Lucy's limbs: skeletal allometry and locomotion in Australopithecus afarensis

Precise information about the bodily proportions of early hominids is crucial for accurate functional and phylogenetic interpretations of early human evolution1–6. The partial skeleton of Australopithecus afarensis (AL 288-1; ‘Lucy’7,8) recovered in 1974 from the Hadar area of Ethiopia9 permits the first direct assessment of body size, limb proportions and skeletal allometry in ancestral hominids that pre-date 3 Myr. Using allometric relationships for limb lengths in non-human catarrhine primates (as a whole and for African apes alone) as empirical base lines for comparison, I show here that the limb proportions of A. afarensis are clearly unique among hominoids. The data indicate that A. afarensis had already attained forelimb proportions similar to those of modern humans but possessed hindlimbs that were relatively much shorter; hence the ‘intermediate’ humerofemoral index of AL 288-1 (85.1) compared with Homo sapiens and great apes9,10. It follows that relative and absolute elongation of the hindlimbs represents one of the major evolutionary changes in later human evolution. The bodily proportions of Lucy are not incompatible with some form of bipedal locomotion, but kinematic identity and functional equivalence with the bipedal gait of modern humans seem highly improbable. Reduced relative stride length in AL 288-1 probably implies both greater relative energy cost and relatively lower peak velocities of bipedal locomotion in A. afarensis.