| The skeleton of Australopithecus afarensis A.L. 288-1 (better known as “Lucy”) is by far the most complete record (approximately 40%) of locomotor morphology of early hominids currently available. Even though researchers agree that the postcranial skeleton of Lucy shows morphological features indicative of bipedality, there are intense discussions regarding the manner in which she could have walked; that is, whether she could have walked in a manner that is similar to that of modern humans (upright, straight-legged locomotion) or whether she could have walked with bent-hips and bent-knees, as chimpanzees do when they walk bipedally. This is partly because several morphological features suggest that Lucy most likely walked like a human, whereas several other morphological features suggest that Lucy most likely walked like a chimpanzee. The purpose of this study was to address this question using the methodology of forward dynamic computer simulation. Two (one human-like and one chimpanzee-like) detailed three-dimensional musculoskeletal models of Lucy were developed. These models were driven by lower limb muscles (52 for the human-like model and 56 for the chimpanzee-like model), with muscle activation profiles as the only inputs. Muscle activation profiles that produce a step of human-like locomotion and a step of chimpanzee-like locomotion, while at the same time minimizing the energy consumed per meter traveled, were searched through numerical optimization. The numerical optimization of the muscle activation profiles resulted in natural-looking locomotor kinematics. The energy consumption per meter traveled during chimpanzee-like locomotion was more than twice the amount of energy consumption during human-like locomotion. On the other hand, the chimpanzee-like locomotion was only slightly more robust against external perturbations. Therefore, from the perspectives of both energy consumption as well as functional stability, it is most likely that Lucy walked in a manner that is similar to the way in which modern humans walk today. |
