The human ability to walk and run efficiently on two feet arose around 2 million years ago with our Homo erectus ancestors. But our earlier relatives, the australopithecines, were also bipedal around 4 million years ago. Given the long arms and different body proportions of species like Australopithecus afarensis, though, researchers have assumed that australopithecines were less capable of walking on two legs than modern humans. In a study published online Dec. 18 in the journal Current Biology, a team of researchers modeled the skeletal and muscular anatomy of Lucy to determine her maximum running speed, the energetic costs associated...