IT WAS one of the features that was supposed to be unique to the human evolutionary branch. We walk upright, while chimps and gorillas 鈥渒nuckle-walk鈥� on all fours. Now it seems that we didn鈥檛 evolve bipedalism from knuckle-walking: apparently walking upright arose far earlier in our history, when our ancestors were still in the trees, and has died out in our closest relatives.
Several scenarios have been proposed to explain why we came to walk on two legs, from the idea that it helped our ancestors feed and carry infants more efficiently, to the theory that it reduced our exposure to the sun, allowing us to forage for longer. Now Susannah Thorpe of the University of Birmingham, UK, and colleagues have made extensive observations of the most arboreal of the great apes, the orang-utan, and come to another conclusion: bipedalism evolved before we even left the trees, to help us move about the forest canopy.
Thorpe spent a year recording orang-utan behaviour in the Gunung Leuser National Park in Sumatra, Indonesia. From nearly 3000 observations of locomotion, the team found that the apes are more likely to walk on two legs 鈥� using their hands to guide them 鈥� when they are on the thinnest branches, less than 4 centimetres in diameter.
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Since orang-utans are fruit eaters, and fruit is more likely to be found on the ends of slender branches, being able to walk out along them is a great advantage, Thorpe says.
Moreover, climate change in the Miocene epoch, 24 to 5 million years ago, opened up gaps in the forest canopy that had a profound effect on our ape ancestors. Some of them 鈥� the ancestors of gorillas and chimps 鈥� crossed the gaps between trees by knuckle-walking, and specialised on climbing high into the canopy for food. Thorpe鈥檚 team suggests that only the ancestors of humans retained their ability to walk on two legs, and specialised on collecting food from smaller trees and the ground (Science, DOI: 10.1126/science.1140799).
鈥淲alking upright and balancing themselves by holding branches with their hands is an effective way of moving on smaller branches,鈥� says team member Robin Crompton of the University of Liverpool, UK. 鈥淚t helps to explain how early human ancestors learned 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.鈥�
Crompton says that the orang-utan is the only ape with a knee joint similar to that of humans. Orang-utans walk by extending their legs and hips to give a straight posture, whereas chimps waddle on two legs with bent knees and torso bent at the hip.
There is also tantalising fossil evidence to suggest bipedalism evolved relatively early. 鈥淥ur argument is supported by the fact that there have been a number of fossils found that show adaptations to upright posture,鈥� says Thorpe. The oldest fossil hominid showing such traits is called Morotopithecus, which lived between 16 and 21 million years ago, yet the idea has persisted that we arose 鈥� literally 鈥� from knuckle-walking apes. 鈥淧erhaps we鈥檝e been too focused on the African apes,鈥� says Thorpe. 鈥淭he trend has been to look at them to explain human evolution.鈥�
鈥淧erhaps we鈥檝e been too focused on the African apes. The trend has been to look at them to explain human evolution鈥�
The idea that human bipedalism evolved in the trees has been proposed before, says Chris Stringer, a palaeontologist at the Natural History Museum in London. 鈥淣evertheless, this is the best observational data on the importance of hand-assisted bipedalism to orangs.鈥� Since all the sites that have yielded fossil evidence of our earliest ancestors were forested rather than open, Stringer says, 鈥渁rboreal bipedalism is certainly a very plausible mechanism for the origins of walking upright鈥�.
Paul O鈥橦iggins of the Functional Morphology and Evolution unit at the University of York, UK, says the finding makes it more difficult to find a feature unique to the human ancestral line. 鈥淚f extended hip and knee bipedalism did indeed arise in the distant past, this makes the task of identifying possible ancestors of the human line much more difficult,鈥� he says.
