Darwin's theory of Common Descent says that all animals originated from an early descendent. This theory is more clearly explained through the tree of life. The very first organism within a certain type or class of organisms is at the base of the tree. From that organism, many new species branch out. These species become so different and unrelated that they are eventually new organisms. This occurs for many different animals, allowing for one descendent to produce a large number of new organisms. There are many trees of life for the different classes of organisms, including mammals, amphibians and others. Each different classification has its own tree with branching organisms that evolve from that common descendent. For example, since it is believed by some that our common ancestor was a tetrapod fish since both humans and these fish have very similar arm bones, the human and the tetrapod...