergence of warfare in prehistory to growing population density, diminished food sources and the separation of people into culturally distinct groups. "It is only after the cultural foundations have been laid for distinguishing 'us' from 'them,'" Haas says, "that raiding, killing and burning appear as a complex response to the external stress of environmental problems."

On the other hand, Haas adds, "groups that are at war in one era or generation may be at peace in the next." War's recent emergence, and its sporadic pattern, contradict the assertion of Wrangham and others that war springs from innate male tendencies, he argues. "If war is deeply rooted in our biology, then it's going to be there all the time. And it's just not," he says. War is certainly not as innate as language, a trait possessed by all known human societies at all times.

Defenders of the demonic-males theory often accuse critics of being peaceniks who hope that war will be easier to abolish if it isn't innate. I am, I confess, a peacenik. But my criticism—and that of other critics I've cited—stems from science, not ideology. The evidence for the demonic-males theory, far from extraordinary, is flimsy.

Image: chimpanzees, courtesy of Wikimedia Commons