Anthropologist Stephen Beckerman was well into his forties before he finally understood how babies are made. He had thought, as most people do, that a sperm from one man and an egg from one woman joined to make a child. But one summer day, as he and his colleague Roberto Lizarralde lounged around in hammocks, chatting with Rachel, an elderly woman of the Barí tribe of Venezuela, she pointed out his error. Babies, she explained, can easily have more than one biological father. "My first husband was the father of my first child, my second child, and my third child," Rachel said, recalling her life. "But the fourth child, actually, he has two fathers." It was clear that Rachel didn't mean there was a stepfather hanging around or a friendly uncle who took the kid fishing every weekend. She was simply explaining the Barí version of conception to these ignorant anthropologists: A fetus is built up over time with repeated washes of sperm— which means, of course, that more than one man can contribute to the endeavor. This interview changed not only the way Beckerman and Lizarralde viewed Barí families but also brought into question the very way that anthropologists portray human coupling. If biological fatherhood can be shared—an idea accepted by many indigenous groups across South America and in many other cultures across the globe—then the nuclear family with one mom and one dad might not be the established blueprint for a family that we have been led to expect. If so, the familiar story of traditional human mating behavior, in which man the hunter brings home the bacon to his faithful wife, loses credibility. And if the Barí and other groups work perfectly well with more flexible family styles, the variety of family structures that are increasingly common in Western culture these days—everything from single-parent households to blended families—may not be as dangerous to the social fabric as we are led to believe. People in this culture may simply be exercising the same family options that humans have had for millions of years, options that have been operating in other cultures while the West took a stricter view of what constitutes a family.

Inez and her children live in the village of Saimadodyi in Venezuela, where anthropologists have spent decades gaining the trust of her people—the Barí—in order to study their unusually free and peaceful sexual and family relationships.

Stephen Beckerman folds his 6-foot-4-inch frame into a chair and turns to the mountainous topography of papers on his desk at Pennsylvania State University. Once he manages to locate a map under all the piles, he points to a spot on the border between Venezuela and Colombia where he spent 20 years, off and on, with the indigenous Barí Indians. The traditional Barí culture, Beckerman explains, has come under attack by outside forces, starting with the conquistadors who arrived in the early 16th century. Today Catholic missionaries interact with the Barí, coal and oil companies are trying to seize their land, and drug traffickers and guerrillas are threats. Western influences are apparent: Most families have moved from traditional longhouses to single-family dwellings, and everyone wears modern Western clothes and uses Western goods. However, the Barí continue to practice their traditions of manioc farming, fishing, and hunting, according to Roberto Lizarralde, an anthropologist at the Central University of Venezuela who has been visiting the Barí regularly since 1960. Lizarralde also says that the Barí still have great faith in traditional spirits and ancestral wisdom, including their notion that a child can have multiple biological fathers. The Barí believe that the first act of sex, which should always be between a husband and wife, plants the seed. Then the fledgling fetus must be nourished by repeated anointings of semen; the woman's body is viewed as a vessel where men do all the work. "One of the reasons women give you for taking lovers is that they don't want to wear out their husbands," Beckerman says. "They claim it's hard work for men to support a pregnancy by having enough sex, and so lovers can help." Just look, the Barí say. Women grow fat during a pregnancy, while men grow thin from all their work.

Arohbarí (on hammock), former headman of the village of Bachichida, talks with Manuel Lizarralde (left) and his father, Roberto, about the cattle ranchers who massacred the Barí in the 1940s and 1950s because they wanted control of Barí land. Sometimes half a village was killed in a single morning. While the men talk, the children of the family eat lunch. Bananas are a staple: Nineteen varieties grow nearby, and five of them hang on this kitchen wall.

Anthropologists study a culture's ideas about conception because those ideas have a profound impact on the way people run their lives. In our culture, for example, conceiving children incurs long-term economic responsibility for both the mother and father. We take this obligation so seriously that when a parent fails to provide for a child, it is usually a violation of law. In the Barí system, when a man is named as a secondary biological father he is also placed under an obligation to the mother and the child. In addition, he is expected to give gifts of fish and game. These gifts are a significant burden because the man must also provide for his own wife and primary children.

