What are you? Obviously, you are a person with human ancestors that can be plotted on a family tree, but you have other identities as well. According to evolutionary biologists, you are a member of the species Homo sapiens and as such have ancestral species that can be plotted on the tree of life. According to microbiologists, you are a collection of cells, each of which has a cellular ancestry that goes back billions of years. A geneticist, though, will think of you primarily as a gene-replication machine and might produce a tree that reveals the history of any given gene. And finally, a physicist will give a rather different answer to the identity question: you can best be understood as a collection of atoms, each of which has a very long history. Some have been around since the Big Bang, and others are the result of nuclear fusion that took place within a star. Not only that, but most of your atoms belonged to other living things before joining you. From your atoms' point of view, then, you are just a way station on a multibillion-year-long journey.