The ancestors of humans began walking upright while they were still living in trees - not out on open land, according to a new theory.
Our upright walking may be related to behaviour shown by orangutans
The traditional view is of bipedalism evolving gradually from the four-legged "knuckle-walking" displayed by chimpanzees and gorillas today.
Now, a study published in the journal Science disputes this idea.
The British authors of the study say that upright walking was always a feature of great ape behaviour.
Humans inherited it without ever passing through a knuckle-walking phase.
They believe that knuckle-walking evolved only recently as a way of getting around the forest floor.
Susannah Thorpe, Robin Crompton and Roger Holder came to their conclusions after analysing the movement of wild orangutans, which spend most of their lives in trees.
They found that orangutans used upright locomotion to fetch food from the small branches of trees and to cross directly from one tree to another.
"Both access to fruits and crossing gaps in the trees would require an ability to navigate very thin, terminal tree branches which are liable to bend under body mass," said Professor Robin Crompton, from the University of Liverpool.
"The logical conclusion from the environmental, fossil, and experimental evidence is that upright, straight-legged walking originally evolved as an adaptation to tree-dwelling."
They suggest the shift made by our ancestors to a terrestrial lifestyle came about as climate change thinned out their forest habitat.
In response, these ancient ape-like creatures, or hominids, may have abandoned the high canopy for the forest floor. Here, they remained bipedal and began eating food from the ground or from smaller trees.
Professor Crompton explained that orangutans walking upright on springy branches act much like athletes running on springy tracks - they use extended postures of knee and hip to give them straighter legs.
The researchers point out that some of the earliest fossil human ancestors combined lower limbs that were adapted for upright walking with an upper body that seems suited to climbing trees.
There is also evidence these bipedal creatures lived in a closed forest environment, not the savannah habitat that would have required them to routinely move on the ground.
Daniel Lieberman, a biological anthropologist from Harvard University, US, told BBC News: "I think it's a neat paper; it's always terrific when people think creatively about the origins of human bipedalism. But it's not going to be the last word.
"The big problem is - what was the selective advantage for that first hominid that stood upright? We know very little about the context in which that occurred."
Dr Lieberman also questioned the idea that the kind of locomotion displayed by chimps and gorillas must have evolved only recently.
Chimps, gorillas and humans are more closely related to one another than they are to orangutans.
"The relationships between the apes are not in question," he said, "unless all those similarities between chimps and gorillas are independently evolved, then the inference is inescapable that the last common ancestor of chimps and humans must have been like a chimp or gorilla."