National Geographic : 2011 Feb
TAKING WING Scientists now know that feathers evolved long before they were used for flight. How did the transition to powered flight take place? Luis Chiappe, an expert on early birds at the Natural History Museum of Los Angeles County, suggests that flight likely occurred as a by-product of arm flapping in ground-dwelling dinosaurs, as the predecessors of birds used their feathered arms to increase their running speed or balance themselves as they made fast turns. Over generations, muscles used for such actions evolved to become stronger, bodies smaller and lighter, and feathers longer and more aerodynamic. Eventually the flapping of feathery arms evolved into the repeti- tive strokes of wings. "Even Archaeopteryx, which is often cast as a poor flier, could have taken off from the ground," says Chiappe. It is also possible that before powered flight, dinosaurs went through a gliding stage, taking advantage of the greater lift associated with the speeds they could achieve by dropping from trees or launching from cliffs. But the old "ground up versus trees down" arguments are likely too simplistic. "The impor- tant question is, how did the ancestors of birds employ their hind limbs and forelimbs to negotiate 3-D environments?" says Ken Dial, of the flight lab at the University of Montana-Missoula. "I would argue that everything we need to know about the origin of the flight stroke is right here in front of us in living birds." Sinosauropteryx Colorful banding in the tail feathers suggests they were for camou- flage or communication. Microraptor This dromaeosaur's feath- ered legs may have acted like airfoils, providing lift for gliding from trees. Caudipteryx Broad feathers in running dinosaurs may have pro- vided bursts of speed or been simply for display. FEATHER EXPERIMENTS The fossils of feathered nonavian dinosaurs (the three at left) and early birds (at right) from northeast China's Liaoning Province are all about 125 million years old, but they show different approaches to feathers and flight. Because they lived at the same time, sorting out stages in the evolution of flight is difficult. Jeholornis This early bird was likely a powerful flier. Its long tail could have been used as a rudder or an airfoil.