Human babies are the wimps of the primate world. Whereas baby chimpanzees are crawling about a month after being born, human infants lag behind behaviorally through much of their first year, remaining helpless little things akin to puppies or kittens.

Around the 1950s, scientists began to think they'd figured out why human gestation seems somehow rushed. The answer was all in the hips.

"There were a couple of papers -- sort of short popular papers, not necessarily meant for a professional audience -- that floated the idea that childbirth must be so difficult because evolution has selected for big-brained babies but  also for upright running and walking," University of Rhode Island anthropologist Holly Dunsworth said in a phone interview.

This hypothesis, known as the "obstrepic dilemma," proposed that the evolution of bipedalism in humans had restricted the size of a woman's pelvis and birth canal, meaning that a big-brained human baby had to exit the womb at a less developed stage than other primates.

"The explanation is that we are born early to escape the horrible crunch of childbirth," Dunsworth explains.

But Dunsworth and her colleagues now say in a paper appearing in the Proceedings of the National Academy of Sciences that the real answer is that women gestate their babies for about as long as their bodies can support the baby. Childbirth is timed to occur just as women approach a "metabolic danger zone" after which they'll be taxed beyond their limits. They've dubbed their concept EGG, which stands for energetics, gestation and growth.

The EGG theory grew out of metabolic data from mothers. In humans, the maximum metabolic rate we can sustain is thought to be between 2.0 and 2.5 times our resting rate, according to the paper.

But a fetus makes large demands, and as it grows, its metabolic tax on the mother multiplies exponentially. By nine months, the demand is around 2.1 times a mother's normal resting metabolic rate. Any more gestation beyond that point could start to seriously threaten a mother's health.

In other words, the competition between the mother's energy needs and fetal energy needs is what has shaped the length of human gestation, not the competition between the size of the mother's birth canal and the baby's head, the authors claim.

"When you focus on the mother" -- rather than the supposedly exceptional human pelvis -- "it all makes sense," Dunsworth says.

Plus, one of the key assumptions of the OD hypothesis -- that bigger hips impede walking on two legs -- might not be true. Coauthor and Harvard University researcher Anna Warrener, who studies how hip size affects a woman's locomotion on treadmills, found that bigger hips are no detriment to a woman's walking or running efficiency.

"So there's no reason to suspect widening the birth canal to accommodate a bigger baby would be a problem," Dunsworth says.

SOURCE: Dunsworth et al. "Metabolic hypothesis for human altriciality." Proc Natl Acad Sci published online 27 August 2012.