The replication of DNA is the process by means of which genetic information is passed from one generation to the next in all living organisms. It is widely believed that a similar process must have become important early in the development of life on the Earth, either before or at the same time as the evolution of protein synthesis. The evolution of any life-form based on familiar chemistry would seem to require the early appearance of some type of residue-by-residue replication, although it might be very different in detail from the one we know.