Footage of a chimpanzee being born has shown that the animals give birth in a way that was thought to be unique to humans.
A team took close-up footage of captive chimps giving birth, which revealed that the newborn emerges from the birth canal facing away from the mother.
Scientists had believed that this birth position evolved in the primate ancestors of modern humans.
These findings could refute that theory.
The researchers filmed three live births for their study, which is published in the Royal Society journal Biology Letters.
The team, from Hayashibara Great Ape Research Institute in Okayama, Japan, lived and even slept in the same enclosures as the captive chimps, to ensure they would be there when they gave birth.
"We have been trying to keep strong relationship with the chimpanzees," lead researcher Satoshi Hirata told BBC News.
"We stayed in the room when they gave birth, so we could record the behaviour with a camera from very close distance."
In non-human primates such as monkeys, the newborn emerges facing the mother. This allows the mother to "safely lift the infant towards her and clear its breath passage soon after the birth", the scientists explained in the paper.
"Anthropologists have argued that the fact that human babies are born facing away from the mothers have led to [the need for] 'midwifery'," said Dr Hirata. "But our observation tells us that this is not true.
"We tend to think that we are unique, without knowing [enough] about other animals."
The researchers stressed that it was possible the three cases they filmed were "exceptions to general rules".
"But [all of] the cases were similar in terms of the direction of the face and shoulders when they emerged," they explained in the paper, which suggests that this is "normal" and regulated in some way by the chimp's anatomy.