DNA is the universal data storage molecule across all cellular life. However, the transition from RNA to DNA-based life may have occurred multiple times in evolutionary history, and had spread laterally since. In apparent agreement with such processes, thymidine which is unique to DNA can be synthesized by two radically different enzymes that have no similarity in sequence or structure. However, it is also possible that one functional analog preceded the other. In this chapter we review the advantages of a transition to DNA and explore the differences between the different thymidylate synthase families. We show how thymidylate synthases have been frequently transferred in evolution, both in viruses and unicellular organisms, and the adaptive potential of their acquisition. We show that the ThyX family is likely to be the more ancient thymidylate synthase family and provide explanations as to why is persists today and is frequently transferred.