Human babies enter the world utterly dependent on caregivers to tend to their every need. Although newborns of other primate species rely on caregivers, too, human infants are especially helpless because their brains are comparatively underdeveloped. Indeed, by one estimation, a human fetus would have to undergo a gestation period of 18 to 21 months instead of the usual nine to be born at a neurological and cognitive development stage comparable to that of a chimpanzee newborn. Anthropologists have long thought that the size of the pelvis has limited human gestation length. New research may challenge that view.

The traditional explanation for our nine-month gestation period is that natural selection favored childbirth at an earlier stage of fetal development to accommodate selection for both large brain size and upright locomotion—defining characteristics of the human lineage. But when Holly M. Dunsworth of the University of Rhode Island and her colleagues tested this so-called obstetrical dilemma hypothesis, their findings did not match its predictions. The researchers argue that instead of fetal brain expansion being constrained by the dimensions of the pelvis, the dimensions of the human pelvis have evolved to accommodate babies, and some other factor has kept newborn size in check.

That other factor, they contend, is Mom's metabolic rate. Data from a wide range of mammals suggest that there is a limit to how large and energetically expensive a fetus can grow before it has to check out of the womb. Building on an idea previously put forth by study co-author Peter T. Ellison of Harvard University, known as the metabolic crossover hypothesis, the team proposes that by nine months or so the metabolic demands of a human fetus threaten to exceed the mother's ability to meet both the fetus's energy requirements and her own, so she delivers the baby. Dunsworth and her collaborators published their findings online in August in the Proceedings of the National Academy of Sciences USA.

When I asked paleoanthropologist Karen Rosenberg of the University of Delaware, an expert on the evolution of human birth, what she thought about the new work, she called it “important and interesting.” Yet “just because there's a metabolic moment when it becomes reasonable to have a baby doesn't mean it isn't also true that the pelvis is a trade-off between giving birth and walking on two legs,” she contends.

Rosenberg additionally noted—and I found this especially fascinating—that the authors mention the possibility that the timing of birth actually optimizes cognitive and motor neuronal development. That idea, described by Swiss zoologist Adolf Portmann in the 1960s, is worth pursuing, she says. “Maybe human newborns are adapted to soaking up all this cultural stuff, and maybe being born earlier lets you do this,” Rosenberg muses. “Maybe being born earlier is better if you're a cultural animal.” Food for thought.

Adapted from the Observations blog at blogs.Scientifi