Asked by: BBC Focus Magazine
Other mammals don’t have the same problems that we do for two reasons: their heads aren’t so large, relative to their body size, and they don’t walk upright. Evolving a fully upright gait required our ancestors to change the angle and shape of the pelvis, which reduced the width of the pelvic canal. Natural selection is therefore pulling the pelvis in two different directions.
To run fast, we need a narrow pelvis but to give birth easily, we need a wide one. This is known as the obstetric dilemma and the pelvis of modern humans is a compromise between these constraints. Our brains grow 3.3 times larger from birth to adult, compared with 2.5 times for chimpanzees, so humans are already born at a much earlier stage of development than other animals. But if women had hips any wider than they do now, they wouldn’t be able to run. Natural selection has probably been a lot more ruthless in eliminating slow-running mothers or underdeveloped newborns, than those women who are put off the idea of childbirth by the pain it causes.