Birth channels are different around the world, countering a long-standing theory of evolution | Science

The shape of a mother's birth canal is a tussle between two opposing forces of evolution: it must be wide enough to allow our babies to pass to the big brain and narrow enough to allow women to walk effectively. At least, it's the common thought. But a new study reveals that birth channels come in various forms among women around the world.

The idea that women's basins have been shaped by an evolutionary compromise – also known as an "obstetric dilemma" – has been a determining factor in anthropology, says Jonathan Wells, human evolution expert at University College London, who was not involved. not at work. But recent studies have challenged him, and new discoveries are adding to this research, he says. If the obstetric dilemma were true, one would expect the birth channels of the world to be relatively standardized, Wells says. But that's not what the researchers discovered.

Lia Betti, biological anthropologist at the University of Roehampton in London and evolutionary ecologist Andrea Manica of the University of Cambridge in the UK, has measured pools of 348 human skeletons from 24 different regions of the world. The birth channels were far from being true copies of each other. Women in sub-Saharan Africa and some Asian populations were generally narrow from one side to another and deep from one front to the other, while Amerindian women had larger canals. Amerindians and Europeans also had the oval most oval channels, reports the team today at Acts of the Royal Society B.

Betti and Manica also found that there was less variability in the shape of the uterine canal in populations further afield from Africa, such as Native Americans. This trend has been observed in other traits, and it is thought that this simply reflects a lower variability in genes and traits among relatively small groups of people who have left Africa to populate the world. Overall, the analysis suggests that a population may have ended up having a particular form of birth canal simply by chance, and not because of any selective pressure.

Temperature could also be a factor. Colder climates favor larger bodies, which retain heat better, which could impact the shape of the birth canal. But pelvic data only weakly follow this trend. Wells argues that other environmental factors may play a role and have yet to be explored.

The work could improve the practices surrounding childbirth, says Betti. For example, a fetus must rotate to negotiate the twisting of the birth canal during labor, and these movements may vary depending on the shape of the birth canal. Betti explains that the midwives with whom she spoke are well aware that women in different parts of the world have marked work differences, although this is not part of their training.

The new findings suggest that if baby's movements differ from what is considered normal in a given area, it is not necessarily a concern. This may simply reflect the range of birth canal shapes seen around the world.