All species are believed to have come from a common descent. Therefore, humans are thought to have evolved from an ancestral species that can be directly related to all forms of life. Evidence of a common ancestor is even evident among fish and humans. For instance, modern day animals have vertebrae, being composed of a bony skull and backbone. The first organisms on earth to have a bone-like structure are fish. The similarities between the bones of fish and humans can be illustrated through homologous structures, which are bone structures that have many similar anatomical commonalities between organisms. This is direct proof of the common ancestor. The most common homologous structure is the arrangement of the limbs of many animals, like birds,
