8.3: Bipedalism
- Page ID
- 62182
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Fossil pelvic and leg bones, body proportions, and footprints all read "bipeds." The fossil bones are not identical to modern humans, but were likely functionally equivalent and a marked departure from those of quadrupedal chimpanzees.
Australopithecine fossils (see image) possess various components of the bipedal complex which can be compared to those of chimpanzees and humans:
A diagnostic feature of bipedal locomotion is a shortened and broadened ilium (or large pelvic bone); the australopithecine ilium is shorter than that of apes, and it is slightly curved; this shape suggests that the gluteal muscles were in a position to rotate and support the body during bipedal walking.
- In modern humans, the head of the femur (or thigh bone) is robust, indicating increased stability at this joint for greater load bearing
- In humans, the femur angles inward from the hip to the knee joint, so that the lower limbs stand close to the body's midline. The line of gravity and weight are carried on the outside of the knee joint; in contrast, the chimpanzee femur articulates at the hip, then continues in a straight line downward to the knee joint
- The morphology of the australopithecine femur is distinct and suggests a slightly different function for the hip and knee joints. The femoral shaft is angled more than that of a chimpanzee and indicates that the knees and feet were well planted under the body
- In modern humans, the lower limbs bear all the body weight and perform all locomotor functions. Consequently, the hip, knee and ankle joint are all large with less mobility than their counterparts in chimpanzees. In australopithecines, the joints remain relatively small. In part, this might be due to smaller body size. It may also be due to a unique early hominin form of bipedal locomotion that differed somewhat from that of later hominins.
Thus human bodies were redesigned by natural selection for walking in an upright position for longer distances over uneven terrain. This is potentially in response to a changing African landscape with fewer trees and more open savannas.