The truth about why we walk on two legs: it saves energy

The truth about why we walk on two legs: it saves energy
By Steve Connor, Science Editor
Published: 17 July 2007
Independent UK

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 proven to be one of the most difficult and contentious issues in evolution.

The study 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 bipedal gait of humans became established because it was more energy efficient and so required less food.

David Raichlen, professor of anthropology at the University of Arizona, said that although in the past scholars have suggested that bipedalism may have something do to with saving energy, there was little hard evidence to support of it.

"For decades researchers have debated the role of energetics and the evolution of bipedalism," Professor Raichlen said. "The big problem in the study of bipedalism was that there was little data out there."

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 bipedalism over knuckle-walking, but that there were also substantial differences between individuals which were masked when the entire group was analysed.

On average, the amount of energy used by chimps to walk on two legs or to knuckle-walk on four legs was about the same. However, the chimps who took longer bipedal strides were more energy efficient than those who took shorter ones.

"We were able to tie the energetic cost in chimps to their anatomy," Professor Raichlen said. "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."

Such an analysis has revealed that some early human ancestors had developed slightly longer legs. "This tells us that energetics played a pretty large role in the evolution of bipedalism," Professor Raichlen said.

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 the central aim of the study was to test the idea that two-legged walking is better than four when it comes to burning calories.

"We now have a tried-and-tested and supported hypothesis for why bipedalism was so successful," Professor Pontzer, a member of the research team, said.

The chimps involved in the study were trained to walk on the treadmill. " It took a long time and a lot of treats [to train them]," Professor Pontzer said. "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."

Rival theories

* Tree walking: A recent theory suggests that our tree-living ancestors walked upright on branches, using their arms for balance. The idea is based on observing modern-day orang-utans, which often move around in trees by walking on two legs, gripping the branches with their hands.

* A cooling effect: Another idea is that it was cooler to walk upright in savannah grasslands because by doing so there was less surface area of the body exposed to the Sun.

* Wading in water: If our ancestors had to wade through water, they would have learned to walk on two legs. Modern-day chimps often adopt a two-legged gait when walking through water.