Anthropologists rely largely on educated guesswork and computer models to figure out if australopithecines and other early hominids walked erect, like we do, or used a bent-hip, bent-knee, Groucho-style gait.
These computer simulations, based on the reconstructed skeleton of the famous Australopithecus afarensis Lucy, model the biomechanics of both motions and effectively rule out the Groucho gait. It wouldn't have worked well for Lucy and her kin, says anthropologist Yu Li of the University of Liverpool, who helped develop the model. First, says Li, the gait is inefficient. The power driving the motion, the model showed, is generated in the hips. "The knee and ankle only absorb energy. As a result, the hip needs to work doubly hard, and the energy absorbed by the other two joints can only be released as heat." This is not just a waste of energy, it raises body temperature by perhaps as much as 12 degrees. "If Lucy ever walked in the bent-hip, bent-knee way, in a transitional period from quadrupedal to full bipedal, the period would have been a very short one," Li says, "because selection pressures"--that is, predators--"would not have allowed it to exist for very long."