Bipedal Locomotion

Among living primates, only humans are bipedal. It is not certain when this unique feature emerged, but it must have been before the 3.6 million-year-old Laetoli footprints were made. Although the prints were not made by completely modern feet, they are [Page 373]unequivocally the prints of bipeds. They are the impressions of feet that lacked a distinctive human rounded ball, or swelling, at the base of the great toe, that had no well-defined arch, and that retained ever so slightly divergent great toes. Somewhat later in time, the well-known 2.9 million-year-old Australopithecus afarensis Lucy fossil is the earliest human ancestor to display the clear skeletal hallmarks of bipedalism. Earlier fossils are either not yet described or lack the two most diagnostic parts, the pelvis and the distal (i.e.,lower) femur.

Figure 1 Torsos and pelves of a modern human, a reconstruction of A. afarensis, and a chimpanzee.

Despite the paucity of fossils near the beginning of the human lineage, most paleontologists regard bipedalism as the earliest distinctively human feature to have evolved in hominins. Hominin refers to members of the taxonomic family Homininae, which consists of human ancestors and collateral species after the lineage split from that leading to chimpanzees. In great apes, the pelvis is vertically elongated so that it makes up much of the back (see figure), leaving the back somewhat inflexible. The human pelvis is vertically short and shaped like a bowl with an open bottom, the opening being the pelvic inlet or birth canal. Humans also possess a valgus femur, a femur that angles in at the knee, whereas ape femora have no angle, giving them a bowlegged appearance. A.afarensis clearly exhibits these two hallmarks of bipedalism. Many other early hominin features, however, are more primitive. Early hominins have a partly divergent great toe, long curved lateral toes, short legs, a knee joint that is small and flexible, a coneshaped rib cage, long robust arms, disproportionately long forearms, a scapula or shoulder blade with a tilted-up joint surface, and curved robust fingers with distinctive concavities on the inner surface to accommodate large finger flexor tendons.

[Page 374]Two features in A. afarensis are neither chimpanzee-like nor humanlike. First, the pelvis, while human shaped, is extraordinarily broad. Second, the joint at the base of the great toe, deep in the foot near the ankle, is shaped like a hinge. As with a hinge, it allows little movement up or down and is therefore a rigid strut for pushing off during walking or running, just as is the great toe of humans. Like a hinge, it swings side to side, perhaps to allow the great toe to grip small branches. In chimpanzees, the whole foot is modified so that the great toe functions like a human thumb.

Disagreement among experts on the locomotor habits of early hominins has little to do with these details themselves and much to do with their interpretation. Some argue that the apelike features are holdovers from an arboreal ancestor, “primitive retentions,” and therefore unimportant for inferring behavior. Others interpret the primitive features more straightforwardly.

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