Carl (Loom) Zimmer reviews an hypothesis about the evolution of DNA. The first cells were thought to be RNA based. How did DNA develop? The hypothesis is that viruses "invented" double stranded DNA as a technique to evade the defence mechanisms of RNA based cells. (The opposite of what RNA viruses do to us.) In time fragments of this double stranded DNA took up residence permanently in cells. (This happens routinely with viruses now).
DNA has a lot of advantages over RNA as mechanism for heredity. In time the DNA encoding replaced the RNA encoding in cells. Modern life was on its way.