Objections and other theories
Not all scientists agree with the birds-from-dinosaurs link. Two vocal opponents of the "Birds Are Dinosaurs" theory are Alan Feduccia, of the University of North Carolina and Larry Martin, from the University of Kansas.Feduccia contends that Archaeopteryx wasn't the ancestor of all birds, but just another of nature's many experiments. He argues that a huge evolution of birds had been going on before Archaeopteryx, and that they evolved from four-legged forest reptiles. In 1996 Feduccia investigated an intriguing bird that lived about 135 million years ago, just after Archaeopteryx. The bird, Liaoningornis, did not look like a dinosaur bird at all. It had a breastbone similar to modern birds, with massive flight muscles that enabled longer flights. It was found alongside fossils of ancient birds not unlike Archaeopteryx. Feduccia believes that birds were very widespread by that date, occupying a variety of habitats. He believes most of them died out with the dinosaurs, about 65 million years ago. The ancestors of all today's birds evolved later, he says, between 65 and 53 million years ago, independently of the dinosaurs. This is the "big bang theory" of birds. Feduccia and his fellow sceptics - it must be stressed they are in the minority - regard any similarity between birds and dinosaurs as an example of convergent evolution, by which two independent groups grow to look alike.
Martin claims that flight is most likely to have started from a tree-climbing (arboreal) ancestor but that all the proposed dinosaurian ancestors were ground-dwellers (cursorial). This is the "trees-down" versus the "ground-up" debate and the trees-downers do have a point. There is a variety of modern animals that are gliders, possibly representing a bridge between flying and non-flying animals, and all gliders are arboreal (including frogs, snakes, lizards and a variety of mammals). But the Birds-Are-Dinosaurs crowd argue that an unknown dinosaurian bird-ancestor could have been arboreal or that birds evolved flight from the ground up by chasing and leaping after insects (there are plenty of little theropods thought to have made a living by doing just that). |