New research suggests that upright walking began high up in the trees.
Walking on two legs, or bipedalism , is a key feature that defines humans and our human ancestors.
A popular view is that bipedalism evolved when ancestors of chimps, gorillas and humans moved from living in the trees to much more on the ground. They would have walked on all four legs but in gorillas and chimpanzees, this gradually evolved to the four-legged 'knuckle walking' you see today. But the human line began walking on two legs and increasingly adapted to open conditions.
However, new research, published today in the journal Science, suggests bipedalism may have evolved when our human ancestors were still living in trees.
A team of British scientists spent a year studying wild orangutans in the Sumatran rainforest. They wanted to find out if walking on two legs benefited the tree-living ape ancestors.
Orangutans are arboreal, spending most of their lives in trees. Susannah Thorpe, Robin Crompton and Roger Holder watched thousands of different orangutan movements.
On the largest branches, orangutans walk on all fours but the team found that when orangutans wanted to reach fruit, which is usually out on the smaller, thin and bendy branches, they walked on two legs. They used their arms for balance, called hand-assisted bipedalism, and to reach out for the fruit. So bipedalism helps orangutans navigate the smallest branches where their food is.
Chris Stringer, human origins expert at the Natural History Museum said, 'Although the idea of an arboreal origin for human bipedalism has been suggested before, including by Crompton's team, this is nevertheless the best observational data on the importance of hand-assisted bipedalism to orangutans, and its possible implications for the evolution of human bipedalism.'
Many of the recent findings of fossil evidence of human ancestors have been recovered from wooded environments.
Chris adds, 'all the sites that are claimed to contain fossil evidence of our earliest ancestors were forested or wooded, rather than open, and arboreal bipedalism is certainly a very plausible mechanism for the origins of walking upright .'
The evolutionary theory is that as the climate in East and Central Africa changed, the rainforest became patchy. This stopped apes from being able to cross gaps while they were high in the forest canopy. Early human ancestors moved to the ground to cross these gaps and then abandoned the treetops altogether. They remained bipedal and adapted to life on the ground.
The ancestors of chimps and gorillas, however, stayed in the trees to a greater extent and developed knuckle walking for crossing the ground in between the gaps in the rainforest.
'Our findings blur the picture even further,' said Crompton of the University of Liverpool, UK. '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 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.'
As well as helping us to understand the origins of bipedalism in human ancestors, knowing more about how orangutans move could be useful in helping conserve these endangered creatures. Habitat destruction is threatening the survival of orangutans as a species.
'If you can understand how they cross gaps in the forest, you can learn about effects that living in logged or degraded habitat would have on their locomotion,' said Thorpe of the University of Birmingham, UK.
'These could affect energy levels, for example, if they have to go to the ground, which is incredibly risky because the Sumatran tiger is down there licking its lips.'
'The Sumatran orangutan population is predicted to be extinct in the next decade if habitat degradation continues. Our research further highlights the need for protecting these animals.'