One of the most famous of all the ancient animals were the dinosaurs—creatures that only lived on land (so far, no dinosaurs are known to have lived in the air or water)—first appearing in the early Triassic period (about 220 million years ago) and all but disappearing at the end of the Cretaceous period (about sixty-five million years ago). It is thought that modern humans (Homo sapiens sapiens) appeared only about 25,000 years ago. Although we know movies that show humans and dinosaurs existing together are only Hollywood fantasies, one caveat remains: Many scientists believe that modern birds are the ancestors of the dinosaurs—and thus, in some ways, we can say that the dinosaurs never truly became extinct! (For more about dinosaur extinctions, see the chapter “Heredity, Natural Selection, and Evolution.”)