THURSDAY, May 31, 2007 (HealthDay News) -- New research with wild orangutans suggests that walking on two legs -- long considered a defining trait of humans and close ancestors -- may have started among tree-dwelling apes.
Traditionally, many scientists have thought that walking on two legs -- called bipedalism -- took hold after the ancestors to chimps, gorillas and humans descended from trees and began walking on the ground on all fours, a theory known as the savannah hypothesis.
"Orangutans, which are the most arboreal and tree dwelling of the great apes, use walking on two legs assisted by one or two hands to move on the fine, small, bendy branches at the edge of trees where the ripest fruits are to be found and there may be close contact with the canopy of a neighboring tree," said study lead author Robin Crompton, of the University of Liverpool Department of Human Anatomy and Cell Biology.
Because orangutans walk upright on the smallest tree branches, Crompton thinks this is evidence that upright walking began in trees.
"There would have been some stage where we [humans] walked on all fours on our knuckles, rather like chimpanzees and gorillas do. And the common ancestor would have been someone who walked on all fours," Crompton said.
But, Crompton added, fossil records don't support this theory. "From 5 million to 2 million years ago, the environments in which early human ancestors lived were not savannah but were woodland or forest," he said. "It's only when our own genus appears about 2 million years ago that humans are found in ancient savannah environments. This suggests that bipedalism evolved in a woodland or forest environment."
In addition, some early hominins, including Lucy (Australopithecus afarensis), lived in woodland environments. And even earlier forms such as Millenium Man (Orrorin) lived in the forest canopy and moved on two legs at the time when humans and apes diverged, Crompton said.
"What we have done is provided an explanation of why walking on two legs could be advantageous in the trees, which is really the ancestral habitat," he said.
But another expert contends that walking on two legs did not evolve exclusively in trees.
"The early habitat that Australopithecus lived in was really a woodland, which is a fairly open habitat with small trees in it, it's not a forest," said Kevin D. Hunt, a professor of anthropology at Indiana University. "Orangutans live in rainforests, but Australopithecus were found in woodlands."
Hunt thinks that bipedalism developed separately in trees and in woodlands. The advantage of bipedalism in woodlands was that it made it easier to reach for low hanging fruit, and on tree branches, he said.
"When humans became meat-eaters and started to travel long distances, that's when we developed the long legs," Hunt said. "We stayed in the Australopithecus stage longer than we have been humans. That lasted 4 million years, while we were still fruit-eaters."
Once humans gave up fruit gathering and started to eat meat and became hunters, humans evolved to be completely bipedal, Hunt said. "It happened over 200,000 years, certainly not a million," he said. "It's relative fast."
More information
For more information on evolution, visit the University of California, Berkeley.
