The evolution of human bipedalism approximately four million years ago has led to morphological alterations to the human skeleton including changes to the arrangement and size of the bones of the foot, hip size and shape, knee size, leg length, and the shape and orientation of the vertebral column. The evolutionary factors that produced these changes have been the subject of several theories. Human walking is about 75% less costly than both quadrupedal and bipedal walking in chimpanzees. Some hypotheses have been supported that bipedalism increased the energetic efficiency of travel and that this was an important factor in the origin of bipedal locomotion. Humans save more energy than quadrupeds when walking but not when running. Human running is 75% less efficient than human walking. A study helped to prove that walking of living hominin bipeds is noticeably more efficient than walking of living hominin quadrupeds, but the costs of quadruped and bipedal travel are the same.