Chimps walking a on a treadmill have provided a clue as to why humans evolved to walk upright. Researchers from the University of Arizona, University of California-Davis and Washington University in St Louis found that walking upright, rather than on both knuckles and feet like chimps, used much less energy. The researchers looked at five chimps and people on a treadmill and observed their oxygen use and biomechanics. The team said that people walking on two legs used a quarter of the energy relative to their size compared to chimpanzees walking on their legs and hands. Taking longer strides increased energy efficiency. Human fossils showed an increase in leg length and changes to the pelvis enabling them to move upright, which suggested that energy efficiency played a role in humans becoming bipedal, the researchers said.