By Joseph Milton of Nature magazine

A key feature of human childbirth, long thought to be unique to Homo sapiens--the arrival of the baby facing backwards relative to its mother--has been observed in our closest living relatives, chimpanzees.

The discovery, reported April 19 in Biology Letters, calls into question the argument that backwards-facing babies were an important factor in the evolution of midwifery in humans. Rather than searching for assistance when they go into labour, pregnant chimps seek solitude.

"It's clear from our observations that chimp babies are born facing backwards, but they give birth alone," says lead author Satoshi Hirata, a behavioural biologist at the Great Ape Research Institute of Hayashibara Biochemical Laboratories in Tamano, Japan. "So the reverse orientation is clearly not a necessary condition for the evolution of midwifery."

Remarkably, before Hirata and his colleagues filmed three captive chimpanzees giving birth, nobody had observed chimp parturition at close quarters, and the animal's young were assumed to be born facing forwards, as are those of many other non-human primates. Click here to see one of the team's videos.

Hirata thinks this is probably because the timing of birth is unpredictable, and because pregnant females do not like company when they give birth. "They get very nervous," he says.

Close relationship

The researchers were able to observe the births only because of their very close relationship with the animals they study. "We even sleep in the chimpanzee enclosures every night," says Hirata, "so we could be in the same room as the pregnant females and record the behavior from a very close distance."

Hirata says that, during the births, he and his co-workers had no idea that they were witnessing something so momentous. It was only thanks to a discussion with a human-childbirth researcher that the importance of their observations came to light. "She was very surprised to see the orientation of the baby, so we decided to write a paper about it," Hirata says.

The idea that babies being born backwards--making it difficult for the mother to pick up and nurture the child--may have been instrumental in the evolution of midwifery was first suggested by anthropologists in the 1980s.

"But their arguments were not based on clear comparative data from non-human primates," says Hirata. "Now our data have clearly shown that's not the case."

Unusual humans

Wenda Trevathan, a biological anthropologist at New Mexico State University in Las Cruces, was one of the first to suggest that fetal orientation played an important part in the evolution of midwifery.

"It's taken 25 years for people to start reporting some observations that help confirm or refute my hypothesis," she says, "so I'm glad that finally we've got some observational data on chimpanzees--it's advancing science."

Trevathan says there are still aspects of human labor that make it "unique, or at least very unusual." "One is the series of rotations that the fetus undergoes as it is born--I'm not sure that's been called into question," she says. "Another is routinely seeking assistance."

She adds that the orientation of the human infant still provides a compelling explanation for the evolution of midwifery in humans because "assistance definitely facilitates delivery when the baby comes out in that position."

She also thinks her arguments have often been misinterpreted. "I have never said assistance is a necessity in human childbirth, but rather that it's beneficial."

Trevathan thinks that the pertinent question is not why humans have evolved midwifery, but rather why chimps have not.

Although the study does not tackle that question, it certainly helps to quash the outmoded idea that humans are distinct from the rest of the animal kingdom. "In a broad sense I think humans tend to believe we are unique," says Hirata, "but that belief is not based on facts."

This article is reproduced with permission from the magazine Nature. The article was first published on April 20, 2011.