When a 10-kilometer-wide hunk of burning space rock slammed into what is now the Gulf of Mexico 66 million years ago, it touched off widespread destruction, wiping out more than 75 percent of life on Earth. The Chicxulub asteroid, as it is called, is best known as the dinosaur killer. But although it doomed Tyrannosaurus rex and Triceratops, the sauropods and the hadrosaurs, the asteroid actually set one lineage of dinosaurs on a path to glory: that of modern birds.