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, according to primatologist Robin Crompton of the University of Liverpool, U.K., co-author of the report in tomorrow's issue of Science .
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 with bent knees. And although orangutans had been spotted walking upright in trees, the behavior had never been well documented. What's surprising about the new observation, made by Crompton's colleague, Susannah Thorpe, 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. "When they move along the branches, they keep their knees and hip joints extended--like what human runners do when moving on springy indoor athletic tracks," says Crompton, noting that this gait could be how humans' ancestors initially began to walk upright.
The new observation supports a 30-year-old hypothesis that upright walking first evolved in the trees, in the ancestor common to all the great apes that usually moved on all fours but occasionally walked upright. Chimpanzees and gorillas would have subsequently adapted a new way of moving on all fours--by walking on their knuckles, as they do now. The authors suggest that early human ancestors in Africa may have abandoned the high canopy when climate change thinned the forest--and retained this familiar, upright mode of walking when they moved down to the forest floor. Still, many changes were needed for habitual use of two legs. For example, their lower limbs and pelvises were remodeled to better balance the weight--and these changes still are found only in hominids that walked upright most of the time, not apes.
But researchers will probably think twice before they use upright walking as their main defining trait for identifying a very early hominid that was still in the earliest stages of becoming bipedal. "Clearly it's emerging that the defining trait is not bipedalism alone," says primatologist Craig Stanford of the University of Southern California in Los Angeles. Other features, such as smaller canines and flatter faces, also suggest a fossil might be a member of the human family. "I think we're only going to be able to distinguish which lineage of apes led to hominins in hindsight, by looking at a suite of anatomical traits," Stanford says.