The human bipedal gait may represent an evolution in energy use
Humans evolved to walk upright because it uses less energy than travelling on all fours, according to researchers.
A US team compared the energy used by humans and by chimpanzees in walking.
The human bipedal gait is about four times more efficient than chimps getting around on either two or four legs, the researchers found.
Writing in Proceedings of the National Academy of Sciences (PNAS), they say this may explain why we walk bipedally, and some of our anatomical features.
Other research groups have proposed alternative explanations for our two-legged gait.
Some suggest it evolved because early humans needed to reach upwards to collect food or pass it to a mate, while others maintain it predates four-legged locomotion in primates, citing the often upright posture of orangutans as they move across slim branches.
On the treadmill
A study from 1973 found little difference in efficiency between two-legged and four-legged walking in primates, but its conclusions had been disputed because only juvenile chimpanzees were used.
So David Raichlen from the University of Arizona in Tucson and colleagues set up a study in which five adult chimps were trained to use a treadmill, either on two legs or four.
The subjects were fitted with masks to collect exhaled air so that parameters such as oxygen use could be measured. Blobs of white paint on critical parts of the body such as elbows and knees allowed researchers to analyse the gait using video.
The results were compared with four human subjects using the same treadmill.
Generally, the humans were about four times more efficient than the chimps.
Three of the chimps found bipedal walking used more energy than going on all fours. But one of the others showed the opposite pattern; and intriguingly, she was the only chimp to lengthen her stride.
"We were able to tie the energetic cost in chimps to their anatomy," noted Dr Raichlen.
"We were able to show exactly why certain individuals were able to walk bipedally more cheaply than others."
The hypothesis, then, is that early humans began to evolve in a direction which allowed for easy bipedal travel.
David Raichlen suggests that early humans should show adaptations such as a longer leg length, and that there are indications of this in fossils of the genus Australopithecus, such as the famous "Lucy" specimen discovered in Ethiopia in 1974.