1. either moving rapidly across the surface and then lifting into the air - the cursorial explanation
2. or descending through the air from a tree or another high vantage point - the arboreal explanation.

the first windsurfers!

The evidence points to the evolution of birds from land-based two-legged carnivorous dinosaurs (theropods) some 150 million years ago.

Most non-avian dinosaurs were large or medium sized animals, but recently the fossil remains of a much smaller dinosaur - Microraptor - have been described by Xu, Zhou, and Wang (2000). This species has several features that may place it on the evolutionary pathway to birds. We can be more confident about Archaeopteryx as a direct ancestor of modern birds. Seven fossilised specimens of Archaeopteryx have been found in Germany. They have feathered wings and tails, but there is still some doubt about how capable they were of flapping wing flight given the relatively modest development of the flight muscles and unspecialised wrist bones (Speakman and Thomson, 1994). Archaeopteryx lacked the supracoracoid muscle of modern birds, but the pectoral muscles may have been adequate for powered flight. Archaeopteryx had a full set of teeth and a long bony tail, unlike modern birds.

Recently, fossils of possibly feathered dinosaurs have been found in China and Madagascar (Unwin, 1998). (For a discussion of the possible evolution of feathers, click here.) A recent finding of the fossil remains of Apsaravis has given insight into the emergence of the flight mechanism, particularly the mechanism by which the hand region of the upper limb is automatically extended as required in the transition from upstroke to downstroke (Norell and Clarke, 2001).

Birds have flight adaptations that are similar to those of pterosaurs: light, hollow bones, keeled sternum for attachment of flight muscles, and short and stout humeri. A difference is that in birds the clavicles are fused to form the furcula (wishbone) which helps to stabilise the shoulder during the wing beat. Another difference is that much of the bird's wing is supported by the radius, ulna, and carpometacarpus rather than by an elongated 4th digit as was the case in the pterosaurs.

Modern birds grow rapidly, reaching full size in about 1 year and starting to fly when growth is nearly completed. Dinosaurs also grew to adult size relatively quickly, especially compared with the growth of lizards which tends to be slower and more prolonged. It has been proposed that a key feature of bird evolution has been acceleration and curtailment of the growth phase inherited from their dinosaur ancestry (Chinsamy and Elzanowski, 2001; Padian, Ricqles, and Horner, 2001; Erickson, Rogers, and Yerby, 2001).

Generally the arboreal hypothesis for the origin of flight in birds has been the more popular, but a recent paper by Burgers and Chiappe (1999) suggests that the apparent gap between the running speed of Archaeopteryx (2 metres per second) and required take-off speed (6 metres per second) could have been made up by the thrust produced by flapping its wings - a cursorial origin of flight. They point out that the structures and functions necessary for wing-generated thrust were already present in the flightless ancestors of birds.

Both the arboreal and cursorial hypotheses for the origin of bird flight have explanatory gaps. For example, gliding tree-dwellers of the present day such as the flying squirrels and lemurs make no effort to prolong their flight by flapping their appendages, raising the question of why tree-living ancestors of the birds may have done so. With regard to the cursorial hypothesis, it is necessary to suggest an explanation as to why natural selection would have favoured the development of protowings in running ancestors of birds. Dial observed that some predominantly ground-living species of extant birds routinely run up tree-trunks and other inclined surfaces to reach safety, and beat their wings to improve traction as they do so (work described by Wong, 2002). This wing-flapping behaviour was also observed in juveniles of these species even before they were able to fly. Thus Dial proposes that the use of wing-beating during inclined running might have provided the necessary incentive for the evolution of wings in ancestors of the birds.

Chiappe, L.M. (1995) The first 85 million years of avian evolution. Nature, 378, 349-355.

Chinsamy, A., and Elzanowski, A. (2001) Evolution of growth pattern in birds. Nature, 412, 402-403 (26 Jul).

Erickson, G.M., Rogers, K.C., and Yerby, S.A. (2001) Dinosaurian growth patterns and rapid avian growth rates. Nature, 412, 429-432 (26 Jul).

Norell, M.A., and Clarke, J.A. (2001) Fossil that fills a critical gap in avian evolution. Nature, 409, 181-184.

Padian, K., Ricqles, A.J. de, and Horner, J.R (2001) Dinosaurian growth rates and bird origins. Nature, 412, 405-408 (26 Jul).

Speakman, J.R., and Thomson, S.C. (1994) Flight capabilities of Archaeopteryx. Nature, 370, 514.

Unwin, D.M. (1998) Feathers, filaments, and theropod dinosaurs. Nature, 391, 119-120.

Wong, K. (2002) Taking wing. Scientific American, (January), 14-15.

Xu, X., Zhou, Z., and Wang, X. (2000) The smallest known theropod dinosaur. Nature, 408, 705-708.

It is clear that pterosaurs were able to fly, but to what extent is still not clear. The hindlimbs formed an important part of the flight apparatus in some species, supporting the flight membranes and probably assisting during tight maneuvres and braking (Unwin and Bakhurina, 1994).