Why is the beginning of human life such a pain? Unlike that of other mammals, human maternal labor is notoriously rough on the female. Since the birth canal is barely large enough for a baby's head, the physical constraints of the pelvis force the bones of the child's nascent skull to overlap slightly, which is why many newborns come out with cone-shaped heads.

But even with this malleability, the skull is simply not slim enough to fit through the birth canal without causing the pain it does, and biologists have long sought a conclusive answer as to why this “design flaw” appears particular to the humans.

Many researchers have traditionally looked to the “obstetric dilemma” (OD) hypothesis, which holds labor pains to be the inevitable result of an unusual evolutionary design compromise stemming from the perpetual tug-of-war between the bipedal necessity for slim hips and the increasingly large skull-size needed to enclose a growing brain. The theory thus also offers an explanation for the incredible helplessness of human babies when compared to newborns in other species, where the gestation period can be much more extensive.

However, some of the assumptions underpinning the OD hypothesis have recently been called into question in a paper authored by Holly Dunsworth, assistant professor of anthropology at the University of Rhode Island. The Guardian reports that Dunsworth became suspicious about the link between bipedal efficiency and hip size, as it appeared to have limited scientific footing beyond the empirical fact that the world’s top male athletes slightly outperform the world’s top female athletes.

In an experiment involving male and female subjects running on a treadmill, the paper’s co-author, Herman Pontzer of Hunter College, measured the oxygen consumption of both genders and concluded that the women, in spite of their relatively larger hips, were just as efficient as the men.

From this, Dunsworth and Pontzer hypothesized that the human gestation period is not necessarily truncated at all, and that the seemingly premature labor is brought on by an “energetic crisis” in the mother’s metabolic system — just like in all other placental mammals. According to the paper, painful labor and helpless babies have less to do with pelvic size and more to do with the child’s growing brain size.

It would simply take too long and too much of the mother's energy for the human brain to develop significantly during the gestation period. Where offspring of other mammals can achieve a 40 percent of their adult brain size, human newborns only reach 30 percent.

In answer to the question why the pelvic structure seems way too narrow, the researchers respond that it appears to be wide enough — as far as natural selection is concerned, at least. A mother-to-be amid the throes of labor, however, might disagree.

Published by Medicaldaily.com