The evolution of easier births means slower walking and pelvis issues

The width of a person’s hips seems to be a result of complex trade-offs between the evolution of larger brains and upright walking, according to the largest study of its kind to date.

“If your brain is getting larger and larger over the same evolutionary time as your pelvis is getting narrower, this, of course, results in conflict,” says Vagheesh Narasimhan at the University of Texas at Austin.

This idea, first proposed in the 1960s, is known as the obstetrical dilemma. More recently, it has been suggested that the risk of pelvic floor conditions also factors in. The pelvic floor is a layer of muscles that keeps our organs in place. If it weakens or tears, it can lead to incontinence and problems during childbirth.

“The obstetrical dilemma has been debated very hotly,” says Narasimhan. There have been many previous studies trying to relate the structure of the pelvis to walking speed and efficiency, for instance, but these studies have usually involved small numbers of people and produced conflicting results, he says.

Now, Narasimhan and his colleagues have used data from the UK Biobank to look at 31,000 men and women. The team measured various aspects of the pelvis based on a type of scan called dual-energy X-ray absorptiometry and looked for correlations with genetic variations and aspects of people’s health, such as requiring emergency caesarians.

The results indicate that having a wider pelvis reduces the risk of complications during birth, but results in slower walking and a higher risk of pelvic floor-related conditions and osteoarthritis of the hip. Meanwhile, a narrower pelvis may speed up walking but increases the risk of birth complications, back pain and knee osteoarthritis.

It has previously been suggested that there is a link between having narrower hips and earlier births, to reduce the risk of birth complications, but the team found no association between pelvis width and the length of pregnancies. “This is in line with other studies showing that human children are not born [relatively] sooner than in the other great apes,” says Narasimhan.

The team did observe an association between pelvic width and the head size of babies at birth. “Individuals who might give birth to children with a wider head often have wider pelvises,” says Narasimhan. “That happens because of natural selection, where we’re continually selecting for individuals which have this correlation.” This selection may now have come to an end because of C-sections, a 2016 study claimed.

Another finding is that most people have slightly asymmetrical pelvises that correlate with their handedness. Being left or right-handed usually determines which leg is dominant, which affects walking and the development of the pelvis, and this may lead to a slight asymmetry as we grow up, says Narasimhan.

“This is an extraordinary contribution to a fundamental aspect of human evolutionary biology,” says Scott Simpson at Case Western Reserve University in Cleveland, Ohio. “By integrating anatomic, genetic, clinical and behavioural data, the authors have provided significant insights into this uniquely human adaptation.”

“It is nice to be able to exploit large datasets,” says Nicole Webb at the University of Zurich in Switzerland, who has shown that the birth canals of chimpanzees aren’t much wider than those of people. Webb points out that all the people in the dataset were aged over 40 and from the UK. “If this work were carried out on younger, more diverse populations, the results could be even more striking,” she says.