Flexi Says: Fossils provide important evidence for the theory of evolution. Fossils and the order in which fossils appear in rock layers is called the fossil record. The fossil record provides evidence for when organisms lived on Earth, how species evolved, and how some species have gone extinct. However, the formation of fossils is a rare phenomenon. Because most parts of organisms decompose rapidly following death, fossilization is an exceptionally uncommon occurrence and usually preserves only hard body parts.