Humans ‘learned to walk in trees’

Humans learned to walk upright in the trees, not on the open land, experts have said.

The new theory marks a U-turn in scientific thinking. Previously it was assumed humans only began to stand upright after moving out of the forests on to the wide open savannahs of East Africa.

Moving on two legs was thought to have evolved slowly from the all-fours “knuckle-walking” displayed by chimpanzees and gorillas today. But a study of orang-utan behaviour, published in the journal Science, suggests this is wrong, according to a British team of scientists from Liverpool and Birmingham universities.

They believe knuckle-walking evolved only recently as a way of getting around the forest floor. Walking on two legs, assisted by the support of branches, appeared to be an older trait which evolved from foraging for food in tree tops.

According to the new theory, bipedalism was always a feature of great ape behaviour. Humans inherited it without ever passing through a knuckle-walking phase. Skeletons of early human ancestors show a combination of short legs and long arms, which are adaptations for tree-living.

To understand why walking on two legs might have evolved in tree-living apes, the scientists turned to the Sumatran orang-utan - the sole modern great ape that only inhabits trees. They found that the orang-utan uses bipedalism to fetch food from the small branches of tree tops, and to cross directly from one tree crown to another.

Professor Robin Crompton, from the University of Liverpool, said: “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 work by the team showed that orang-utans use the natural springiness of branches to save energy, especially when crossing from one tree to another on two legs. “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,” added Prof Crompton.

“It helps to explain how early human ancestors learned 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.”

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