To walk upright is to be human. At least that’s what paleoanthropologists have thought for decades. But now, researchers have observed orangutans walking in a way that resembles human locomotion–albeit along the branches of trees. This suggests that the earliest stages of upright walking evolved in apes living in the trees rather than in hominids walking on the ground.
Researchers had seen other primates walking on just two of their fours before. Chimpanzees sometimes stroll upright during foraging, for example, but they do it bowlegged and bent-kneed. And although orangutans had been spotted walking upright in trees, the behavior had never been well documented. What’s surprising about the new observation was that the orangutans were walking upright on thin, flexible branches, which are springy, like spongy ground. What’s more, the orangutan walk was similar to the straight-legged bipedalism seen in humans.