Paleontological evidence to date the tree of life

The role of fossils in dating the tree of life has been misunderstood. Fossils can provide good minimum age estimates for branches in the tree, but maximum constraints on those ages are poorer. Current debates about which are the best fossil dates for calibration move to consideration of the most appropriate constraints on the ages of tree nodes. Because fossil-based dates are constraints, and because molecular evolution is not perfectly clock-like, analysts should use more rather than fewer dates, but there has to be a balance between many genes and few dates versus many dates and few genes. We provide hard minimum and soft maximum age constraints for 30 divergences among key genome model organisms; these should contribute to better understanding of the dating of the animal tree of life.