Tree-dwelling humans were first to start walking upright
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 onto 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 orangutan behaviour, published in the journal Science, suggests this is wrong, say 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 orangutan — the sole modern great ape that inhabits only trees.
They found that the orangutan uses bipedalism to fetch food from the small branches of treetops, and to cross directly from one tree crown to another.
Professor Robin Crompton, from the University of Liverpool, said: “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.”
