Scientists have explained the fact that humans, unlike wildebeests, for instance, are born as helpless and vulnerable as shell-less mussels by pointing to the relatively small size of the female pelvis and collectively hissing, "It's all your fault!" New research, however, suggests that the apparently insufficient baking time for human babies (whose brains are only 30 percent of their adult size at birth) has less to do with the size of a mother's pelvis and more to do with the speed of her metabolism.


It's like this: babies are worthless. They'd can't do ANYTHING for themselves. Not only that, but their heads, undersized though they may be, require almost constant support because their neck muscles aren't strong enough to keep their craniums from rolling around like a sixteen-pound bowling ball stuck on the fingers of a chubby second-grader. They're also too stupid to compensate for their clumsiness. If their brains were just a little bigger, however, babies would get stuck in the birth canal on their way towards a whole lifetime of womb-separation issues. Chimpanzee babies, the closest relatives to human babies, are born with 40 percent of their brain capacity, right from the get-go, a number that would require another 18 to 20 months of gestation for humans to reach.

Babies were thought to be born a little underdone because women's pelvises had to be narrow enough to facilitate bipedal walking, an adaptation that gave rise to an "obstetrical dilemma" and chiropractors. In order to walk upright, babies, so the logic went, had to be born a little earlier than was ideal so their heads would slide right through nature's original luge. University of Rhode Island anthropologist Holly Dunsworth, however, has come up with a new and improved interpretation about the seemingly abbreviated human gestation period.


Dunsworth contends that, when one considers body size, humans aren't cutting gestation short at all — compared to the higher apes, human pregnancies have the second-longest gestation period (orangutans take the top banana). Human mothers also invest a lot of energy into incubating a baby, so much so that their brains, when newborn, are 47 percent larger than gorillas' brains (human babies are also twice as big as gorilla babies, so there, gorillas). This data has prompted Dunsworth to suggest that humans haven't cut pregnancy short at all — they've extended it so that babies can get even bigger and beat their gorilla counterparts at feats of intellect as well as strength.

Pregnancies last nine months instead of some other random span of time, contends Dunsworth, because a mother's metabolism, by the six-month mark, is working like a nuclear power plant on the verge of melting down. By that point, a woman is expending twice her usual energy just keeping all the basic metabolic functions going, and that strain only gets more intense the larger the baby grows. Humans can only sustain about a metabolic rate of between 2 to 2.5 times their usual rate for so long, meaning that babies are born at the nine-month mark because mothers' bodies just can't take the metabolic strain anymore.

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Dunsworth's findings essentially invert the idea that pelvic size and the need to run/walk around determines the length of pregnancy and the size of the baby. Interestingly, babies probably didn't need such a long gestation and labor time before the advent of agriculture because mothers didn't have the energy to grow babies quite so big. It's very possible, therefore, that before farming gave us all of our delicious, civilization-inducing grain-based foodstuffs, pregnancy and birth weren't quite as protracted and onerous as they are today.

Why Pregnancy Really Lasts 9 Months [LiveScience]