 Each node on a tree is the origin of a monophyletic group. For example, the claim that the mammals form a single, united monophyletic group just means that all and only mammals share (i.e., are descended from) a common ancestor. We have very good reasons to believe that this is true. For example, all and only the mammals have certain traits (or characters) such as mammary glands, hair, and ossicles (three bones in the middle ear). Each of these traits are homologues, i.e., their similarity is due to shared ancestry. (Homologous characters are typically contrasted with analogous characters, i.e., traits whose similarity is not explained by common ancestry, but by some other process, e.g., convergent evolution or reversal). The mammary glands in humans and in elephants are homologous because the trait has been inherited from their common ancestor. In  figure 1,  the ancestor of the mammals is located at the node with branches leading to the platypus and the rest of the mammals. Once traits emerge (as evolutionary novelties) on a branch they can be passed down (unchanged or modified) to the descendants of that lineage. Since platypuses and mice have mammary glands, but no taxa stemming away from earlier nodes do, we can infer that mammary glands evolved after the node where mammals and birds split, but prior to the mammalian node. That is one way phylogenetic trees support inferential claims in biology. If mammary glands had evolved prior to the node joining mammals and birds, then we might expect to find mammary glands on birds or lizards. Platypuses and mice are also born inside an amniotic sac, as are turkeys and lizards. However, frogs do not have an amniotic sac. This tells us that the amniotic sac evolved after the frog lineage branched off from the other tetrapods. Parallel reasoning allows us to infer the history of the evolution of bones, tetrapod limb structure, feathers, etc. These homologies form a nested hierarchy of traits just as the monophyletic groups are nested, so, for example, the mammals are a part of amniotes which themselves are a part of the tetrapods. On the other hand, while the mammals and the birds are both part of the amniotes, they do not overlap at all. And indeed, no organisms have both mammary glands and feathers.