Extinction is the process of a coherent lineage dying out. It is a major topic in evolutionary biology, particularly fields such as paleontology that deal with the distant past. Most of the species that have ever lived are now extinct, and this has been the case for the entire history of life. In every time period, the multitude of living things that exist are descendants of a comparatively small number of organisms. Since evolution means that descendants can become distinctly different from their ancestors, the species that gave rose to extant life are themselves usually extinct.
