Lucy’s lower body (pelvis and hindlimbs) indicates that when Lucy was on the ground (terrestrial), she walked bipedally. For example, Lucy’s innominate blades (pelvic blades), and those of modern humans, are short, broad, and curved forward, so that the iliac crest courses toward the front of the body. In contrast, the chimpanzee innominate blade is tall, narrow, and flat, so that the iliac crest lies flat across the back. This difference in the orientation of the pelvic blade affects the way some of the gluteal muscles attached to the iliac crest (and the femur’s greater trochanter) and action at the hip joint. In chimpanzees, a flat iliac blade causes these muscles to extend the leg backward at the hip. This action helps move the limbs front-to-back during quadrupedal locomotion. In humans, and in Lucy, the curved iliac blade moves these muscles to the side of the body, causing these muscles to pull the leg to the side at the hip. This action helps to maintain the body’s support and prevent collapsing toward the side that is unsupported by a leg during bipedal walking. That Lucy’s pelvic blades appear closer to the human condition indicates she walked bipedally.
