Evidence from a wide variety of sources supports the theory of evolution of life on Earth. Some of this evidence also enables us to determine relationships among living species.

Transition fossils demonstrate the change from one taxonomic group to another, as seen in the evolution of whales from terrestrial mammals to highly evolved marine forms. The fossil record is but one way that evolutionary pathways can be deduced. Similar evidence for the history of relationships among species can be derived from a variety of sources.