The popular Darwinian concept of man evolving from a hunched, knuckle-walking ape to a straight-backed two-legged runner needs radical revision, according to a study.

Far from lumbering around on four legs and gradually standing upright over millions of years, man’s ancestors could stand tall from the moment they came down from the trees.

A study into the way that apes move combined with analysis of fossils shows that man’s ancestors learnt to walk upright while still in the trees because of their love of fruit.

They inherited the ability from an ape similar to modern orangutans, which use an upright stance to grasp thin, flexible branches where fruit grows.

The finding means that scientists will have to reconsider how man evolved to walk – and will cause T-shirt producers to think about replacing the images of crouching apes incrementally standing upright to become man.

It reverses the assumption that man’s ancestors went through a stage of walking on their knuckles like chimpanzees and gorillas. Instead, chimps and gorillas were considered by the researchers to have evolved knuckle walking after separating from man’s direct ancestor.

Scientists at the Universities of Birmingham and Liverpool concluded that man’s ancestors had been walking upright in the trees for between 17 and 24 million years, rather than beginning to learn the ability four to eight million years ago when they came down from the trees as commonly claimed.

“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 study suggests the opposite,” said Professor Robin Crompton, of the University of Liverpool.

“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.

“These techniques were later used by human ancestors to allow them to adapt to walking on two feet on the ground.”

He said that the findings, made after 3,000 observations of orangutan movements, will make it harder to tell from fossil records which remains are man’s direct ancestors and which are from other apes: “If we’re right, it means you can’t rely on bipedalism to tell whether you’re looking at a human or other ape ancestor.”

The finding ties in with fossil discoveries of bipedal hominids, including Lucy, Australopithecus afarensis, and more recently Millennium Man, Orrorin tugenensis, which walked on two legs yet lived in woods.

Changes in the climate eight to 12 million years ago caused tropical forests to thin out over thousands of years and then grow back again in cycles. Man’s ancestors would, said researchers, have first come to the ground when tree cover was lost and would have walked upright between forest areas, perhaps staying on the ground to hunt for food.

The study, reported in the journal Science, showed that a common ancestor of great apes, which separated from monkeys 23 to 24 million years ago to seek fruit instead of leaves for sustenance, was the first to evolve the ability to walk upright on two legs. The Morotopithecus fossil dated 16 to 21 million years ago displayed the bone structure necessary to walk on two legs.

It stayed in the trees but would walk upright among the branches. The fruit it ate would have been at the end of thin branches too insubstantial to take its weight.

However, by grabbing several of the twigs with each hand and foot the ape, hanging in an upright position, would have found its weight supported and it was able to reach the fruit.

Observations in Sumatra revealed that this method of walking through the trees is today employed by orangutans, which spilt from humans in the ape family tree about 9 million years ago.

Dr Susannah Thorpe, of Birmingham University, spent a year with the orangutans in Sumatra. She said: “We were surprised to find that, when they use bipedalism in the branches, they keep their legs very straight. We have shown how bipedalism could originally have evolved in the original ape, probably to get to foods.”

Dr Joanne Baker, of Science, said: “This scientific paper addresses one of the most important questions on human evolution.”

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