Several hypotheses have been advanced to account for this extinction, but in recent years some scientists have been claiming evidence that an asteroid or comet collided with the earth at the end of the Cretaceous Period. The impact may have caused so much dust and smoke to enter the atmosphere that the sun was obscured for a time, producing a pronounced cooling and interruption of vegetative growth. The result was the rapid extinction of the dinosaurs, according to the theory. Other scientists view this scenario with caution, contending that other factors forcing climatic change may have played a role in the extinction. That an important extinction did occur is agreed upon; in fact, this extinction defines the end of the Mesozoic Era. It is worth noting that extinctions are the rule in nature. More than 99 percent of all the species that ever lived are now extinct.

Paleontologists generally agree that dinosaurs and birds are closely related because they share many common anatomical features. Birds have dinosaur ancestors. In a sense, then, the birds are their true descendants.