Energy efficiency in bipedal walking

A bipedal walking mode distinguishes even the earliest hominins from other apes and may have provided a crucial advantage. However, the selective pressure that originally drove the adaptation is unknown. Michael Sockol et al. now report that upright hominins would have enjoyed a saving in the energy required for locomotion and thus would have been able to forage extensively. The authors measured how much oxygen five chimpanzees and four human volunteers burned as they walked on a treadmill: the chimpanzees on four legs or two legs, the humans on two. The humans were far more efficient than the chimpanzees, consuming just one-quarter of the energy per unit weight. On average, the chimpanzees used as much oxygen on two legs as on four; however, one chimpanzee in the study, with a longer stride, was more efficient when walking upright. According to biomechanical equations, energy consumption increases with shorter steps or with more active muscle mass. Humans’ long legs and a more dorsally projecting pelvis result in a reduction in both factors. The fact that variation exists even among a small population of chimpanzees, the authors say, makes it reasonable to assume that human ancestors may have been selected for their efficiency in walking. — K.M.