Our ancient tree-dwelling ancestors stood upright on two legs - a trait modern humans have retained, while other great apes have evolved four-legged knuckle-walking, researchers say.
It was one of the characteristics that was supposed to define the ancestral human line from our great ape cousins: We walk upright, while chimps and gorillas walk on four legs, using their knuckles.
Now it seems that we did not evolve from knuckle-walkers - bipedalism apparently arose far earlier in evolutionary history, when our ancestors were still in the trees.
Several scenarios have been proposed to explain why we came to walk on two legs, from the idea that it allowed our ancestors to feed more efficiently and carry infants, to the idea that the posture reduced our exposure to sunlight and so gave us longer to forage.
Susannah Thorpe, of the University of Birmingham, UK, and colleagues have made extensive observations of the most arboreal of the modern great apes, the orang-utan, and come to another conclusion, that bipedalism evolved to help us move about the forest canopy.
"The orang-utan is the only great ape still living in its ancestral habitat," says Thorpe, making them good models for understanding the selection pressures on ancestral apes.
Thorpe spent a year recording orang-utan behaviour in the Gunung Leuser National Park in Sumatra, Indonesia, and from nearly 3000 observations of locomotion, the team concluded that the apes were more likely to walk on two legs - using their hands to guide them - when they are on the thinnest branches, less than 4 centimetres in diameter.
On medium-sized branches - those greater than 4 cm but less than 20 cm diameter - the apes tended to walk bipedally, but used their arms to support their weight by swinging or hanging.
Only on the largest branches, with a diameter greater than 20 cm, did the animals walk on all fours.
Fruits of bipedalism
Since orang-utans are fruit eaters, and fruit is more likely to be found on the ends of thinner branches, the ability to walk out along them is advantageous. Since they also spend most of their time in the trees, the ability to move over thinner branches helps when it comes to crossing from tree to tree and traversing the canopy, says Thorpe.
When walking bipedally, orang-utans extended their legs at the knee and hip to give them a straighter posture, in contrast to what happens when chimps try to walk on two legs. Chimps are forced to waddle with bent knees and their torso bent over at the hip. When humans run on springy surfaces they also keep their legs straight, like orang-utans, Thorpe points out.
"Walking upright and balancing themselves by holding branches with their hands is an effective way of moving on smaller branches," says Robin Crompton of the University of Liverpool, UK, who was also involved in the study.
"It helps to explain how early human ancestors learnt to walk upright while living in the trees, and how they would have used this way of moving when they left the trees for a life on the ground."
So, rather than evolving to walk on two feet after scrabbling around on the floor on all fours, the theory suggests our ancestors already had the rudimentary means of walking on two feet before they even left the trees.
When the ancestors of chimps and gorillas left the trees, however, they needed to maintain the ability to climb tree trunks. This need for tree-climbing strength and anatomy guided their evolution at the expense of more efficient terrestrial movement, and therefore led to knuckle-walking, says Crompton.
Orang-utans are the most distant of our relatives among the great apes, followed by gorillas, and then bonobos and chimpanzees. The ancestors of the latter two species split from the human line around 6 million years ago; the orang-utan ancestor split from the human ancestor around 10 million years ago.
Thorpe and colleagues suggest that at sometime in the Miocene epoch - 24 to 5 million years ago - the increased gaps in the forest canopy that came about as a result of climate fluctuations had a profound effect on our ape ancestors.
Some of them - the ancestors of chimps and gorillas - specialised on climbing high into the canopy and crossing the gaps between trees by knuckle walking. Others - the ancestors of humans - retained their ability to walk on two legs, and specialised on collecting food from smaller trees and the ground.
The idea of an arboreal origin for human bipedalism has been proposed before, says Chris Stringer, a palaeontologist at the Natural History Museum, London. "Nevertheless, this is the best observational data on the importance of hand-assisted bipedalism to orangs, and its possible implications for the evolution of human bipedalism."
Since all the sites which have yielded fossil evidence of our earliest ancestors were forested or wooded, rather than open, Stringer says, "arboreal bipedalism is certainly a very plausible mechanism for the origins of walking upright."
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