When you're born, you have absolutely no defense against predators, and we humans can't really fend for ourselves for years. Why is this?
One longstanding theory among evolutionary biologists says human babies are so completely dependant after birth, as a consequence of our bipedal architecture. Our upright stance limits the width of the birth canal, and thus the size of the baby's head that can pass through it. But now, new research has thrown this idea into question by suggesting that there's a more important limiting factor involved — namely, the mother's metabolic rate. According to this idea, a fetus can only grow so large and consume so much energy before it needs to make a hasty exit.
And indeed, as Kate Wong of Scientific American reports, human babies are extremely underdeveloped at birth compared to other species. In fact, a pregnant mother would have to carry a baby for 18 to 21 months, instead of the usual nine, for it to have the neurological and cognitive development of a newborn chimpanzee. Our babies' brains, says Wong, are less than 30% the size of an adult's — and up until this point this discrepancy has been attributed to the narrow birth canal.
But researchers at the University of Rhode Island have poked several holes in this theory, including the observation that many women have pelvic inlets wide enough to support the birth of babies that have 40% of adult brain size, and that wide hips should infer a biomechanical disadvantage for women, as compared to men — which it does not. And as Wong reports, there may be another explanation:
That other factor, they contend, is mom's metabolic rate. "Gestation places a heavy metabolic burden (measured in calories consumed) on the mother," Dunsworth and her co-authors explain. 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. Once outside of the womb, the baby's growth slows down to a more sustainable rate for the mother. 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 "energetic constraints of both mother and fetus are the primary determinants of gestation length and fetal growth in humans and across mammals." By nine months or so, the metabolic demands of a human fetus threaten to exceed the mother's ability to meet both the baby's energy requirements and her own, so she delivers the baby.
In their report, the researchers conclude that "if the human reproductive system poses a dilemma between competing needs, then fetal energy needs and maternal energy supply are the competitors, rather than [brain expansion] and bipedalism."
Check out Wong's entire article at SciAm. The study will appear online later this week at the Proceedings of the National Academy of Sciences USA, but the citation reads: Dunsworth HM, Warrener A, Deacon T, Ellison P, & Pontzer H (2012) Metabolic hypothesis for human altriciality. I'll be sure to add a link once it becomes available.