Humans Evolved to Walk on Two Legs Simply Because It's More Efficient

Posted Jul 17, 2007 by Digital Journal Staff

Chris Hogg in Science | 43 comments | 1400 views

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Anything that involves monkeys, juice boxes and treadmills is a must-read and this is no exception: A new study suggests human ancestors evolved to walk upright rather than "knuckle-dragging" because it requires 75 per cent less energy.

Digital Journal — One of the most contentious issues in evolutionary debate, is answering the question as to why early human ancestors evolved to walk on two legs rather than dragging their knuckles like chimpanzees and gorillas.

In an attempt to study the benefits of walking upright, a team of scientists from Washington University in St. Louis put a chimp on a treadmill (rewarding it with sweet treats and juice boxes of course) and studied how much energy adult chimps used compared to humans while walking.

As it turns out, it seems humans might have evolved to walk upright simply because it more efficient, using less energy to move around to find food.

Collecting metabolic and biomechanical data from four humans and five chimps, researchers learned humans use 75 per cent less energy than chimps by walking upright compared with chimps who walk on all fours.

Published in this week's online edition of the Proceedings of the National Academy of Sciences, Herman Pontzer (a researcher with the university's department of anthropology) said the study suggests human ancestors began walking upright because it burns fewer calories. The result of being able to walk upright, and thus using less energy, meant human ancestors required less food.

David Raichlen, an anthropology professor at the University of Arizona, told The Independent:

For decades 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.

During the experiment, both humans and chimps had to wear a mask to measure how much oxygen was used, and Pontzer said the chimps were not happy about it. Scientists also measured the force exerted by walking on the treadmill, allowing them to calculate muscle power used.

Dave Raichlen of the department of anthropology at the University of Arizona in Tucson, told the Globe and Mail the chimps were brilliant during testing, going so far as to hit the big red "stop" button when they wanted to stop the walk on the treadmill. However researchers say it was a long process to teach chimps to cooperate, saying they weigh as much as a human but are five times stronger (thus, if they don't want to cooperate there is not much researchers could do about it). Raichlen admits the juice boxes were key to having the chimps stay on the treadmill for at least three minutes.

When observing the humans, scientists found a notable difference between individuals but a solid advantage in bipedalism.

They also learned that chimps walking on two legs, or all fours, used the same amount of energy. But with chimps who took longer strides while on two feet, they were more efficient than those who took short strides.

In the same interview with the Independent, Raichlen said:

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. 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.

This process showed some early humans developed longer legs than others, indicating that "energetics played a pretty large role in the evolution of bipedalism."

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Humans Evolved to Walk on Two Legs Simply Because It's More Efficient

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