A complete knee joint (AL 129-1), minus the knee cap (patella), from a single individual was found at Hadar, which showed that the species Australopithecus afarensis made a habit of walking bipedally.

AL 129-1 is a complete knee joint, consiting of the distal femur (lower end of the thigh bone, at the top of the photo) and proximal tibia (upper end of the shin bone, at the bottom of the photo) from a single individual. This discovery was conclusive proof of bipedal walking in early humans as old as 3 million years ago. (More recent finds push that benchmark of human evolution back to at least 4 million years ago.)

The Hadar knee shows several characteristics that reflect adaptation to bipedal locomotion. First, the end of the femur has an area of bone (called the lateral condyle, on the left side of the photo) that is elliptically shaped, as in humans, rather than spherical like a chimp's lateral condyle. This shows that the movement of the femur on the tibia was like that of a bipedal human. Second, the patellar groove, which is a depression in the femur that allows space for the kneecap (patella), is deep and had a high lip on the outside. This is important because bipedal legs have to lock straight when walking.

Finally, look at the angle of the femoral shaft, the part of the top bone that rises above the knee. There is a definite angle, relative to the tibia. This oblique femoral shaft is an adaptation that allowed early humans to walk upright by placing the foot under the center of the body when walking. A chimpanzee knee shows both bones lining up in a straight line.