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Walking on two legs uses up a quarter of the energy it takes to walk on all fours according to a study that could explain why early human ancestors adopted bipedalism rather than the knuckle-walking of chimpanzees and gorillas.
Explaining why humans went from a four-legged gait to a two-legged, upright posture has been one of the most difficult and contentious issues in evolution.
The latest idea suggests that it all comes down to energy expenditure and how costly it is to move around in terms of the food required.
Scientists compared the amount of energy expended by humans and chimps when walking on a treadmill and found that a two-legged gait is about 75 per cent less costly compared with walking on all fours.
The results provide powerful evidence in support of the idea that the distinctive bipedal gait of humans became established because it was a more energy-efficient form of locomotion and so required less food.
David Raichlen, professor of anthropology at the University of Arizona, said that although in the past scholars had suggested that bipedalism might have something do to with saving energy, there was little hard evidence in support of it.
"For decades now researchers have debated the role of energetics and the evolution of bipedalism. The big problem in the study of bipedalism was that there was little data out there," Professor Raichlen said.
The study, which is published in the journal Science, measured the oxygen used by four adult humans and five adult chimps as they walked on a treadmill. The scientists also measured the force exerted on the treadmill, which enabled them to calculate the amount of muscle power being used.
The scientists found that there was a clear advantage in human bipedalism over knuckle walking among chimps, but that there were also substantial differences between individuals.
On average, for instance, the amount of energy used by chimps to walk on two legs or to knuckle-walk on four legs was about the same. However, there were large differences between individuals, with the chimps who took longer bipedal strides being more energy efficient than those who took shorter steps.
"We were able to tie the energetic cost in chimps to their anatomy. We were able to show exactly why certain individuals were able to walk bipedally more cheaply than others, and we did that with biomechanical modelling," Professor Raichlen said.
"What those results allowed us to do was to look at the fossil record and see whether fossil hominins [ancestors] show adaptations that would have reduced bipedal energy expenditure," he said.
Such an analysis, for instance, has revealed that some early human ancestors had developed slightly longer legs.
The study can explain the advantage of a bipedal gait once it had come about, but is less convincing when it comes to explaining why our ape-like ancestor went from a four-legged to a two-legged gait in the first place.
Herman Pontzer, professor of anthropology at Washington University in St Louis, said that the central finding of the study was to test the idea that two-legged walking is better than four when it comes to burning calories.
The chimps involved in the study were trained to walk on a treadmill either using their knuckles or with a bipedal gait. "It took a long time and a lot of treats [to train them]," he said.
"It's so hard to work with adult chimps if they don't want to work with you. They weigh as much as you do and are five times as strong - so they won't work with you if they don't want to."
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