A new study discovered that humans’ ability to walk upright was developed from ancestors foraging for food in forest tree tops. Until now it was believed that this ability was developed from walking on all fours on open land.

After examining the skeletons of early human ancestors the study conducted by the scientists at the University of Liverpool concluded that the combination of short legs and long arms is an adaptations for moving amongst tree tops, with hindlimbs adapted for walking on two legs. To understand why bipedalism – walking on two legs – would be necessary for the tree-living ancestors of humans, scientists studied the movement of the only completely arboreal great ape, the Sumatran orang-utan. It appears that they use bipedalism to forage for food from small branches of tree tops, and to cross directly from tree top to tree top.

Professor Robin Crompton explains: “We found that orang-utans walking bipedally on springy branches act much like athletes running on springy tracks; they use extended postures of knee and hip to give them straighter legs. Other recent work by the team shows that orang-utans use the natural springiness of branches to save energy in movement, especially when crossing from one tree to another, and this may also be the case when they move bipedally in small branches.”

“Walking upright on two legs, gripping branches with the feet and balancing themselves by holding or touching higher branches with their hands is actually a very effective way of moving on smaller branches. It helps to explain how early human ancestors learnt to walk upright whilst living in the trees and how they would have used this way of movement when they left the trees for a life on the ground.” he added

Daniel Lieberman, a biological anthropologist from Harvard University, US, told BBC News: "I think it's a neat paper; it's always terrific when people think creatively about the origins of human bipedalism. But it's not going to be the last word.

"The big problem is - what was the selective advantage for that first hominid that stood upright? We know very little about the context in which that occurred."