Beckerman and his colleagues have discovered that naming secondary fathers has evolutionary consequences. A team of ethnographers led by Beckerman, Roberto Lizarralde, and his son Manuel, an anthropologist at Connecticut College who has been visiting the Barí since he was 5 years old, interviewed 114 Barí women past childbearing years and asked them about their full reproductive histories. "These interviews were a lot of fun," Beckerman says, laughing. "Randy old ladies talking about their lovers." In total, the researchers recorded claims of 916 pregnancies, an average of eight pregnancies for each woman. But child mortality was high—about one-third of the children did not survive to age 15. Naming secondary fathers was a critical factor in predicting which babies made it to adulthood. Secondary fathers were involved in 25 percent of pregnancies, and the team determined that two fathers were the ideal number. Children with one father and one secondary father made it to their teens most often; kids with only one father or those with more than two fathers didn't fare as well. The researchers also found that this decrease in mortality occurred not during the child's life but during fetal development: Women were less likely to have a miscarriage or stillbirth if they had a husband and an additional male contributing food. This result was a surprise because researchers had expected that help during childhood would be more important. "The Barí are not hungry; they are not close to the bone. But it must be the extra fat and protein that they get from secondary fathers during gestation that makes the difference," Beckerman explains as he points to photographs of Barí women who look well nourished, even downright plump. Barí women seem to use this more flexible system of paternity when they need it. Within families, some children have secondary fathers, while their siblings belong to the husband alone. The team discovered that mothers are more likely to take on a secondary father when a previous child has died in infancy. Manuel Lizarralde claims the strategy makes perfect sense, given the Barí belief that the best way to cure a sick child is for the father to blow tobacco smoke over the child's body. "It is easy to imagine a bereaved mother thinking to herself that if she had only provided a secondary father and so more smoke for her dead child, she might have saved him—and vowing to provide that benefit for her next child." Beckerman says extra fathers may have always been insurance for uncertain times: "Because the Barí were once hunted as if they were game animals—by other Indians, conquistadors, oilmen, farmers, and ranchers—the odds of a woman being widowed when she still had young children were one in three, according to data we gathered about the years 1930 to 1960. The men as well as the women knew this. None of these guys can go down the street to Mutual of Omaha and buy a life insurance policy. By allowing his wife to take a lover, the husband is doing all he can to ensure the survival of his children." Barí women are also freer to do as they wish because men need their labor—having a wife is an economic necessity because women do the manioc farming, harvesting, and cooking, while men hunt and fish. "The sexual division of labor is such that you can't make it without a member of the opposite sex," says Beckerman. Initially, the researchers worried that jealousy on the part of husbands would make Barí women reticent about discussing multiple sexual partners. "In our first interviews, we would wait until the husband was out of the house," says Beckerman. "But one day we interviewed an old couple who were enjoying thinking about their lives; they were lying in their hammocks, side by side, and it was obvious he wasn't going anywhere. So we went down the list of her children and asked about other fathers. She said no, no, no for each child, and then the husband interrupted when we got to one and said, 'That's not true, don't you remember, there was that guy . . .' And the husband was grinning."

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Sakbonbó, below, is involved in local and national politics. The campaign buttons on his hat support his candidate, incumbent president Hugo Chávez. As gatekeeper of the northern boundary of the Barí territory, Sakbonbó tries to stop local rivals, the Yukpa, from stealing Barí land. He also takes responsibility for his older brother's older son in his role as secondary father. Map by Matt Zang

Not all women take lovers. Manuel Lizarralde has discovered through interviews that one-third of 122 women were faithful to their husbands during their pregnancies. "These women say they don't need it, or no one asked, or they have enough support from family and don't require another father for their child," Lizarralde says. "Some even admit that their husbands were not that happy about the idea." Or it may be a sign of changing times. Based on his most recent visits to the Barí, Lizarralde thinks that under the influence of Western values, the number of people who engage in multiple fatherhood may be decreasing. But his father, who has worked with the Barí for more than 40 years, disagrees. He says the practice is as frequent but that the Barí discuss it less openly than before, knowing that Westerners object to their views. After all, it took the anthropologists 20 years to hear about other fathers, and today the Barí are probably being even more discreet because they know Westerners disapprove of their beliefs. à "What this information adds up to," Beckerman says, "is that the Barí may be doing somewhat less fooling around within marriage these days but that most of them still believe that a child can have multiple fathers." More important, the Barí idea that biological paternity can be shared is not just the quirky custom of one tribe; anthropologists have found that this idea is common across South America. The same belief is shared by indigenous groups in New Guinea and India, suggesting that multiple paternity has been part of human behavior for a long time, undermining all previous descriptions of how human mating behavior evolved.

At dawn a woman walks back home from the river after bathing with these children. Domesticated pigs abound now: They replace wild game that dwindled away as forests were cut to make way for cattle and farming.

