Skeptics of the dinosaur-bird link say it would have been physically impossible for running dinosaurs to overcome gravity and get off the ground. It made more sense for flight to evolve from the trees down, with small, arboreal reptiles that glided from the treetops on their way to becoming full-fledged fliers. And that seemed to rule out dinosaurs, which, presumably, couldn't climb trees.

On NOVA, Mark Norell of the American Museum of Natural History is one of the proponents of the "birds-are-dinosaurs" hypothesis, which is the predominant view among most paleontologists, while Larry Martin of the University of Kansas speaks out for the minority view that birds descended from non-dinosaur tree dwellers.

Tantalizingly, Microraptor is the unexpected missing link that has reignited the debate and just might settle the issue--or at the very least deepen our understanding of the long-ago era when the ancestors of birds first took to the air.


Images: Producer Mark Davis (in background) scopes out a scene with puppeteer Fred Stinson. Davis first explored the dinosaur-bird connection in his 1991 NOVA "Case of the Flying Dinosaur."

Resembling a creature from Jurassic Park, the lappet-faced vulture offers striking visual evidence of how closely akin birds are to dinosaurs.