A study of humans and chimpanzees has provided new evidence to support the theory that our ancestors evolved to walk upright for the simple reason that doing so saves energy.
The study, which used treadmills, shows that people walking on two legs use 25% of the energy used by chimps who "knuckle walk" on all fours. Researchers hope that future fossil findings will reveal the precise anatomical changes that enabled our ancestors to take up bipedalism.
There are numerous ideas to explain human bipedalism, some of which stand on firmer ground than others.
Some experts, for example, have suggested that it helped our ancestors move about the forest canopy more easily. Others propose that this adaptation provided a more efficient way to feed and carry young at the same time.
But for decades some scientists have suggested we evolved to walk upright because doing so helped our ancestors conserve precious calories under harsh environmental conditions, when food was scarce. "The critical piece that's been missing is how to link the anatomy to the energy cost," says Herman Pontzer at Washington University in St. Louis, Missouri.
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.
According to Pontzer, chimps require more energy when walking on two feet because of their crouched gait – their bent joints use a lot of calories. The fact that humans have longer legs, and that our hamstrings attach further towards the back of the pelvis bone, might be the reason that we can comfortably walk upright with our legs straight, minimising the muscle energy needed to support our joints.
Pontzer says that future fossil findings will tell us for sure if it energy efficiency was the key advantage and, if so, shed light on exactly which changes in anatomy allowed early hominins to conserve calories by walking upright.