KEY POINTS:
Walking upright on two legs - a quintessentially human trait - started when our ape ancestors still lived in trees rather than evolving from knuckle-walking on the ground, scientists say.
A study of how wild orang-utans living in the forests of Sumatra "walk" around the treetops has led researchers to conclude that the conventional view of how human ancestors started to walk on two legs is probably wrong.
The scientists said instead of being an adaptation that evolved from knuckle-walking on all fours, as seen in modern-day chimps and gorillas, bipedalism first came about when the ancestors of all great apes learnt to move around in the forest canopy on two legs, using their arms for balance.
This latest theory about how humans came to walk upright is certain to be resisted by many other experts who claim the adaptation came about for other reasons, but the evidence has been judged good enough to be published in the journal Science.
Professor Robin Crompton, of Liverpool University, said it raised the prospect that few features of human behaviour might be unique - walking upright on two legs was considered to be one of the principal traits that separated humans from the great ape.
"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. It's getting more difficult for us to say what's a human and what's an ape, and our work makes that much more the case," Professor Crompton said.
Susannah Thorpe, of Birmingham University, spent a year studying the behaviour of orang-utans in Sumatra and found they frequently walked along branches using two straightened legs - rather than the bent legs used when chimps and gorillas engage in limited bipedalism.
Although orang-utans live in Indonesia rather than Africa - the cradle of humanity - and are less related to humans than chimps and gorillas, Dr Thorpe nevertheless believes their gait is an ancestral trait that evolved long before gorillas and chimps developed knuckle walking.
"The crux of our argument is that bipedalism was an adaptation for living in trees - it is an ancient, ancestral trait," Dr Thorpe said.
"Our results suggest that bipedalism is used to navigate the smallest branches where the tastiest fruits are and also to reach further to help cross the gaps between trees." Scientists believe that as successive bouts of climate change caused repeated forest retreats, there was pressure on the fruit-eating primates to adapt to the changing environment.
One adaptation was to use bipedalism to walk through the thinning treetops. Another was to climb up and down trees and move around on the ground, which is how gorillas and chimps learned knuckle walking.
But the third line of adaptation was to exploit the tree-walking "arboreal" bipedalism of ancestral apes and use it to learn to walk on the ground with two legs.
- INDEPENDENT
