Human babies come helpless and dependent into the world, more so than the young of any other species. A baby elk can stand a few hours after birth, and run within a day. On the other hand, human babies, with their eager, adaptable brains, are able to learn far more in their first two years of life than an animal ever will. They can even master the basics of the miraculously complex, uniquely human skill language.
A leading theory is based on the fact that humans walked upright while their ancestors used their arms as forelegs, walking on their knuckles as the apes do. Perhaps the first humans stood upright so they could see above the grasses in the savannas in order to spot predators and game. Upright posture may also have allowed their bodies to keep cool by venting heat through the head into the moving air above the grasses.
In the course of evolution the upright position must have shifted weight to the pelvis, which thickened to bear it, closing down the birth opening. As the pelvis was thickening, the head of the fetus continued to grow larger to accommodate the constantly enlarging brain. Whatever the reasons, the human fetus had to be born well before the brain and head had reached maturity so it would still be small enough to pass through the birth canal.
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