Lucy’s femur, or thigh bone, provides some of the best evidence for bipedal locomotion. In humans (shown on the left) the shaft of the femur slants downward and medially toward the body’s midline so that the knees are held close together, forming an angle from the knee to the hip joint. This angle is called the bicondylar angle. In this way, the foot is closer to the body’s center of gravity during single-foot support, relieving stress on the knee joints and ensuring balance during bipedal locomotion. Quadrupedal locomotion lacks a single support phase. Thus, in quadrupedal animals, the shaft of the femur is straight so that the knees are separated and the legs are positioned directly underneath the hip joints. By contrast, the morphology of the human knee is called a valgus knee, meaning “knock-kneed”. Lucy’s femur also exhibited a bicondylar angle similar to that seen in modern humans, and thus Lucy also had a valgus knee.
