InvisblePinkUnicorn writes: "There have been various explanations for the evolution of upright walking, such as the need to get above tall grass and look across longer distances. Now a study published in the PNAS points to a simpler explanation — efficiency. According to the study, humans walking on two legs consume only a quarter of the energy that chimpanzees use while 'knuckle-walking' on all fours. The energy saved by walking upright gave our ancient ancestors an evolutionary advantage over other apes by reducing the costs of foraging for food. The abstract is available from PNAS."