To explain evolution and how different forms of life are related, paleontologists often use a tree as a model. Each branch shows where a different life form evolved from the earlier life form. While paleontologists would like to place every species that ever lived in this model, the "tree of life" contains gaps.

A gap can only be filled in when an important new transitional fossil is found—a fossil that has characteristics of an ancestor (an earlier species) as well as those of a descendant (a species that come after it).