Scientists at the University of Liverpool have found that humans’ ability to walk upright developed from ancestors foraging for food in forest tree tops and not from walking on all fours on open land.

It was traditionally thought that humans became upright walkers in a slow process which had its origins in ‘knuckle-walking’ – movement on all fours – just as chimpanzees and gorillas walk today. It was believed that this developed once human ancestors moved out of the forests into the savannahs of East Africa.

Study at the University of Liverpool, in collaboration with the University of Birmingham, into the behaviour of the orang-utan, has now suggests that knuckle-walking evolved quite recently in chimpanzees and gorillas, as a way of moving on the forest floor, whilst walking on two legs – assisted by the support of tree branches – is an older trait and evolved from tree walking. The study suggests that walking on two legs was always a feature of great-ape behaviour and human ancestors never passed through a knuckle-walking phase.

Skeletons of early human ancestors show a combination of short legs and long arms, which are 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.

“The traditional theory of human origins states that we evolved to walk upright from ancestors who walked on all fours when on the forest floor. This new study suggests the opposite. Upright walking evolved in the ancestors of all apes, including humans, as a means of foraging for food in the small branches of the tropical forests and these techniques were later used by human ancestors to allow them to adapt to walking on two feet on the ground.

“Around 15 million years ago the tropical forests which once covered East Africa began to break up, and although the forest sometimes grew back temporarily, eventually trees became separated and further apart, preventing our ape ancestors from swinging from one tree to the next. This forced them to go down to the ground in order to move between trees.

“Our ancestors made use of the way they moved through the trees to adapt to their life on the ground. Ancestors of chimps and gorillas, however, tried to maintain access to the canopy as well as the ground by developing very strong arms to climb vertically up and down tree-trunks and as a consequence became ‘top-heavy’. When they are on the ground, therefore, they move predominantly by knuckle-walking, propping themselves on their long, heavy forelimbs.”

Source: University of Liverpool