Mankind's earliest ancestors may have started walking on two feet to free up their arms to carry as much food as possible, British and Japanese researchers say.

A joint study of chimpanzees by the University of Cambridge and Kyoto University found them switching to moving on two limbs instead of four to monopolize a food resource, usually because it was not in plentiful supply in their habitat.

Standing on two legs allowed them to carry much more at a time because it freed up their hands.

Our earliest hominin ancestors may have lived in shifting environmental conditions in which certain resources were not always easy to come by, researchers said.

Bursts of bipedal activity may have led to anatomical changes that in turn became the subject of natural selection where competition for food or other resources was strong, a Cambridge release reported Tuesday.

"Bipedality as the key human adaptation may be an evolutionary product of this strategy persisting over time," Cambridge anthropologist William McGrew said. "Ultimately, it set our ancestors on a separate evolutionary path."