Chimpanzees show surprising similarities to humans in the way they rotate their bodies when walking on two legs, finds a study in Nature Communications. This suggests that the early chimp-like ancestors of humans would have been capable of more efficient upright walking than previously thought.
Human bipedal walking is characterised by coordinated movements of the hips (pelvis) and upper body (thorax), which move in opposite directions during a stride to allow longer steps and to utilise arm swinging for counterbalancing pelvic rotations. In contrast, it has been assumed that chimpanzees, which have a much shorter abdominal (lumbar) region, have trunks (pelvis, lumbar and thorax regions combined) that remain rigid when walking on two legs.
Nathan Thompson and colleagues use kinematic analysis to track the independent motions of these three body regions (hips, lumbar and thorax) in both humans and chimpanzees trained to walk on two legs. They find that although the direction in which the upper body is aligned during a stride differs between humans and chimpanzees, the magnitude of their movements relative to the pelvis is nearly identical.
A key difference between these movements is that chimpanzees rotate their pelvis much more than humans, although the motion of the thoracic region maintains the same function of countering excessive pelvic rotations. This similarity indicates that the chimpanzee-like skeletal morphology would not have hindered bipedal locomotion in the hominin Australopithecus afarensis, and suggests that it is likely this ability would also have been present early in hominin evolution.