The war between Bisaasi-teri and Konabuma-teri started with an axe murder. The two Yanomamö villages, several days walk from each other in the forests around the unmapped headwaters of Orinoco river, had been moving towards an alliance when disease killed several children in Bisaasi-teri. The Yanomamö believe that disease is caused by evil spirits cast from other villages: when it falls upon them their shamans attempt to drive out the invading demons and to return them to the villages responsible. After the shamans of Bisaasi-teri concluded that the disease in their village had been cast from the purportedly friendly village of Konumba-teri, a respected visitor arrived from Konabuma-teri. He was greeted in the normal way: the men of the Bisaasi-teri came out with their weapons to yell at him intimidatingly until he had stood calmly for long enough to prove himself fearless. Then he was invited into the village — a single, circular, comunal roof covering family houses arranged around an open space — and given a gourd of soup to drink in front of the headman’s house. As he squatted on his haunches, drinking the soup, he was approached from behind by Mamikininiwä, who is described by the anthropologist Napoleon Chagnon as "a mature man of about forty, whose decisions few would challenge", and who carried an axe whose worn steel head had been traded over the course of years in from the coast. He smashed it into the visitor’s head without any warning, and the man died almost at once.

The story comes in the latest edition of Professor Chagnon’s classic work on the Yanomamö, subtitled until recentlyThe Fierce People, which goes on to trace a thirty-year pattern of migration, alliance, and emnity, that resulted from this murder in 1950: the Konumba-teri retaliated by arranging for mutual allies to hold a feast for the people of Bisaasi-teri and once their guests were comfortably immbiolised by the hammocks they were offered to rest in, attacked them with clubs and bowstaves, before pursuing the survivors with a flight of arrows. Around a dozen men were killed in this massacre. It’s difficult to be more precise because Chagnon learned of it only thirty years later and in any case, the Yanomamö counting system runs one, two, more-than-two.

The war within the American Anthropological Association probably started in 1994, with a shouting match between Professor Chagnon and Professor Terry Turner, of Cornell University, who called him "a sociopath" at a meeting arranged to reconcile Chagnon with the Salesian missionaries. It’s difficult to be more precise, because Chagnon and Turner had been enemies for years before then. But their equivalent of a murderous axe blow came this summer, with the release of an email, signed by Turner, and Professor Les Sponsel, of Hawaii University, in which they accused Chagnon of participation in "crimes beyond the imagination of Joseph Conrad, though not, perhaps, of Josef Mengele." In particular, he was supposed to have taken part in an experiment which started a deadly measles experiment among the Yanomamö in 1968, killing thousands and refusing medical treatment to those they had infected with a vaccine known to be potentially lethal. Both Turner and Sponsel are full professors, who have held high office in the AAA, "mature men, whose decisions few would question." The effect of their email was devastating. Had the charges been true, they would have finished Chagnon’s reputation as surely as an axe to the back of his head would have ended his life.

What is truly extraordinary about the story is that the charges set out in Turner and Sponsel’s email, are not only untrue; they could not possibly not be true, as reports of a full and thorough scientific investigation made plain in front of an enthralled crowd of four or five thousand anthropologists crammed into a ballroom at the Hilton Hotel in San Francisco last month — yet despite this, a sizeable minority of the profession clearly wish that one of their colleagues was guilty of genocide, and feel his moral guilt is established whatever the scientific and historical facts may be.

Napoleon Chagnon, the target of this animosity, is one of the most famous anthropologists alive. A large, bearded, weatherbeaten man, he lived and worked with the Yanomamö for a total of five years between 1964 and 1988 and his books and films about his time with them have become a set text in thousands of anthropology courses while at the same time provoking deep theoretical debates at the top of his profession and well outside it. He managed to combine dramatic human stories with a rigorous, but deeply controversial theory of the role of warfare and sexism in human nature which became one of the cornerstones of Evolutionary Psychology and all the modern Darwinism promoted by people such as Richard Dawkins, Dan Dennett and Steven Pinker. He argued that violence or the threat of violence shaped Yanomamö life in all sorts of ways and that the root of this violence was the competition between men for women.

But he had also made plenty of enemies. Some were professional. Chagnon’s explanation of Yanomamö violence was deeply controversial, both among people who understand it as a general theory of human nature (which it is meant to be), and among those who see it only as a particular explanation of Yanomamö culture. The Yanomamö are not in fact exceptionally violent by the standards of aboriginal people — which is to say that homicide is far more common among them than it is among better-armed, but more settled societies: it is something like eight times as high as New York’s, for example. Among the tribe, a quarter of all adult males die as a result of violence, according to Chagnon’s figures. But this is about in line with the rate for the !Kung bushmen of South Africa, and far below some figures recorded for other tribes in the Amazon basin, or in New Guinea and among some aborigines. But plenty of anthropologists who accept the facts of Yanomamö violence as recorded by Chagnon reject his interpretation of it: some believe they are fighting over food supplies; others that they fight for access to Westerners and Western artefacts, among them the machetes with which Chagnon and other antrhopologists trade for information.

Some of his enemies were personal. He is by all accounts a boisterous man. Moving among the Yanomamo, Chagnon had, or discovered in himself, the sort of personality that could thrive and impose itself on a brutal and treacherous political environment. For the first months of his fieldwork, while he was learning the language, the Yanomamö systematically lied to him. He needed to collect genealogies in order to trace the histories of the people he moved among, yet among the Yanomamö there is a tabu against using people’s names, and especially the names of the dead. He got around this difficulty by offering machetes to people who would talk to him: they squared their consciences by accepting the gifts but giving him false names for their fellow villagers. It took him five months to discover that the secret names for the chief’s family that he had so carefully collected actually translated as "long dick", "hairy cunt" and their daughter "fart breath". He overcame this obstacle with characteristic determination. It did not make him more trusting of the Yanomamö, and he later wrote: "It became indelibly clear to me shortly after I arrived that … I had to become like the Yanomamö to be able to get along with them on their terms: somewhat sly, aggressive, intimidating, and pushy.

"Had I failed to adjust in this fashion I would have lost six months of supplies to them in a single day … more importantly, had I failed to demonstrate that I could not be pushed around beyond a certain point, I would have been the subject of far more ridicule, theft, and practical jokes than the actual case." However effective this kind of behaviour may be among the Yanomamö it wins him little respect in the post-modern common room. He also lacks piety, both old-fashioned and modern. He is rude to and about missionaries, but when he discusses wife-beating among the Yanomamö he does not bother to tell us it is a wicked thing. He observes instead that it provides a low-cost way for a man to demonstrate his ferocity to other men. It is a common theme of his opponent that he is much more like the brutes he describes than the real Yanomamö are: they are not "the fierce people"; he is "the fierce anthropologist". Frank Salamone, a Catholic anthropologist who tried to broker a truce between Chagnon and the Salesian missionaries, says: "I believe the enmity is the result of Chagnon's harmful field methods, ‘fierce’ personality, and his tendency to lie. He has harmed the Yanomami by his false depiction of them and his failure to defend them in the press."