A theory of childbirth’s evolution may not be what you’re expecting

It's a question on every new parent's exhausted mind: Why are babies born so helpless? In 1960, an American anthropologist laid out an influential explanation rooted in human evolution.

As our early ancestors began walking upright, Sherwood Washburn argued in 1960, they evolved a narrower pelvis to make walking long distances more efficient. At the same time, those hominins were evolving larger brains. And babies with big heads could get stuck in a tight birth canal during delivery, threatening the lives of mothers and babies alike.

According to Washburn, evolution dealt with this "obstetrical dilemma," as he called it, by shortening pregnancies, so that women delivered babies before the infant brain was done growing.

Washburn's theory was hugely influential and became a common lesson in biology classes. "Sapiens: A Brief History of Humankind," a 2011...