Making babies is hard work for Curripaco men who live in the tropical rainforest of north-west Amazonia. One sex act and one male partner are never enough to produce a child and the men who share in this difficult task each wins a claim to being the biological father.
The Curripaco, in common with an increasing number of hunter-gatherer peoples being discovered around the world, subscribe to "partible paternity", which holds that every man who contributes sperm during a pregnancy contributes biologically to the child.
Partible paternity "is an uncomfortable concept" in our society, explained Dr Stephen Beckerman, associate professor of anthropology at Pennsylvania State University.
It also flies in the face of theories on the evolution of humans from the point when we broke away from the practices of apes and set up a new way of organising ourselves.
"The existence of partible paternity as a common concept raises serious questions about the presumed evolutionary bargain between men and women," he said.
"The bargain, that men supply the resources in return for female fidelity and guaranteed paternity, may not exist.
"Partible paternity appears to be quite common in South America and exists in at least 18 groups widely separated in distance and culture in the lowland forests and savannahs. 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."
The practice has definite advantages. Two previous studies indicated that children with multiple fathers had higher survival rates than children with one recognised father, he said. The secondary fathers publicly accepted a responsibility and might supply meat for the child or protect it from dangers. Two or more dads are obviously better than one. It also allows a degree of picking and choosing your patrilineage. "Your biological paternity is negotiated," he said.
Everyone had to agree who contributed during the pregnancy. In most cases this soliciting for contributions did not begin until after the pregnancy was established, however, so in the majority of cases the spouse was also the biological progenitor.
Dr Paul Valentine of the department of sociology and anthropology at the University of East London conducted studies of the Curripaco, who establish a ranking for the multiple fathers. In effect, the more access to the woman and the greater the contribution of semen, the higher the rank from "major father" to the "lesser fathers".
Dr Robin Baker, formerly of Manchester University, author of Sperm Wars and expert in human mating and procreation strategies, pointed out that while westerners didn't have partible paternity, they certainly had similar behaviour patterns.
His research had shown that 10 per cent of children born in Britain were not the children of the husband.
"Although we have a different set of beliefs, there isn't such a difference in behaviour," he suggested.