French anthropologist Philippe Descola tells Lucy Hodges about a lively two years spent with an isolated South American Indian tribe of wife-beaters and reformed headhunters with a penchant for vicious vendettas. Only in France would a 400-page book about a legendary tribe in the Amazonian jungle famous for headhunting and murderous conflicts with their neighbours become a great critical success.

Here The Spears of Twilight: Life and Death in the Amazon Jungle, for all its fascinating detail and fine writing, is likely to be read only by a few anthropological cognoscenti. That is a pity, because Philippe Descola has achieved what few others have done before - enabled us to smell, hear and feel what it was like to live with South American Indians who had somehow avoided the white man since the Spanish conquest.

During the two years he and his saintly companion, Anne-Christine, spent with the Achuar, they lived as members of the tribe, eating monkeys, toucans and wild pig, drinking the local beer, throwing up in the daily sacrifice, hallucinating on local drugs, learning the local language, listening to endless stories of vendettas and painting their faces red. They were plagued by mosquitoes, jiggers and lice. He got malaria and hepatitis. But Descola, professor of anthropology at the Ecole des Hautes Etudes en Sciences Sociales, Paris, does not complain. He tells his story non-judgementally and with endearing self-deprecation.

As a French intellectual and a graduate of the elite Ecole Normale, he had hardly been trained for life in the wild. "I myself was trained for textual criticism and reflective studies," he explains in the book. "I knew how to establish a genealogy and identify a kinship terminology. I had been taught how to measure a field using a compass and a surveyor's chain, but nothing in my earlier life had prepared me for the role of a trapper."

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As the son of intellectuals he grew up surrounded by the complete collection of Le Tour du Monde, the equivalent of The National Geographic at the end of the 19th century, which was beautifully illustrated with woodcuts in the style of Jules Verne. Which is why he has illustrated his book in similar style - with pen drawings. These books collected by his parents were also full of accounts of travels in the Amazon. Descola was hooked. When he read Tristes Tropiques, the classic text by Claude Levi-Strauss, who also wrote about anthropology in a personal narrative style, he was smitten.

Anthropological personal narrative is almost a literary genre in France. "My ambition in this book was not only to describe a very interesting culture, very original, which had very few contacts with the exterior world, but at the same time also to try to make the uninitiated understand what anthropology is about," he says.

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Many people think anthropologists work a bit like natural scientists. In fact, the way they build their knowledge, Descola shows, is through participant observation - through chance. It depends on circumstances, on interactions with people. "The subjective aspect of our interactions is extremely important in our interpretations of a culture," he says. "That is why this is written as a chronicle which highlights certain moments in the process of learning what another culture is about." Thus he begins with describing the daily ritual of this subgroup of the Jivaro tribe who live on the border of Peru and Ecuador - how you sit, eat, where you can go, where you cannot go. At the beginning he was passively observing what was going on as he could not understand a word anyone was saying.

He took no interpreter with him because no one speaks this particular Indian dialect.

As the months rolled by the fog cleared. Slowly, he began to understand the language. That enabled him to get the hang of the social life, the relationships between people and different local groups.

Magic plays a pivotal role in the lives of the Achuar. Nankui is, for example, the goddess or spirit of the vegetable patch, appearing in people's dreams and watching benignly over the cultivation of the staples of the tribal diet. The women, whose job is to grow manioc root and other vegetables, sing to her. They invest their plants with souls - sweet potato and courgettes have feminine souls, the banana tree a masculine one. When plants fail, foul play - curses and jealousy - is suspected. When roots acquire red streaks, it is assumed the plant has sucked blood.

In fact, it was the tribe's relationship to the environment that struck Descola most. "Because they think most plants and animals have souls they are treated as persons," he says. "This means plants and animals are part of the society in the sense that you can communicate with them through magical songs, through dreams and trances."

He found the tribal penchant for internecine warfare, assassination and pulling the trigger on whoever you think has it in for you, harder to take. Domestic violence was common too, he discovered, as evidenced by the criss-cross scars all over the skulls of women. Men practise polygamy and keep their wives in order by taking a swing at them or reaching for their machete. It is considered normal for a husband to beat his wife when angry.

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Descola describes how one brute of a husband, not content with his three wives, took over the other three wives of his brother when his brother's head was carried off by a neighbouring tribe. Of the three new wives, he killed one, suspecting adultery, the second ran away to escape ill-treatment and the third tried to commit suicide by taking poison. When bandaging the women's wounds, poor Descola found his neutrality as an ethnologist severely tested.

Given the lurid reputation the Achuar enjoy, Descola was disappointed to discover they had not practised headhunting for a long time. In fact, he was told they had never made shrunken heads and that practice was associated with a neighbouring tribe. They were, however, the only group locally to continue with internal vendetta warfare, a practice linked to shamanism.

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Vendettas break out typically because a wife flees a brutal husband to join another man. The brute and his family feel justified in seeking revenge against the new man and his kin, and go in search, armed with weapons. Or maybe a relation dies. The tribe does not accept the notion of natural death. They believe either you are killed through violence or you die as a result of a shamanic spell. If the latter, there is always someone responsible. Which means a vendetta. Descola emphasises, however, that the violence is ritualised and that daily life is extremely peaceful most of the time.

What did the Achuar think of him? They were sorry for him because he had only one wife, and were keen to give him another. But Descola said Anne-Christine's father would not like it. Similarly Anne-Christine was considered unfortunate in having such a spindly fellow for a husband - and one so lacking in hunting prowess.

The tribe had no idea it was a tiny island in a mostly westernised world. Descola told them he had come from far away over the sea, but they had no idea what the sea was. And, of course, it was difficult to explain. They have managed to remain so isolated because they are warriors and because of geography. It is very hard to reach them. Descola and his companion had to travel by canoe along a river and on foot through the jungle for several days, tripping on roots and slipping on damp clay. When the intrepid pair reached the Achuar they were abandoned instantly by their guides because of the tribe's terrifying reputation.

It took them months to build up a relationship with their hosts, but they managed it, partly because they conformed to their customs but also because they did not boss them about like the missionaries did. They gave them multicoloured glass beads - just like explorers in 19th-century engravings - as well as lengths of cloth, fishing hooks and axe-heads. They became so close in fact that Descola was made an amik - a kind of brother - by the man he lived with.

Their two years went by in a blur of gardening, hunting, drinking manioc beer, listening to ritual dialogues, taking notes by torchlight and the occasional wild party. Descola makes no secret of how bored he became, shut off from the outside world with the same faces day after day bringing the same stories. "I sometimes wonder whether the vendettas that punctuate their lives are not a means for them to escape from the everyday grey uniformity," he writes. Eventually his desire to understand the Achuar eroded. It was time to pack his bags and head back to Paris. One might think he would have been relieved to get home to creature comforts, but he was not. It was extremely hard to adjust to the materialism of the West.

He came away having learnt some encouraging lessons - that people can be at one with nature rather than continually striving to conquer it, and that it is possible to live without divine or historical transcendence, or national consciousness. Descola has built his academic career on a study of the Achuar and this book is an attempt to convey the excitement of what he discovered to a wider audience. It took him 15 years to write. One can see why his compatriots lapped it up.

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The Spears of Twilight, by Philippe Descola, HarperCollins, Pounds 20.