Dino becomes bird

According to a US scientist, flight may have evolved in two-legged dinosaurs that flapped their feathered fore-limbs to climb slopes.

They eventually developed true wings and became flying birds, says Kenneth Dial of the University of Montana.

The idea is based on a study of the habits of modern flightless birds, which beat their wings to scurry up hills and get away from predators.

It turns out that the physics of this sort of flapping motion is different from that of aerial flight.

Foot traction

Professor Dial says it helps push the birds' feet against the slope, thus improving traction - in the same way that spoilers work on a racing car.

The work adds a new dimension to the whole debate on how flight evolved Dr Angela Milner

He came to this conclusion by studying partridges running up hills and measuring their speed.

Even chicks with downy fluff were better at getting up steep slopes than those whose flight feathers had been trimmed or removed.

By modifying these wing movements, birds or their ancestors - the dinosaurs - may have been able to launch themselves into the air.

Fossils show that some dinosaurs had feathered fore-limbs but were unable to fly - something that has puzzled palaeontologists.

Professor Dial studied the partridge

"The big dilemma has been, 'How do you explain the partial wing?'," says Professor Dial.

"It turns out the proto-wings - precursors to wings birds have today - actually acted more like a spoiler on the back of a race car to keep the animal sure-footed even while climbing up nearly vertical surfaces."

Rival theories

Professor Dial believes that what he calls wing-assisted incline running was first seen in prehistoric times.

But the idea is likely to ruffle a few feathers. There has been heated debate about how dinosaurs learned to fly.

One camp believes ground-dwellers grew feathers that helped them run faster and eventually become airborne.

Feathered or furry dinosaurs are a puzzle to scientists (Photo: The Geological Museum of China)

A more recent school of thought favours the idea that flight arose from the tree down - as small meat-eating dinosaurs leapt from branch-to-branch in the canopies.

Dr Angela Milner, a dinosaur expert at London's Natural History Museum, says the latest theory is a "third way".

"The work adds a new dimension to the whole debate on how flight evolved," she told BBC News Online.

"A predator escape mechanism using wing-assisted incline running fits with what we see in the fossils."

The research is published in the journal Science. Professor Dial's theory is featured in an on-going exhibition, Dino Birds, the feathered dinosaurs of China, at London's Natural History Museum.