The biomechanical modeling revealed that more energy is used with shorter steps or more active muscle mass. Indeed, the chimpanzee with the longest stride was the most efficient walking upright.

What those results allowed us to do was to look at the fossil record and see whether fossil hominins show adaptations that would have reduced bipedal energy expenditures, said Raichlen. We and many others have found these adaptations [such as slight increases in hindlimb extension or length] in early hominins, which tells us that energetics played a pretty large role in the evolution of bipedalism.