Walking in Trees
A research team observing wild orangutans in Sumatra, Indonesia, found that walking on two legs may have originated in ancient, tree-dwelling apes, rather than in more recent human ancestors who lived on land, as current theory suggests.

Walking upright, or bipedalism, has long been considered a defining feature of humans and our closest ancestors. One of the most popular explanations, the "savannah hypothesis," suggests that chimps, gorillas and human ancestors descended from tree-swinging primates and began walking on the ground on all fours.

Over time, this four-legged gait would have evolved into the "knuckle-walking" that chimps and gorillas still use today, and then into upright, two-legged walking in humans.

Paleontologists have conventionally used signs of bipedalism as key criteria for distinguishing early human fossils from those of apes. But this distinction is complicated by recent fossil evidence that some early humans, including Lucy, lived in woodland environments, while even earlier forms, such as Millennium Man, might have lived in the forest canopy and moved on two legs.

To collect the data, Susannah Thorpe of the University of Birmingham, UK, spent a year living in the Sumatran rainforest, recording virtually every move the orangutans made. She and her colleagues then used these observations to test the hypothesis that bipedalism would have benefited our tree-dwelling ape ancestors.

-    American Association for the Advancement of Science

Jessica Jones  |  Assistant Editor