Since life first appeared on Earth some 3500 million years ago, species have been evolving from one another to create the diversity that is seen today. An analogy between this branching pattern of evolution and the growth of a tree has been popular since the time of C. Darwin—thus the “tree of life” metaphor. Phylogeny is the study of how species are related to one another and involves reconstructing or mapping the historical pattern of branching that best explains the patterns of similarity among organisms.