Evolution by natural selection is a scientific theory, and as such cannot be proven beyond a shadow of a doubt, since there is always the possibility, however remote, that evidence could be uncovered to contradict it. However, no such evidence has ever been found, and an abundance of evidence uncovered over more than a century has supported the theory.
Gradual changes in form can be seen in the fossil record, indicating that evolution has taken place over millions of years. Many animals share structures that seem to point to a common ancestor, such as the pentadactyl limb. Similarities between different animals at the fetal stage also suggest some common ancestor from which many animals have evolved. In addition, the fact that a wide variety of animals share the same DNA structure, as well as certain structures within cells, is cited as strong evidence that species have evolved from other species.