Our two-legged life may have its roots in the trees. Researchers have witnessed wild orangutans standing straight-legged on slender branches as they grab for fruit, suggesting that bipedalism, or upright walking, may have arisen in the tree-dwelling ancestor of apes and humans and passed down to us—proving, if true, that the biped doesn't fall far from the tree.

The evidence comes from a yearlong field study of Sumatran orangutans in Indonesia's Gunung Leuser National Park. Paleoanthropologist Susannah Thorpe of the University of Birmingham, England, spotted apes clambering on trees a total of 2,811 times, including numerous instances of bipedalism. In 75 percent of these cases the animals maintained balance with their hands, and over 90 percent of the time their legs were stiff, unlike the bent-knee, bent-hip shuffle of chimps and gorillas, which also stand upright in trees sometimes.

Thorpe and her colleagues report in a paper published online today by Science that the orangutans reared up on two legs mostly as they reached for fruit while perching on one or more smaller branches totaling four or fewer centimeters in diameter. They speculate that straight legs help the apes balance on pliable branches, much the way gymnasts steady themselves on a trampoline.

Researchers had previously taken bipedalism as distinctive of humans and their closest extinct relatives, together called hominins, speculating that early hominins adapted to two legs on African savannas some five million years ago, after descending from the trees and outgrowing a phase of knuckle walking similar to that of modern chimps and gorillas. "Most of us had assumed that the only place where it's sensible to be bipedal is on the ground," says paleoanthropologist Bernard Wood of The George Washington University in Washington, D.C.

A handful of fossil species dating from five million to 28 million years old, mostly before chimpanzees split from hominins, showed signs of upright posture and bipedalism, but "the evidence has been pretty flakey," Wood says.

The orangutan finding, he says, suggests these fossils may indeed represent bipedal apes. Thorpe and company propose that bipedalism evolved to help such animals reach tasty fruit just out of reach, and as forest cover shrank, chimps took to shimmying and knuckle walking while hominins began stretching their legs on the ground.

The researchers "present a plausible and elegant argument in favor of the emergence of bipedalism in an arboreal rather than terrestrial context," according to an accompanying commentary by researchers Paul O'Higgins and Sarah Elton, both of the Hull York Medical School in England.

They say the discovery strengthens the case that differences in foot structure and limb proportion in early hominins reflect different paths from orangutanlike bipedalism.

It "reopens the debate," they write, "about the origins of our own peculiar commitment to bipedal locomotion."