For any collection of species, given information on their attributes and a phylogeny that describes their shared hierarchy of descent, it is possible to reconstruct the characteristics of the ancestors of the species. This is an intriguing idea that offers the possibility of glimpsing the past, discovering how traits evolve, and understanding their function. Reconstructions of the ancestral states of organisms are increasingly used to investigate ancient features of life on Earth and to test ecological and evolutionary hypotheses. They are also being used to reconstruct proteins and genes that existed millions of years ago.