The discovery of Microraptor and other small, exquisitely preserved feathered dinosaurs from China also helps to settle a century-old controversy over whether avian flight began in trees (trees-down theory) or on the ground (ground-up theory). These fossils show various transitional stages?from wingless, tree-dwelling theropod dinosaurs to fully winged, active flyers, Chatterjee said.
The central theme of the trees-down theory is that gravity was the source of energy: a small climbing dinosaur first parachuted down, then began to stay aloft longer by gliding, and finally acquired powered flight. As those abilities developed, feathers became larger and more specialized, providing greater lift and thrust. The Chinese feathered dinosaurs show these transitional stages of flight.
In contrast, the ground-up theory has a theropod struggling toward flight directly from the ground, against gravity, without any gliding stage. Such long feathers around the feet would make it hard for Microraptor to run on the ground, says Chatterjee, supporting the idea that it was a tree dweller, thus reinforcing the trees-down theory.