Since the 1960s, when anthropologists began to construct scenarios of early human mating, they had always assumed that the model family started with a mom and dad bonded for life to raise the kids, a model that fit well with acceptable Western behavior. In 1981 in an article titled "The Origin of Man," C. Owen Lovejoy, an anthropologist at Kent State University, outlined the standard story of human evolution as it was used in the field—and is still presented in textbooks today: Human infants with their big brains and long periods of growth and learning have always been dependent on adults, a dependence that separates the humans from the apes. Mothers alone couldn't possibly find enough food for these dependent young, so women have always needed to find a mate who would stick close to home and bring in supplies for the family. Unfortunately for women, as evolutionary psychologists suggest, men are compelled by their biology to mate with as many partners as possible to pass along their genes. However, each of these men might be manipulated into staying with one woman who offered him sex and a promise of fidelity. The man, under those conditions, would be assured of paternity, and he might just stay around and make sure his kids survived.

This scenario presents humans as naturally monogamous, forming nuclear families as an evolutionary necessity. The only problem is that around the world families don't always operate this way. In fact, as the Barí and other cultures show, there are all sorts of ways to run a successful household. The Na of Yunnan Province in China, for example, have a female-centric society in which husbands are not part of the picture. Women grow up and continue to live with their mothers, sisters, and brothers; they never marry or move away from the family compound. As a result, sisters and brothers rather than married pairs are the economic unit that farms and fishes together. Male lovers in this system are simply visitors. They have no place or power in the household, and children are brought up by their mothers and by the mothers' brothers. A father is identified only if there is a resemblance between him and the child, and even so, the father has no responsibilities toward the child. Often women have sex with so many partners that the biological father is unknown. "I have not found any term that would cover the notion of father in the Na language," writes Chinese anthropologist Cai Hua in his book A Society Without Fathers or Husbands: The Na of China. In this case, women have complete control over their children, property, and sexuality. Across lowland South America, family systems vary because cultures put their beliefs into practice in different ways. Among some native people, such as the Canela, Mehinaku, and Araweté, women control their sex lives and their fertility, and most children have several fathers. Barí women are also sexually liberated from an early age. "Once she has completed her puberty ritual, a Barí girl can have sex with anyone she wants as long as she doesn't violate the incest taboo," Beckerman explains. "It's nobody's business, not even Mom's and Dad's business." Women can also turn down prospective husbands. In other cultures in South America, life is not so free for females, although members of these cultures also believe that babies can have more than one father. The Curripaco of Amazonia, for instance, acknowledge multiple fatherhood as a biological possibility and yet frown on women having affairs. Paul Valentine, a senior lecturer in anthropology at the University of East London who has studied the Curripaco for more than 20 years, says, "Curripaco women are in a difficult situation. The wives come into the village from different areas, and it's a very patrilineal system." If her husband dies, a widow is allowed to turn only to his brothers or to clan members on his side of the family for a new husband.

Ashirokó (left), one of the oldest of the Barí, uses a shugda, the traditional spear made from palm wood, to catch fish, which account for 75 percent of the protein in their diet. His great-grandson (right) uses a spear made of rubber and metal as well as a diving mask, popular new technology from the West.

The relative power of women and men over their sex lives has important consequences. "In certain social and economic systems, women are free to make mate choices," says Valentine. In these cultures women are often the foundation of society, while men have less power in the community. Sisters tend to stay in the same household as their mothers. The women, in other words, have power to make choices. "At the other extreme, somehow, it's the men who try to maximize their evolutionary success at the expense of the women," says Valentine. Men and women often have a conflict of interest when it comes to mating, marriage, and who should invest most in children, and the winners have sometimes been the men, sometimes the women. As Beckerman wryly puts it, "Anyone who believes that in a human mating relationship the man's reproductive interests always carry the day has obviously never been married." The Barí and others show that human systems are, in fact, very flexible, ready to accommodate any sort of mating system or type of family. "I think that human beings are capable of making life extremely complicated. That's our way of doing business," says Ian Tattersall, a paleoanthropologist and curator in the division of anthropology at the American Museum of Natural History in New York City. Indeed, such flexibility suggests there's no reason to assume that the nuclear family is the natural, ideal, or even most evolutionarily successful system of human grouping. As Beckerman says, "One of the things this research shows is that human beings are just as clever and creative in assembling their kin relations as they are putting together space shuttles or symphonies."

This little boy climbs on a kirora, a wall built of stones and palm leaves that the Barí construct to entrap fish in pools and so increase their catch.