Our ancestors may have ended up on two legs rather than four simply because it meant they could carry more food.
This helped to ensure their survival when resources were scarce, say scientists.
Anthropologists studying chimpanzees found the great apes walk upright and free their hands for carrying when they need to monopolise hard-to-find food.
They can grab more with a single swipe in the face of fierce competition from other apes.
The team from the University of Cambridge and Japan's Kyoto University believe the benefit of 'first-come, first-served' and getting a bigger share of scarce food supplies could, over a long period of time, have led some of our earliest 'hominin' ancestors to evolve into bipedal primates - walking on two legs permanently instead of four. Professor William McGrew, of Cambridge's department of archaeology and anthropology, said: 'Bipedality as the key human adaptation may be an evolutionary product of this strategy over time.
Ultimately, it set our ancestors on a separate evolutionary path.'
The research, published in the journal Current Biology, suggests our earliest hominin ancestors may have lived in shifting environmental conditions where certain resources were not easy to come by.
The scientists conducted two studies of chimpanzees in Bossou Forest in Guinea, west Africa, finding that when supplies of highly prized coula nuts were scarce, the chimps were more likely to walk on two feet in an attempt to carry off more in a single trip.
They also found that when the chimpanzees went crop raiding, 35 per cent of their activity involved some sort of bipedal movement, and 'once again, this behaviour appeared to be linked to a clear attempt to carry as much as possible in one go'.
By studying the behaviour of chimpanzees, they believe that over time, intense bursts of bipedal activity in early hominins may have led to anatomical changes that in turn became the subject of natural selection where competition for food or other resources was strong.
- DAILY MAIL