The new study hypothesises that walking on two legs, or bipedalism, evolved because it required only a quarter of the energy used by scampering on all fours.

The study, Chimpanzee Locomotor Energetics and The Origin of Human Bipedalism, is to be made public this week in the Proceedings of the National Academy of Sciences.

'For decades now researchers have debated the role of energetics and the evolution of bipedalism,' said Mr David Raichlen, an assistant professor of anthropology at The University of Arizona, who was one of the lead scientists on the study.

Bipedalism is seen as a critical evolutionary difference between humans and apes.

Scientists theorise that the reduced energy expended in walking upright would have provided evolutionary advantages by allowing early man to forage for food more easily.

Scientists studied the gaits of four adult humans walking on a treadmill, and also studied five chimpanzees who were trained to walk quadrupedally (on four legs) as well as bipedally.

AGENCE FRANCE-PRESSE