Scientists question the theory of the evolution of childbirth – 03/08/2023 – Science

It’s a question on the weary mind of every new parent: why are babies born so helpless? In 1960, an American anthropologist put forward an influential explanation rooted in human evolution.

When our first ancestors started walking upright, Sherwood Washburn argued in the 1960s, they evolved a narrower pelvis to make walking long distances more efficient. At the same time, these hominids were developing larger brains. And babies with large heads can get stuck in a tight birth canal at birth, threatening the lives of mothers and babies alike.

According to Washburn, evolution dealt with this obstetric dilemma, as he called it, by shortening pregnancies so that women gave birth before the infant brain had finished growing.

Washburn’s theory was extremely influential and became a common lesson in biology classes. The book “Sapiens: A Brief History of Humankind”, by Yuval Noah Harari (2011), presented the obstetrical dilemma as a fact. Many researchers still adopt it.

But a detailed analysis of the evidence, due to be published soon in the journal Evolutionary Anthropology, threw cold water on the idea.

In the review, Anna Warrener, a biological anthropologist at the University of Colorado, Denver, argued that the evidence to date does not provide strong support for the obstetric dilemma and that scientists have not paid enough attention to possible alternatives. What’s more, she said, the idea sends a pernicious message to women that pregnancy is inherently dangerous.

“It perpetuates a narrative of bodily incompetence,” Warrener said.

In college, Warrener saw no reason to doubt the obstetric dilemma. For her dissertation, she investigated one of Washburn’s main assumptions – that women walk less efficiently than men because their pelvis is wider for childbirth. But in 2015, after studying volunteers walking on treadmills, Warrener found that having a wider pelvis did not create a greater demand for oxygen.

“Looking at the data, I thought, ‘Wait a minute – I may have got part of the story wrong,'” she recalled.

Holly Dunsworth, a biological anthropologist now at the University of Rhode Island, also became disenchanted with the obstetric dilemma when she looked closely at the evidence. “I was scandalized,” she said.

In 2012, she and her colleagues published a study on the length of pregnancy in humans and other primates. They found that, in general, larger primates tended to have longer pregnancies than smaller ones. For their size, humans don’t have short pregnancies. In fact, human pregnancies are longer than might be predicted for primates of their size.

Dunsworth has since become a strong critic of the obstetric dilemma, arguing that the timing of delivery is determined by the size of babies’ bodies, not their heads. The birthing process begins when the fetus requires more energy than the mother’s body can supply, she proposes. “We are giving birth to huge babies,” she said.

Other scientists, however, came out in defense of the theory, while admitting that its original conception was overly simplistic.

In a study published last month, a team of researchers argued that the difference between male and female pelvises shows signs of natural selection acting in different directions. While human males are larger and taller on average than females, certain parts of their pelvises are relatively smaller. The biggest differences are in the bones that surround the birth canal in human females.

Despite these differences, the female pelvis still creates a tight fit between the baby’s head and the birth canal, sometimes putting the baby and mother in danger.

“So why couldn’t natural selection resolve this situation and make birth a little less risky?” asked Nicole Grunstra, an evolutionary anthropologist at the University of Vienna (Austria) and one of the study’s authors.

“It evolved to be an evolutionary compromise between competing demands,” she said — in other words, to solve an obstetric dilemma.

But Grunstra acknowledged flaws in Washburn’s original version of the theory. She suspected that walking may not have been the most important factor in the evolution of the pelvis. Simply standing, she said, could have put pressure on the pelvic floor, preventing a more spacious birth canal from evolving.

Skeptics are not convinced by these arguments. In her new review, Warrener questioned whether babies who get stuck in the birth canal pose a major threat to women’s lives. It is much more common for new mothers to die from blood loss or infections, she noted.

She was also critical of the way Grunstra and other advocates of the obstetrical dilemma defend their hypothesis. In her opinion, they assume that every piece of human anatomy has been tuned by natural selection for a specific job.

Sometimes, Warrener said, adaptations are a fluke. For example, some of the genes that build the pelvis are also active in the development of other parts of the skeleton. If another bone in our body were to evolve a new shape, the pelvis might simply change as a by-product — not because it was evolving to walk or give birth.

“I think the differences in the pelvis between the sexes have been kind of a red herring,” Dunsworth said.

Like other bones, the pelvis does not have a fixed shape encoded in a genetic blueprint. Their development is influenced by the tissues around them, including the uterus, ovaries and other organs. The proportions of the female pelvis can result in part from all the organs that grow within it.

Dunsworth and Warrener fear that the obstetrical dilemma will lead to a widespread notion of the female body as inescapably defective.

“It makes us feel like problems that need to be solved by medicine,” Dunsworth said.