Bipedalism Made Human Birth Difficult

When our ancestors started walking upright on two legs millions of years ago, their skeletons had to shift into a new configuration that reshaped the pelvis and narrowed the hips. In most primates, the birth canal is relatively straight, but in early hominins, it became a distorted cylinder that changed size and shape along its length. That narrowed, twisted canal made it harder for large-brained babies to twist and turn their way out, which turned childbirth into a longer, more painful, and more dangerous process than in most other mammals.

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