Replication first: RNA world
Main article: RNA world Even the simplest members of the three modern domains of life use DNA to record their " recipes " and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance, and self-replication. The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA.They could have formed an RNA world in which there were individuals but no species , as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with.RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a si...