One of the main differences between apes and humans, as anyone can observe, is that humans are bipedal, or walk upright, while apes walk on all fours.

According to the accepted story of human evolution, known as the savannah hypothesis, early humans first began walking on two legs when they came out of the trees and colonized the ground, where it was advantageous to free their hands to look for food. Now, according to one study, we may have to revise the story of how our earliest ancestors became bipedal.

British researchers found that orangutans in the Indonesian rain forest regularly walk upright while foraging for food in trees. Their gate was not the low, momentary, two-legged crouch of some chimps and other apes. Rather, the orangutans walked upright in a way similar to humans.

The researchers think that orangutans evolved upright walking for the same reason that early human species did: it gave them an advantage for food gathering. At some point, early humans did descend from the trees to the savannah, but the researchers suggest the basic features of bipedalism may have come with them, rather than evolved as a consequence of living on the ground.

If the researchers are right, scientists could have a more difficult time distinguishing early human fossils, prior to 4.5 million years ago, from ape fossils. Since both may have walked upright, a bipedal skeleton could just as easily be the remains of an ape ancestor as it could the remains of an early human.