"One of the key realizations of the last century was that during our evolution, walking upright preceded having a big brain," said Paul O'Higgins, an anthropologist at the University of York in England who is unaffiliated with the research.
"Ever since then people have been wondering what types of behaviors predisposed walking on two legs."
Prominent theories have suggested that relatively recent human ancestors developed the ability to walk upright after leaving the treetops to live on the African savanna.
These ancient animals may have first employed an all-fours gait, the familiar knuckle walk, to get around before hominids later developed an upright stance.
Gorillas and chimps retain the knuckle-walk technique to this day.
The new study will appear in the June 1 issue of the journal Science.
The orangutan evidence adds a new twist to the debate of arboreal versus terrestrial bipedalism theories. But it certainly won't end them.
"The study pushes bipedalism further back into the Miocene [a period that lasted between 24 to 5 million years ago] and up into the trees," O'Higgins of York University said.
Brian Richmond is a biological anthropologist at George Washington University in Washington, D.C., who was not involved in the study.
"This is interesting and a very good study of how orangutans climb and walk in trees, but it's not a study of our distant ancestors," he cautioned.
"It tells us something about how primates other than ourselves use bipedalism. That could tell us something about how bipedalism might have been used by our distant ancestors. But it doesn't tell us what actually happened," Richmond said.
"It doesn't show us the origins of our own human bipedalism."
(See related: New Fossils Help Piece Together Human Origins [January 21, 2005].)
Richmond also noted there is still compelling evidence for the theory that bipedalism first appeared on the ground.
Recent fossil finds have pushed back the earliest known date by which human ancestors walked on two legs to some six million years ago.
"Our closest living relatives are chimps and gorillas, and since both of them climb trees and walk on their knuckles, it's most likely that our common ancestor with them did that too," he said.
"If our common ancestor with chimpanzees was not a knuckle walker, it means that chimps had to evolve knuckle walking independently from gorillas," he added.
But if the new study is correct that bipedalism occurred in very ancient apes, fossil evidence for the trait might no longer be considered a smoking gun that identifies fossils as belonging to a human ancestor, according to Crompton.
"It's going to make it very much more difficult to tell what's a hominid and what's not," Crompton said.
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