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.

Introduction : Your multiple identities --Part I : Your deep ancestry. Your people -- You and I are related -- You have a great throwing arm -- Your place on the tree of life -- Your sex problem(s) -- The neanderthal in your family tree -- The code(s) by which you live -- Your (alien?) roots -- Part II : The cellular you. Your are complex -- Your ancestors were boring -- Your "cellmates" -- Your "boarders" -- Part III : The atomic you. Your are what you eat, ate -- Your windblown past -- Your cosmic connection -- Pulling yourself together -- Part IV : Your place in the universe. You are a gene machine -- You are (merely) part of life -- Your many afterlives -- Why are you here?