bipedality

The hominid pelvis is much shorter than ape pelves, with muscle attachments reoriented for effective walking.

The most dramatic evolutionary change underlying human bipedality is the change in shape of the pelvis. The pelvis is rarely preserved as a fossil, but several partial pelves are available from australopithecines, including the "Lucy" skeleton and several partial pelves from later South African sites. The pelvis consists of three bones — the sacrum, which lies at the bottom of the spine and is composed of several fused vertebra-like elements, and the two os coxae, or hip bones. In early hominids, both the sacrum and hip bones are relatively short compared to apes. The upper portion of each hip bone, called the ilium, is short and curved compared to the long, flattened ilium of chimpanzees and other apes. The curvature places the attachment of the quadriceps muscle closer to the front of the body, allowing the muscle greater leverage in pulling the femur forward in an upright posture.

Lucy (AL 288-1) skeleton.

Although the ilia of Australopithecus were short from top to bottom compared to a chimpanzee, they extend more broadly to the side, resulting in a pelvis that is very broad overall. Lucy’s pelvic width was within the range of today’s women, despite her very small body size. As a result, her body was differently shaped from recent people — very broad for its short height.

The width of the pelvis affects the muscular requirements of walking. Whenever one leg supports the body, gravity tends to tilt the upper body away from the supporting leg. The muscles on the opposite side must counteract this force to prevent the body from falling over. These muscles attach to the lateral part of the ilium and to the femur, pulling the trunk upward around the hip joint. A wide ilium tends to make these muscles more effective, by positioning the point of force further from the joint. A long femur neck also helps, just as long handles on a pair of scissors greatly increase the force with which they can cut. The configuration of these muscles in australopithecines is more extreme than the condition found in living people.

A wide pelvis and long femur neck may have helped australopithecines to maintain a long stride with short legs. Two things add up to determine the length of a step: how much the leg swings, and how much the pelvis rotates. A wider pelvis rotates farther and thereby increases the length of a step. Another explanation is that widely spaced legs may allow a greater mechanical advantage for the muscles that draw the legs toward the midline. This configuration might help the style of climbing that requires the legs to clamp around a branch or trunk. This kind of climbing would be more necessary to bipeds who lacked the prehensile feet of living apes.