Prepare for Takeoff
The forces of evolution are mighty indeed, having sculpted flora fauna as diverse as magnolia trees and algae, dust mites and cheetahs, through natural selection, also known as survival of the fittest. But even the most ardent evolutionists do not claim that natural Selection has access to a psychic hot line: it cannot foretell what might be a really nifty trait and then start assembling it from rudimentary structures before finally creating the full-blown, useful characteristic. How then, do complex traits requiring multiple stages for their development arise? Creationists have had a field day with this paradox. Surely, they scoff, evolution could not produce structures as complex as, say, eyes or wings when the rudimentary forms would not have enabled the creatures that owned them to see or fly or otherwise become more "fit."
Scientists may have an explanation. In a paper in the current issue of the journal Nature, Argentine paleontologists Fernando Novas and Pablo Puerta announce the discovery of a missing link between birds and their dinosaur ancestors. Unenlagia (Mapuehe Indian for "half bird") comahuensi ("of northwest Patagonia"), they report, boasts several features that "are more birdlike than in any other [nonbird] yet discovered." Yet Unenlagia, 90 million years old and found in the sandstone of an ancient riverbed in Patagonia, was no bird. It was, rather, "a true mosaic," says paleontologist Lawrence Witmer of Ohio University. Much too massive to be lofted by its puny arms, it stayed forever grounded. Unlike in other dinosaurs, however, the structure of its shoulder would have permitted the fore-limb to flap. And that, says Novas, is "a prerequisite for powered flight."
Natural, selection could not know that this kind of arm might eventually turn into a wing. Why, then, did Unenlagia evolve a versatile shoulder "pre-adapted" for flight? The joint had to be good for something right away, not in the unseeable future. Noyas suggests that Unenlagia used its arms, to catch and manipulate prey, control its posture while running and provide thrust while attacking. This suggests that flight did not evolve from the top down, as climbing animals glided to earth from trees, but rather from the bottom 'up, when one mighty, running leap sent an ancient creature airborne millions of years ago.