The period of time during which life emerged on the primitive Earth has been narrowed thanks to the geological record and to a better understanding of the primitive terrestrial atmosphere and hydrosphere. Terrestrial life probably appeared between 4.0 and 3.8 billion years ago. The inventory of organic molecules of prebiotic interest that were likely to be present at that time is satisfactory except for the RNA sugar component, ribose. Delivery of extraterrestrial organic molecules is of special interest since it is a process that still occurs today. The existence of such an import process on the early Earth required only an atmosphere to decelerate the particles. Such an atmosphere existed 3.8 billion years ago, as illustrated by, for instance, the Greenland sediments.

Among these organic molecules processed by liquid water, some began to transfer their molecular information and to evolve by making a few accidental transfer errors. For the sake of simplicity – for chemists, prebiotic chemistry was simple – one is tempted to assume that the chemical information and the transfer machinery were provided by the same molecules. Self-replicating RNA molecules fulfill these requirements. However, RNA molecules are not really simple, and whether they started life on Earth is still questionable. Self-replicating RNAs are, by definition, autocatalytic molecules that transfer their linear sequence information by an accurate residue-by-residue copying process, thanks to the template chemistry of complementary strands. Other examples of short autocatalytic molecules capable of making more of themselves by themselves are known in chemistry.