An orangutan at the Borneo Orangutan Survival Centre in Mantangi, Kalimantan. A detailed new study of orangutan movement suggests that apes may first have become bipedal in the trees.
Credit: AFP
PARIS: Tree-dwelling apes may have been the first to begin walking on two legs, says a new study, questioning the current theory that more recent human ancestors were the first bipeds.
The study, published today in the U.S. journal Science, says apes may have walked on two feet with support from their arms to traverse thin branches to collect food.
"If we're right, it means you can't rely on bipedalism to tell whether you're looking at a human or other ape ancestor," said evolutionary biologist Robin Crompton of the University of Liverpool in England, one of the study's authors.
"It's been getting more and more difficult for us to say what's a human and what's an ape, and our work makes that much more the case," he said.
Savannah hypothesis
Crompton and colleagues Susannah Thorpe and Roger Holder of the University of Birmingham observed wild orangutans in Sumatra, Indonesia for their study. Thorpe spent a year living in the rainforest.
Looking at 3,000 examples of orangutan movement, the team found that they were more likely to walk upright, with the help of their arms, on thin branches. The orangutans tended to walk on all fours only on the largest branches, the researchers said.
They decided to observe orangutans because they spend their lives in trees and could serve as models for how human ancestors moved millions of years ago.
One of the most popular explanations for upright walking has been the so-called savannah hypothesis, which suggests ancestors to chimps, gorillas and humans descended from trees and began walking on all fours.
That type of walking would have eventually become the knuckle-walking that chimps and gorillas still use, followed by upright walking by humans, according to the hypothesis.
Previous research has said that apes would have begun encountering gaps between trees in eastern and central Africa toward the end of the Miocene era – which spanned the period from 24 to five million years ago – as the climate alternated between wet and dry.
Knuckle-walkers
The British researchers suggest human ancestors then descended to the forest floor, finding food there and remaining bipedal. Ancestors of chimps and gorillas may have become good climbers, going up and down trees while using knuckle-walking when moving between trees on the ground, they say.
However, French anthropologist Yvette Deloison of the National Centre for Scientific Research (CNRS) in Paris said the orangutan model was an incorrect one to use. She formulated a theory several years ago that the human form could have only derived from an ancestor that was already a biped.
"If the ancestor of man had an anatomy allowing him to do the same thing as orangutans, with hands and feet so perfectly adapted to climbing and suspension, he would have been far too specialised to allow for the development of what we are today," commented Deloison.
