Luis Chiappe and Paul Barrett debate the origins of flight

The claim is a new twist in the old argument over how birds first got airborne. It comes from researchers in California who have studied the aerodynamics of the oldest known, primitive bird called Archeopteryx, which lived about 150 million years ago.

Many scientists believe birds evolved from dinosaurs. This means at some stage in history the ancient ancestors of birds must have evolved wings and feathers that enabled them to take off. But opinion is sharply divided over how they actually first took to the air.

Ground up or tree down

Some claim tree-dwelling dinosaurs developed the capacity for flight by gliding from branch to branch. Others think fast-running, bipedal dinosaurs gradually evolved the wings, feathers, and muscle structure necessary to get off from the ground.

Philip Burgers of the San Diego Natural History Museum, and Luis Chiappe of the Natural History Museum of Los Angeles County, ally themselves to this second hypothesis and have used the fossil records of Archaeopteryx to model how the creature might have got airborne.

The creature had an estimated running speed of two metres per second, but would have needed to achieve a minimum of six metres per second to get fully airborne.

Velocity gap

Writing in the journal Nature, the Californian scientists show how Archaeopteryx could have generated enough energy by flapping its wings during its take-off run to close the "velocity gap". Indeed, they calculate the primitive bird could have managed a runway speed of 7.8 metres per second.

"Conceptually at least our theory is very sound, " says Dr Luis Chiappe.

"When you put the aerodynamic data that we have based on extinct birds along with what we know about the origin of birds, and what we know of fossil animals and dinosaurs we have found in recent years, I think that it's quite clear that the origin of flight evolved from animals that were predominantly terrestrial."

Thrust generation

But Burgers and Chiappe say that while most discussion about the origin of flight has focused on lift generation, the importance of thrust should not be ignored.

They argue their model for Archaeopteryx can be applied to the non-flying dinosaurs in the fossil record which had feathered forelimbs and long, veined plumage. These creatures would have flapped in a similar way to Archaeopteryx purely to gain more speed along the ground.

"We wonder what were they doing with those feathered forelimbs if they were not flying," says Dr Chiappe. "I think the answer that we provided in this paper is that they were flapping their wings - they were not flying, but they were increasing their running speed."

Such creatures would have been able to catch prey or flee more easily. And their escendents probably kept going faster and faster and eventually were able to take off.

Counter view

The Chiappe and Burgers paper will fuel the ongoing arguments about the origins of flight.

Commenting on the research, Paul Barrett, who lectures in Earth Sciences at the University of Cambridge, says the study is a very interesting contribution to the debate. However, Barrett, who favours the alternative tree-down hypothesis, says he remains unconvinced.

"I still have some problems with how Archaeopteryx use their wings to get into the air," he says.

"Although Chiappe and Burgers seem to have good evidence to make Archaeopteryx run fast enough, the creatures still need the muscular effort provided by their wings to get them up off the ground.