According to scientists at Washington University in St. Louis, Missouri, humans took to standing upright for the sake of efficiency. The researchers used treadmills to discover that walking humans use 75% less energy than chimps that walk on all fours. From the New Scientist:
He and his colleagues attempted to provide this missing piece of the puzzle by analysing human and chimp biomechanics. The chimps in the study all knew how to walk on two feet as a result of training. “Chimps can walk bipedally, they just choose not to,” he says.
He adds: “Chimps aren’t built to be efficient. They’re built to be safe.” Using all four limbs can help steady them on branches, he explains. “Fall out of the fruit tree and you’re dead.”
A mask worn by human volunteers and the chimps as they walked on the treadmill enabled the researchers to calculate the number of calories burned, based on oxygen consumption.
Researchers also used non-toxic paint and reflective badges to mark the leg joints of both groups. This allowed a machine to track the subjects’ joints as they walked on the treadmill and to characterise their gait. A complex algorithm also revealed the muscle force exerted on the joints in each step.
The results of the experiment revealed that while a 50 kilogram (110 pound) human uses roughly 13 kilocalories to walk a kilometre (above and beyond the energy needed to keep sustain the body at rest), a similarly-sized chimp uses about 50 kcal to walk the same distance on two feet. And when chimps move on all fours they require slightly less energy – 46 kcal, on average – to cover the same distance.