Two theories have dominated the long-running debate over how bird flight evolved. In the so-called cursorial scenario, the ability to fly emerged in terrestrial dinosaurs that raced across the ground with their arms outstretched and leaped into the air after prey or out of harm’s way, their wing feathers providing lift. The arboreal scenario, in contrast, supposes that flight arose in tree-dwelling dinosaurs that were built for gliding and started flapping their arms in order to stay aloft longer.
In 2003 a feathered dinosaur fossil came to light that was purported to elucidate the question of how flight evolved. The roughly 125-million-year-old specimen exhibited evidence of feathers on its hind limbs in addition to its forelimbs, prompting researchers to describe the crow-size animal,Microraptor gui, as a four-winged dinosaur. A startling artist’s reconstruction accompanied the description of the fossil remains, showing the bird flying with its hindlimbs spread out to the side, as if doing a split. The authors argued that the feathered hindlimbs, together with the forelimb wings, acted as an airfoil to help the animal glide. Critics begged to differ.