That has always struck some scientists as too pat an explanation, though it is only in the last decade or so that the theory, which still has many subscribers, has received substantive pushback. Today, challenges abound for the idiosyncrasies of human gestation and birth—including new notions that look beyond evolution to more proximate and modern factors like poor diet and obesity.

Of course, rigorous debate over the relative strengths and weaknesses of theories in this cul-de-sac of physiological science will surely continue. But for all the back-and-forth, one thing seems quite clear: The days of simply describing the human birth process—and women themselves—as evolutionarily compromised seem to be coming to an end.

For some researchers, that change in thinking is long overdue.

* * *

Sherwood Washburn, the physical anthropologist who coined the phrase “obstetrical dilemma,” first published his theory in the September 1960 issue of Scientific American. He argued that, “in man, adaptation to bipedal locomotion decreased the size of the bony birth canal at the same time that the exigencies of tool use selected for larger brains. This obstetrical dilemma was solved by the delivery of the fetus at a much earlier stage of development.”

Early delivery, he concluded, foisted far greater responsibility on the “slow-moving mother,” who was now forced to hold her “helpless, immature infant,” while the men went out hunting.

The assumption that “women are compromised bipedally in order to give birth,” is widely accepted, says anthropologist Holly Dunsworth of the University of Rhode Island. But Dunsworth sees flaws in this premise. Women already have a range of dimensions in their birth canal, she thought, and they are all walking just fine. Indeed, research on human skeletons by anthropologist Helen Kurki of the University of Victoria in Canada has shown that the size and shape of the human birth canal varies very widely, even more so than the size and shape of their arms.

So in 2007, Dunsworth went looking for evidence to support the obstetrical dilemma as it has traditionally been understood.

“When I couldn’t, I thought I was crazy,” she says. Intrigued, she enlisted Anna Warrener, a professor of biology and biomechanics, then at Harvard University, to test the notion that wider pelvises in women decrease the efficiency of locomotion. After measuring the chain reaction of forces moving through the body—from the foot to the leg to the hip—Warrener and her colleagues found that wider hips do not increase the cost of locomotion. Indeed, both women and men are equally efficient at walking and running, and in hunter-gatherer societies, women walk, on average, 5.5 miles per day, often while carrying and feeding infants as well.

“The obstetric dilemma, in its definition, has housed this idea that women aren’t as good as men in some things because they have to give birth,” adds Cara Wall-Scheffler, an evolutionary anthropologist who studies human locomotion at Seattle Pacific University. “I have a number of papers that show that women are great walkers, and in some particular tasks women are better—they don’t use as much energy, they don’t build as much heat, they can carry heavier loads with less of an energetic burden.”