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There are currently more than 9600 species of birds. They appear to be a relatively recent arrival in our biosphere. The fossil record has been elusive because birds have light, hollow bones that rapidly decompose.

Insects had the skies to themselves for over 100 million years. Then, several hundred million years ago, the huge Pterosaurs took over. These were flying dinosaurs with wings of skin. About 150 million years ago, they were joined by much more aerodynamic feathered creatures, the avians, who eventually replaced them.

The oldest known bird fossil is the Archaeopteryx lithographica, about 150 million years old, discovered in 1861, soon after the publication of Darwin's Origin of Species. This crow-sized creature had wings and feathers, long legs and three toes tipped by claws. Its jawbone and teeth resembled those of a small dinosaur, and its extended spine formed a tail.

This primal bird is thought to have evolved from theropod dinosaurs, which scurried about in roadrunner fashion. They had hollow bones, a wishbone, a backward-pointing pelvis, and a three-toed foot. In the course of their evolution, the forelimbs and hands became longer, and they could whip their hands forward in a swift snatching motion similar to the movement of the wing of a bird in flight.

Recent fossil evidence suggests that feathers developed in theropods before the appearance of birds, perhaps as insulation or as an impressive display for attracting mates. Scientists are divided whether flight began when dinosaurs leaped upwards, giving themselbes additional lift by flapping their feathery forelimbs, or when they began to glide from tree to tree, learning to control their trajectory. The evolution of the alula, a tuft of feathers attached to the thumb, allowed more precision for takeoffs, landings, and flying at low speed.

Not all scientists are convinced that birds evolved from theropods, and are suggesting other possible ancestors, such as the flying Pterosaurs and some types of four-legged forest reptiles. The discovery of the more modern-looking fossil bird Liaoningornis, from 135 million years ago, has raised the possibility that birds had evolved much earlier than believed. Alan Feduccia of the University of North Carolina believes that birds evolved independently of dinosaurs, that most of them died out with the dinosaurs about 65 million years ago, and that the ancestors of all of today's birds evolved later, between 65 and 53 million years ago. The recent discovery of two

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