New theory rejects popular view of man's evolution
· Ancestors learned to walk in trees, says UK team
· Researchers base claims on watching orang-utans
Ian Sample, science correspondent
Friday June 1, 2007
Humanity's earliest ancestors did not drag their knuckles along the ground before mastering life on two feet, but learned to walk upright while still living in the trees, according to a team of British scientists. The claim challenges the belief that humans evolved from chimp-like creatures that descended from the trees to roam the savannahs of east Africa, using their knuckles for support, before slowly rising to the upright posture of more modern humans.
The theory marks a dramatic twist in evolutionary thinking that suggests some of our earliest ancestors may have begun walking on two legs up to 24m years ago, rather than shortly after the human lineage split from chimpanzees around 6m years ago. It suggests early humans adapted rapidly to open landscapes by honing the basic walking skills they developed to move around the forest canopy.
The team, led by Robin Crompton at Liverpool University and Susannah Thorpe at Birmingham University, claim our tree-dwelling ancestors learned to walk on two feet because it helped them edge along outer branches while having their hands free to grasp ripe fruit. The tactic also enabled them to clamber between neighbouring trees without having to descend to the forest floor.
The scientists reached their conclusions after spending a year observing the movements of orang-utans in Sumatra. The great apes of the region are the only species to spend their entire lives in the trees. Footage of nearly 3,000 movements showed the apes consistently walked on two legs to reach the outer branches of trees, using their arms primarily for balance. Unlike gorillas and chimps, which bend their knees to walk on the ground, the orang-utans straightened their legs to adopt a more human-like gait.
Professor Crompton said such skills would have benefited early human ancestors enormously between 24m and 5m years ago, when eastern and central Africa experienced dramatic climatic cycles and the forests first thickened and then died back. "As the forests became sparse, the strategy of our human ancestors was more or less to abandon the canopies and come down to the ground, where they could use this bipedalism immediately to get around," he said.
Writing in the US journal Science, the team describes how the ancestors of other great apes, such as gorillas and chimpanzees, may have continued living in the trees, but later evolved to walk on their knuckles, to help them move quickly between one tree and another on the forest floor. "While they were more innovative and developed this new way of moving, our ancestors were more conservative, using a form of walking that was already in their repertoire."
Carol Ward, a palaeoanthropologist at the University of Missouri-Columbia, said the study fundamentally altered our view of human ancestry.
"Our entire conception of human evolution has included us going through this chimp-like, knuckle-walking phase, but this is saying maybe the trajectory of changes that led to humans didn't look like that at all. It's profound," she said.
Other researchers were more sceptical. "The main evidence is that our closest living relatives are not orang-utans, they're chimps and gorillas, and since both climb trees and walk on their knuckles, it's most likely our ancestors did that too," said Brian Richmond, an anthropologist at George Washington University.
"One of the only anatomical features we share explicitly with chimps and gorillas is that we only have eight wrists bones, while almost all other primates have nine. In humans, chimps and gorillas, two bones have fused into one to stabilise the wrist, making it stronger for knuckle-walking. It's not a smoking gun, but it's the best evidence we have."
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