bipedalism When an animal is capable of walking or running on two feet, it is said to be bipedal or to show bipedalism. Hence, bipedalism is a style of locomotion. Many animals show bipedalism, some of them habitually (for example, birds and humans) and some sporadically (e.g. some lizards and anthropoid apes). Animals that are capable of bipedal locomotion show anatomical adaptations to this mode of gait.

It is instructive to compare a gorilla with a modern human. When a gorilla stands on all fours — its habitual position — its spinal column is at an oblique angle to the ground, not parallel to the ground as in most completely quadrupedal animals. In this position the gorilla's weight-line (or ‘centre of gravity’ line) falls between the forelimbs and the hind limbs. When the human being stands or walks in the upright position, the axis of the body mass passes from the joint at the base of the cranium, close to the vertebral column, through the hip joints, and down the lower limbs to meet the ground between the feet.

The conversion of the gorilla's pattern of structure and function to that of humans involves changes of bones, joints, ligaments, and muscles. Adjustments of the skeleton include alterations at the base of the cranium and of the head–neck alignment: these modifications result in a head which does not hang forward from an oblique spinal column (as in apes), but which is well balanced on an upright column.

Secondly, the spinal column itself develops structural mechanisms for the transmission of body weight down the spinal column to the upper part of the sacrum, and thence through the sacro-iliac joints (the joints between the sacrum and the left and right hip-bones).

Thirdly, substantial differences are seen between the hip bones of quadrupedal and bipedal primates, involving also the locomotor and postural muscles attached to the hip bones.

Fourthly, there are striking structural differences in the size and form of the head and neck of the femur (thighbone), in the length, curvature, and form of the shaft of this bone; in the structure and function of the knee joint; and in the ankle, foot, and toes.

There are also other skeletal differences. For instance, there is a restructuring of the upper limb, from shoulder to fingers, as that limb ceases to be employed for locomotion, and becomes an instrument for carrying and for manipulating.

How did bipedal locomotion develop in human evolution? Fortunately, remains of relevant parts of the skeleton from South and East Africa have thrown light on the way in which bipedalism evolved.

The fossils show that elements of the human bipedal complex were developed early in hominization. However, it would be wrong to imagine that all of the anatomical adaptations to bipedalism appeared at the same time. There is much evidence that many structural and functional complexes evolved piecemeal: some items appeared early, others later; some metamorphosed rapidly, others slowly. We call such patterns of change mosaic evolution. The structural complex which makes bipedalism possible is an example. Bipedalism did not arrive on our planet ready-made, but developed in a stepwise manner. For example, the ilium (the flared upper part) of the hip bone is distinctive in humans of today, compared with apes. The ilium of ape men of the genus Australopithecus, which lived in Africa from four to one million years ago, was much more like that of a modern human than that of an ape; yet it showed features in which it was not perfectly human. The muscles that attached to this bone must have operated in a somewhat different manner from ours. Further changes must have affected the hip bone in later stages, bringing it into line with the structure found in today's human bipeds. Similarly, the foot — an exquisite organ, which permits skilled movements such as ballet dancing and karate — did not arrive at one giant leap. There is evidence for several stages in the conversion of an ape-like foot to a human-like foot. At least some early African hominids, it seems, possessed feet in which the great toe was separated from the lateral four toes, somewhat as in a chimpanzee's foot. It was a more mobile big toe than in modern humans. Yet such a foot was part of the skeleton of a hominid which was bipedal. The knee joint of Australopithecus, too, was in some respects chimpanzee-like and not fully humanized. The shoulder complex of Australopithecus showed the persistence of ape-like features, which has given support to the view that the ape men were capable of functioning in the trees or arboreally, for part of the time, while going bipedally on the ground at other times.

The bipedalism practised by modern humans is a striding gait. In this form of locomotion a major part is played by the tilting of the pelvis: alternately it tilts downwards to the right, enabling the left limb to clear the ground as it moves forward; and downwards to the left, so that the right limb can stride forwards without the sole of the foot scraping the ground. This tilting of the pelvis is effected by the gluteal muscles, which connect the ilium of the hip bone to the lower limb. If the tilting mechanism is less well developed, as may have been the case in some australopithecines, there is an alternative way in which a lower limb may move forwards without scraping along the ground. In this method, the lower limb that is going forwards is simultaneously abducted (swung sideways). This enables it to clear the ground and it is free to move forwards. The process is then repeated for the other limb. This form of bipedal locomotion is a waddling gait. It was probably the kind of locomotion used by some or all species of Australopithecus, before the striding pattern emerged.

We realize that not only are there different kinds of bipedalism, but also there are differing degrees of adaptation to bipedalism. Moreover, the attainment of bipedalism must have occurred without jeopardy to the other crucial role of the pelvis, as the birth canal.

P. Tobias

bipedalism The ability to maintain a standing position, and to move about, using only two legs. Birds and humans are bipedal. The development of bipedalism was an important factor in hominid evolution.