The main reason our ancestor started walking on two limbs instead of four was a bid to monopolise resources and to carry as much food as possible in one go, researchers from the University of Cambridge and Kyoto University discovered.
Researchers found this while studying the behaivour of modern-day chimpanzees. They found that chimpanzees switch to moving on two limbs instead of four in certain situations where they need to monopolise a resource, usually because it may not occur in plentiful supply in their habitat, making it hard for them to predict when they will see it again. Standing on two legs allows them to carry much more in one go because it frees up their hands.
To prove this, the researchers conducted a study in Kyoto University's "outdoor laboratory" of a natural clearing in Bossou Forest, Guinea. Here, the researchers allowed the chimpanzees access to different combinations of two different types of nut - the oil palm nut, which is naturally widely available, and the coula nut, which is hardly available.
Researchers monitored the chimpanzees' behaviour in three different situations: when only oil palm nuts were available, when a small number of coula nuts was available and when coula nuts were the majority available resource.
When the rare coula nuts were available only in small numbers, the chimpanzees transported far more in one go. Similarly, when coula nuts were the majority resource, the chimpanzees ignored the oil palm nuts altogether. Clearly, the chimpanzees regarded the coula nuts as a more highly-prized resource and competed for them more intensely.
In such high-competition settings, the chimpanzees started moving on two legs. Researchers found that bipedal movement allowed them to carry more of this precious resource.
The study concludes that unpredictable resources, like the coula nut in the field survey, are seen by chimpanzees as more valuable. When these resources are scarce and access to them is on a "first-come, first-served" basis, they are more prone to switch to bipedal movement, because it allows them to carry more of the resource at once.
Researchers believe probably this was the main reason humans started walking on two legs.