It’s a query on each new mother or father’s exhausted thoughts: Why are infants born so helpless? In 1960, an American anthropologist laid out an influential clarification rooted in human evolution.

As our early ancestors started strolling upright, Sherwood Washburn argued in 1960, they advanced a narrower pelvis to make strolling lengthy distances extra environment friendly. On the identical time, these hominins have been evolving bigger brains. And infants with large heads might get caught in a decent delivery canal throughout supply, threatening the lives of moms and infants alike.

In accordance with Dr. Washburn, evolution handled this “obstetrical dilemma,” as he known as it, by shortening pregnancies, so that girls delivered infants earlier than the toddler mind was carried out rising.

Dr. Washburn’s concept was massively influential and have become a typical lesson in biology lessons. “Sapiens: A Transient Historical past of Humankind,” a 2011 best-selling ebook, offered the obstetrical dilemma as truth. Many researchers nonetheless embrace it.

However an in depth overview of the proof, slated to be revealed quickly within the journal Evolutionary Anthropology, threw chilly water on the concept. Within the overview, Anna Warrener, a organic anthropologist on the College of Colorado Denver, argued that the proof up to now didn’t provide sturdy assist for the obstetrical dilemma, and that scientists had not paid sufficient consideration to attainable options. What’s extra, the scientist mentioned, the concept sends a pernicious message to ladies that being pregnant is inherently harmful.

“It perpetuates a story of bodily incompetence,” Dr. Warrener mentioned.

In graduate college, Dr. Warrener didn’t see any motive to doubt the obstetrical dilemma. For her dissertation, she investigated one in every of Dr. Washburn’s key assumptions — that girls stroll much less effectively than males do as a result of their pelvis is wider for childbirth. However in 2015, after learning volunteers strolling on treadmills, Dr. Warrener discovered that having a wider pelvis didn’t create an even bigger demand for oxygen.

“The information got here in, and I used to be like, Wait a minute — I’ll have gotten among the story flawed,” she recalled.

Holly Dunsworth, a organic anthropologist now on the College of Rhode Island, additionally turned disenchanted with the obstetrical dilemma when she took a detailed take a look at the proof. “I used to be scandalized,” she mentioned.

In 2012, she and her colleagues revealed a examine on the size of pregnancies in people and different primates. They discovered that, normally, greater primates tended to have longer pregnancies than smaller ones. For his or her measurement, people don’t have shortened pregnancies. If something, human pregnancies are longer than one would predict for a primate of their measurement.

Since then, Dr. Dunsworth has turn into a robust critic of the obstetrical dilemma, arguing that the timing of childbirth is set by the scale of infants’ our bodies, not their heads. The birthing course of begins when a fetus calls for extra vitality than a mom’s physique can present, she proposes. “We’re giving delivery to large infants,” she mentioned.

Different scientists, nevertheless, have come to the idea’s protection, whereas admitting that its authentic conception was overly simplistic.

In a examine revealed final month, a crew of researchers argued that the distinction between the female and male pelvis exhibits indicators of pure choice appearing in numerous instructions. Whereas human males are greater and taller on common than human females, sure elements of their pelvises are comparatively smaller. The largest variations are within the bones that encompass the delivery canals in human females.

Regardless of these variations, the feminine pelvis nonetheless creates a decent match between a child’s head and the delivery canal, generally placing each the infant and mom in peril.

“So why did pure choice not handle to sort of resolve this example and make delivery rather less dangerous?” requested Nicole Grunstra, an evolutionary anthropologist on the College of Vienna and one of many examine’s authors. “It has advanced to be an evolutionary compromise between competing calls for,” she mentioned — in different phrases, to resolve an obstetrical dilemma.

However Dr. Grunstra acknowledged flaws in Dr. Washburn’s authentic model of the idea. She suspected that strolling could not have been an important issue within the evolution of the pelvis. Merely standing upright, she mentioned, may need put stress on the pelvic flooring, stopping the evolution of a extra spacious delivery canal.

The skeptics aren’t satisfied by these arguments. In her new overview, Dr. Warrener questioned whether or not infants getting caught in delivery canals have posed a significant menace to ladies’s lives. It’s way more frequent, she famous, for brand new moms to die from blood loss or infections.

She additionally criticized the best way wherein Dr. Grunstra and different defenders of the obstetrical dilemma make the case for his or her speculation. In her view, they assume that each piece of human anatomy has been fine-tuned by pure choice for a selected job.

Generally, Dr. Warrener mentioned, variations are flukes. For instance, among the genes that construct the pelvis are additionally energetic within the improvement of different elements of the skeleton. If one other bone in our physique have been to evolve into a brand new form, the pelvis may change merely as a byproduct — not as a result of it was evolving for strolling or childbirth.

“I believe intercourse variations within the pelvis have been considerably of a crimson herring,” Dr. Dunsworth mentioned. Like different bones, the pelvis doesn’t have a hard and fast form encoded in a genetic blueprint. Its improvement is influenced by the tissues round it, together with the uterus, the ovaries and different organs. The proportions of the feminine pelvis could lead to half from all of the organs that develop inside it.

Each Dr. Dunsworth and Dr. Warrener fear that the obstetrical dilemma results in a widespread notion of the feminine physique as inescapably faulty.

“That simply makes us really feel like issues that should be solved by drugs,” Dr. Dunsworth mentioned. That narrative could play a component within the medicalization of childbirth in latest many years, she added.

The World Well being Group has warned that medical doctors are more and more performing pointless medical intervention on moms, whereas persistent issues that may threaten maternal well being — comparable to hypertension, weight problems and diabetes — get little consideration.

“The way in which we reside now in all probability doesn’t lead us to fulfill the problem of childbirth in addition to our our bodies did once they developed in a different way previously,” Dr. Dunsworth mentioned.

However recognizing the over-medicalization of contemporary being pregnant doesn’t finish the talk about its origins, Dr. Grunstra mentioned. “That doesn’t in itself imply that evolutionary explanations are flawed,” she mentioned.