Discovering the Timetree of Life

Abstract With great delight, we can say that the time has come, and that it is now. Certainly, many important details remain to be worked out, such as deep branching patterns among major taxonomic groups and the interrelationships of many species, but much of the tree of life already has taken shape (2). 7is revolution in evolution has occurred largely through advances in molecular biology over the last half century, building on a foundation laid by paleontology and comparative biology. It would not have been possible without many discoveries, progressively building on previous work, such as the structure of DNA (3), methods to sequence proteins and DNA (4–6), a technique to rapidly amplify DNA (7), and advances in statistical methods of data analysis. Some—perhaps most—of the resulting molecular phylogenies have corroborated trees based on morphology and cell biology, but many Andings were unexpected including the discoveries of archaebacteria (8) and an African clade of mammals (9) to name just two. Our current understanding of the tree of life draws from fossils, morphology, and—especially in the last two decades— many molecular phylogenies.