Pterosaurs: dragons of the air
Stage 1: falling
Falling is a long way from flight, but anything that allows survival during a fall or even gives an element of control, offers an evolutionary advantage. Thus living in a high place and falling from it, may have been a stimulus for the evolution of gliding.
Stage 2: parachuting
Slowing the descent of a fall to a velocity sufficient to allow survival is parachuting. It is achieved by increasing the surface-area-to-weight ratio during descent, which produces resistance.
Many animals parachute, including a snake that throws itself from a branch, flattens its body, coils up and reduces the speed of its fall sufficiently to fall onto a lower branch without injury.
Stage 3: gliding
Gliding is a form of flight that relies on a flight surface to control the velocity and angle of descent, with no energy used to create lift. It beats parachuting, but is essentially precision falling. Usually, the gliding surface is a membrane stiffened by some part of the skeleton, such as ribs or bones within the skin.
Stage 4: flight
The evolution of true powered flight is a tough one. Living flying animals – insects, birds and bats – are expert flyers with millions of years of evolutionary refinement behind them. And there are few gliding animals that appear to show a transition from gliding to flapping flight.
In flying animals the flight surface provides lift and thrust. For man-made machines the wings certainly provide the lift, and flaps on the wings and tail control the direction, but engines provide the thrust.
Two models of powered flight evolution have been proposed: trees down and ground up. In the former it is assumed that an arboreal animal evolves flight as an extension of gliding between trees. In the latter, it is thought that ground-dwelling animals evolved flapping flight as an extension of terrestrial locomotion, perhaps while escaping from predators or chasing prey. It is quite likely that aspects of both these scenarios were involved in the evolution of animal flight.