The evolutionary scenario that led to powered flight in birds, or rather proto-birds, has been the focus of controversy for more than a century. The enigmatic blend of reptilian and avian characters in Archaeopteryx – the oldest known bird fossil – has certainly resulted in strongly diverging interpretations among scientists
1
. Two main theories explain how flight evolved: (1) the arboreal, or trees-down, scenario; and (2) the ground-up theory. The arboreal scenario hypothesizes that flight evolved in animals that climbed trees or rocks. These animals then simply parachuted down to the ground, and increased aerodynamic adaptations (such as lift-generating surfaces on forelimbs) enhanced gliding performance, eventually leading to the evolution of powered (flapping) flight
2
. In this scenario, powered flight evolved via gliding flight, which is less complicated than flapping flight. The ground-up theory assumes that the bird ancestor, proavis, was a cursorial dinosaur (i.e. adapted for running), probably a maniraptor, that evolved distal-lifting surfaces for increased control when running and catching flying insects
1
,
3
. Subsequently, the aerofoil function developed further to allow extended jumps and eventually self-powered flight evolved.
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© 1999 Elsevier Science Ltd. Published by Elsevier Inc. All rights reserved.
