A NEW ARTICLE in Nature casts light on the evolution of flight. We know that some dinosaurs had feathers, and we know that Archaeopteryx descended from the dinosaurs and retains primitive features that would have been characteristic of the first birds. However, Archaeopteryx is capable of powered flight, although not as adept at flight as later birds. The question is how the ancestors of Archaeopteryx and other early birds made the transition to flight. The issue has largely been dominated by a dichotomy between the arborial (trees down) and cursorial (ground up) models.
According to the arborial model, early birds evolved in the trees, climbing about and then gliding from one resting point to another like a flying squirrel. The hitch with this model for me has been that gliding and flying are two very different things. It’s easy to see how an organism could start out flying and pick up gliding afterwards, but how to go from gliding to flying? According to the competing cursorial model, proto-birds started out as ground-dwelling animals that used their feathered forelimbs perhaps first to trap prey, and then later to assist in ground running by flapping the forelimbs to create lift.
I read about a new hypothesis only a few months ago in the excellent Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, which everyone should immediately go out and buy. This hypothesis is based upon the observation of ground-dwelling birds that use wing-assisted incline running (WAIR). This type of locomotion is not often seen, usually occurring in very short bursts when a bird attempts to escape a predator. By rapid flapping of the wings, the bird can create enough traction to run up a vertical surface.
Kenneth Dial has been studying WAIR for several years. In the current study he tested chukar partridges (and them’s good eatin’!) from first-day hatchlings to adult birds. He found that even newly hatched birds will climb ramps using their wings to paddle along and will leap off a drop flapping the wings to (poorly) control descent. Older birds become more adept at climbing in this manner and controlling descent on the other side of the ramp, until adult birds are able to climb a vertical surface and take off into powered flight.
Dial found the the orientation of the wingbeat remains constant from day 8 to adulthood and is different from that used in flight, and proposes that this wingbeat used in WAIR is a fundamental wing-stroke that predates and is ancestral to flight.
Based on our results, we put forth an ontogenetic-transitional wing (OTW) hypothesis for the origin of flight. The hypothesis posits that the transitional stages leading to the evolution of avian flight correspond both behaviourally and morphologically to the transitional stages observed in ontogenetic forms. Specifically, from flightless hatchlings to flight-capable juveniles, many ground birds express a ‘transitional wing’ during development that is representative of evolutionary transitional forms. Our experimental observations reveal that birds move their ‘proto-wings’, and their fully developed wings, through a stereotypic or fundamental kinematic pathway so that they may flap-run over obstacles8, 9, 10, 11, control descending flight24 and ultimately perform level flapping flight (Fig. 1). The OTW hypothesis provides a simple adaptive argument for the evolution of flight and can be tested and observed in extant fledglings. This hypothesis differs from other published accounts in that it is flap-based (in contrast to requiring a gliding precursor), involves an aerodynamically functional proto-wing11, incorporates both the simultaneous and independent use of legs and wings8, 9, 10 and assumes that a fundamental wing-stroke (described herein) was established for aerodynamic function early in the bipedal ancestry leading to birds. Such an evolutionary pathway provides a parsimonious explanation for numerous non-avian theropod morphologies (for example, semilunate carpal, delto-pectoral crest, furcula, proto-wings, symmetrically vaned feathers, long bipedal hindlimbs, etc.) that have not been discussed in a synthetic context.
Since WAIR has utility in 8 day old birds that are incapable of flight, Dial proposes that WAIR would have been useful in proto-birds with similar proto-wings, incapable of flight but capable of using WAIR for rapid escapes to elevated surfaces. As the proto-birds evolved to perform WAIR more efficiently, the adaptations acquired would also enable the proto-bird to make its first attempts at powered flight, starting a new round of selection and resulting ultimately (so far!) in modern birds.
Dial, Kenneth P.; Jackson, Brandon E.; Segre, P. “A fundamental avian wing-stroke provides a new perspective on the evolution of flight.” Nature advance online publication, 23 January 2008 (doi:10.1038/nature06517)