Sim­ple life appeared on Earth almost as soon as the plan­et cooled suf­fi­cient­ly to sup­port water-based organ­isms. To be detectable from a dis­tance, how­ev­er, life has to evolve to the point where it dom­i­nates the plan­e­tary sur­face chem­istry and has sig­nif­i­cant­ly changed the atmos­phere, cre­at­ing chem­i­cal “biosig­na­tures” that can in prin­ci­ple be detect­ed remotely.