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, for example, you are a member of the species Homo sapiens. To a microbiologist, though, you are a collection of cells, each of which has its own cellular ancestry. A geneticist might point out that besides these identities, you can be understood as a gene-replication machine, which can be plotted on a "genetic tree." Finally a physicist will give a rather different answer to the identity question: you can 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. You: A Natural History o
