STUDIES OF the sex lives of remote South American tribes have thrown into question the orthodox view of how the nuclear family has become the preferred arrangement for rearing children as far apart as Boston and Beijing.

Conventional views of human biological evolution have assumed that the nuclear family came about because of the universal belief that only one man could be the father of a child. This view has permeated western society since the Greeks and Romans, long before it was proved a biological fact just over 100 years ago.

Scientists assumed that nuclear families - unique to humans in the family of apes - were the result of a Darwinian trade-off between men and women. Marriage was the contract whereby women would provide men with a guarantee of paternity, and men would provide women with food and resources for rearing children.

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The theory explained how early humans evolved a social organisation for rearing children that was substantially different from the ape-like norm of females being entirely responsible for offspring with the males contributing little or nothing to their upbringing.

However, several studies of South American tribes living in the lowlands of Amazonia have now thrown this grand theory into confusion. Anthropologists have discovered that many people living the sort of primitive existence in which nuclear families were supposed to have evolved actually believe that children are fathered by more than one man.

"In lowland South America, the core belief is that all men who have intercourse with a woman during her pregnancy share the biological fatherhood of her child," said Stephen Beckerman, an anthropologist at Pennsylvania State University who has studied the Bari tribe of Venezuela.

"The woman's husband, if he cohabited with her during pregnancy, is usually considered the primary biological father. The lovers are secondary fathers," he told the meeting of the American Association for the Advancement of Science in Los Angeles.

Anthropologists have called the phenomenon "partible paternity" to reflect how unusual it seems to westerners brought up on a tradition of one father, one child. "But no one knew this scientifically until 1879. Before this, although western law and custom assumed that each child had a single biological father, that premise was simply a folk belief. It was just a lucky guess," he said.

Several studies have now shown that the notion of partible paternity is widespread. At least 18 geographically separated groups in South America's lowland forests believe in partible paternity and other researchers have found evidence for the belief in indigenous peoples of New Guinea and possibly India, Dr Beckerman said.

"This finding suggests that modern evolutionary scenarios that assume certainty of paternity as a crucial element in the evolution of modern humans from African hominids may be incorrect," he said.

William Crocker, an anthropologist at the Smithsonian Institute in Washington DC, discovered partible paternity in the Canela tribe of Amazonian Brazil, who believe that it takes many inseminations from several men to make a baby.

"In addition, they believe that the eventual adult offspring has grown to express the characteristics of the men who contributed the most semen," Dr Crocker said. "Thus, a pregnant woman seeks affairs with the men besides her husband whom she wants her foetus to be like."

Another South American tribe, the Curripaco, who live on the border between Colombia and Venezuela, also believe children are the product of many fathers.

"They believe if a woman copulates with more than one man, then they are all, to a greater or lesser extent, depending on the amount of sexual access they have had with her, the biological fathers of her child," said Paul Valentine, a social anthropologist at the University of East London.