Flight in the animal kingdom takes place when an animal flaps its wings and thus creates aerodynamic forces that enable it to soar through the air (Rayner 1968).[i] The evolution of vertebrate flight is in many respects a black box. Studying vertebrate flight after the fact requires scientists to make educated guesses about how and why flight developed. The only data with which modern scientists have to work is the fossil record, so scientists must fill in the gaps by hypothesizing how today's flying vertebrates evolved from their ancestors through a series of adaptations (UCMP Berkeley [Date unknown]).[ii] This explains why there are still unanswered questions about how flight developed and why opposing viewpoints proliferate.
The most widely accepted theories for explaining the evolution of flight include the arboreal scenario and the cursorial scenario (Hedenström 2002).[iii] The arboreal theory posits that the first flying animals developed flight from the top down by climbing on trees or rocks that they could glide down from, while the cursorial theory asserts that the birds' ancestors started out by running, then progressed to leaping into the air to catch insects, and finally to actual flight (Hedenström 2002). The two theories differ in terms of the scientific principles they are based on, with arboreal theories operating according to the force of gravity and cursorial theories depending upon the animal's ability to overcome gravity (Taylor 1999).[iv] As one writer phrases it, "You might just as well try to invent a sport somewhere between skydiving and the triple jump" (Taylor 1999). Aerodynamic modeling has been used to support both theories, though, and both theories exhibit good logic, although they are based on differing premises and largely in disagreement with each other (Hedenström 2002).
The only thing the two theories do agree on is that the reason wings evolved was to keep an an...