Scientists say early human ancestors may have begun walking on two legs on tree branches, not on the ground as commonly believed. The evidence contradicts the long-held belief that early humans first began walking upright on the ground. VOA's Jessica Berman reports.

The defining feature of human evolution is bipedalism which means walking on two legs.

Known as the Savannah hypothesis, paleontologists have long believed that bipedalism began after knuckle-walking primates, primates walking on all fours, descended from trees to the ground.

While chimps and gorillas continue to use all fours to get around, human ancestors eventually learned to walk upright. It has been thought that bipedalism began at least 4.5 million years ago.

Now, two teams of British researchers are challenging the Savannah hypothesis.

Robin Crompton of the University of Liverpool in England says upright walking may have occurred between 17 and 24 million years ago, much earlier than anyone imagined.

"It goes back to whenever the date was for the common ancestor for all great apes," said Robin Crompton.

Crompton and colleagues at the University of Birmingham in England collected data for one year on tree-dwelling orangutans in the Sumatra rainforest in Indonesia.

Susannah Thorpe of Birmingham University describes what researchers call hand-assisted bipedalism, in which the orangutans moved their legs and used their arms primarily for balance on thin branches.

The primates were also observed straightening their legs on bending branches to keep from losing their balance, much the same way humans stiffen their legs while running on springy surfaces.

Thorpe says the observations are important, because they propose a less complicated theory of how humans came to walk upright.

"Our results remove the need for complicated scenarios like the Savannah that require human ancestors to move into a completely differently habitat," said Susannah Thorpe. "Instead straight-legged bipedalism simply relocates from the trees down to the ground."

The study on bipedalism is reported this week in the journal Science.