A focus on western women has skewed our understanding of evolution and twisted ideas of what birth should look like, scientists say.

Researchers have found a large variation in the shape of the birth canal between women from different parts of the world – a finding they say has implications both for our understanding of how the pelvis has evolved and our ideas about how babies should move as they leave the womb.

The commonly held belief is that when humans evolved to walk on two feet, our pelvises underwent a radical change. “The whole pelvis had to change and become much smaller and compact, and this is basically to allow for better balance on two feet,” said Lia Betti, first author of the study from the University of Roehampton.

Later, she added, humans started to develop very big brains. The upshot was that babies were born with bigger heads, creating what is known as the “obstetrical dilemma” – opposing requirements on the pelvis, and in particular the birth canal. This, people thought, led to a twisted birth canal to make delivery easier.

“However, that was based – basically exclusively – on European women, who have a particularly twisted canal,” said Betti.

Betti said the study also shows there are problems with standard textbooks, as they base their descriptions of the anatomy of the birth canal – and how a baby must rotate as it passes through – on European women.

“A different pattern of childbirth that is considered odd for European women and therefore seems to ring alarm bells could be perfectly normal for other populations,” said Betti. She added that there were “horrific” stories from the 1930s and 40s of African American women having forceps used during birth because their babies were not turning as expected.

“Actually you can have several different shapes in modern women, and everybody seems to walk fine and the mothers give birth in the right way,” said Betti.

Writing in the Proceedings of the Royal Society B journal, Betti and co-author Andrea Manica from the University of Cambridge describe how they looked at the remains of 348 women from around the world, dating from 2000BC onwards: taking measurements of the birth canal, they note, is difficult in living women.

The results reveal that the bony birth canal varies in shape significantly between women from different geographical regions. Among the discoveries, the team say the pelvic inlet – the large aperture seen as one looks down at the pelvis – is more oval for women from Europe and north Africa or the Americas, and rounder in women from Asia or sub-Saharan Africa.

Indeed, the variation in measurements of the birth canal between women from different parts of the world appears to be greater than that seen for many other body parts, such as the length of leg bones.

Further analysis, involving information of where the women were from and the conditions in those locations, showed that the variation seems to be largely down to a process known as genetic drift. In other words, populations outside Africa may have carried only a fraction of the original genetic diversity for birth canal dimensions, meaning particular versions of such genes were lost due to chance.

What’s more, local climate did not seem to be behind the differences seen for women from different parts of the world.

Overall, the team found distance from Africa explained more than 43% of the diversity seen for the human birth canal.

Betti said the findings cast doubt on the theory that evolutionary pressures linked to walking on two legs have greatly restricted the size and shape of the birth canal. Instead, she said, other explanations for the tight fit of the baby in the birth canal need to be considered, including that it might be down to babies becoming bigger with the advent of agriculture and changes to nutrition.

Dr Philipp Mitteröcker from the University of Vienna said the study convincingly showed geographic differences in birth canal dimensions. But he said it was too soon to rule out that women in different parts of the world might have faced different evolutionary pressures that could also have shaped the birth canal.

Dr Ashley Hammond of the American Museum of Natural History, who was not involved in the study, said it suggests that scientists should be careful about interpreting all features of the pelvis as arising from natural selection.

“For instance, the pelvic shapes of Neanderthals and Homo erectus are often [categorised] as cold-adapted or warm-adapted anatomies, but they may actually reflect past population history more than functional adaptations for climate,” she said.

But she also noted that genetic drift might not be the only thing behind the variation in birth canals around the world: romantic liaisons with other human species might also have introduced differences.

“Specifically,” she said, “we know based on genomic data that Neanderthals and a mysterious hominin population called the Denisovans both interbred with modern humans after they left Africa – have these hominins contributed to regional diversity as well?”