How did flight evolve in birds? While the origin remains a debate among scholars, evolutionary biologist Robert Dudley of the University of California, Berkeley says that the evidence points to the arboreal hypothesis. This hypothesis suggests that the first fliers were gravitationally assisted by falling from trees, as opposed to running up inclines and working against gravity.

"The competing hypothesis is what’s called the arboreal hypothesis for flight origins, whereby animals are falling out of trees, jumping out of trees, being chased out of trees or cliffs or whatever the context and are accelerated by gravity. Gravity’s a very important and powerful force. So it’s much easier to work with gravity than against gravity and to come up to speeds at which aerodynamic forces become important."

Dudley adds that recent fossil discoveries are consistent with the arboreal hypothesis.

"They had aerodynamic structures on the hind legs as well as on the front legs and they also had long, flattened tails. All of that screams aerodynamic function and falsifies running hypotheses for the origins of flight."