The chimpanzee pelvis is taller relative to its width and the iliac ala are more flat than seen in modern humans, lying roughly parallel with the plane of the back. In modern humans, the iliac alae are curved forward and flare more to the sides of the body. Lucy has the broader hips and the flaring iliac alae, which are all hallmarks of bipedal locomotion. However, the pelvis is wider in modern humans than in Lucy. The more widely spaced hip joints and the broader pelvic outlet is a critical adaptation that allows enough space for the larger brained babies of modern humans to pass through the birth canal. During the widening of the pelvic outlet, the hip joints (acetabulum) are moved further away from the body’s center of gravity. This exerts more force on the femoral head during locomotion. An increase in the size of the femoral head helps to counteract these forces.