The Homo genus includes not only humans, the only extant member, but also quite a few other species of extinct hominids. The exact ancestral-descendant order is difficult to discern with certainty, but is briefly examined in the article on human evolution. What is apparent, however, is that other species existed and that these provided the foundation for humans. This is in keeping with the view that life proceeds by stages, just as one sees in an individual species (the complete metamorphosis of a butterfly) or the history of life on Earth (see geologic time scale). From a Unification Thought perspective, development of an individual, life on earth, and the human species is not by chance, but reflects the desire of God working through natural law.