While developing a theory to explain how species can change over time is key to the study of evolution, such a theory must be supported by evidence from the scientific study of the natural world. One of the difficulties faced by early theories, such as those of Buffon and Lamarck, was a lack of such evidence. Even Darwin's theory, which is the most widely accepted today, had to wait several decades until scientific advances and fossil evidence convinced the scientific community of its legitimacy.

Today, evidence for evolution comes from several different fields. Paleontology shows us that gradual change in organisms can be seen in the fossil record. The study of biogeography gives us an idea of how species spread to new habitats. Comparative anatomy, a technique that has been used since the very beginning of the study of evolution, allows us to judge dissimilar organisms in an effort to see whether they share a common ancestor. Most recently, the field of molecular biology has shown us that even the molecules of life change over time, and these can be used to tell just how far back in history any two organisms last shared an ancestor.