Among the most commonly encountered ideas concerning the origin of life is the one that it involved an "RNA world" at an early stage (1). The term was coined by Gilbert (2), who also stated "The first stage of evolution proceeeds, then, by RNA molecules performing the catalytic activities necessary to assemble themselves out of a nucleotide soup." The existence of such a soup has generally been taken for granted. For example, Eigen and Schuster (3) wrote "The building blocks of polynucleotidesthe four bases, ribose and phosphate were available too under prebiotic conditions. Material was available from steadily refilling pools for the formation of polymers, among them polypeptides and polynucleotides." The experimental evidence to date, however, does not appear to support such claims.
Many problems have arisen with both the prebiotic synthesis and the stability of ribose (4-9). To avoid the need for ribose, some authors have preferred to invoke an RNA-like polymer, with a simpler or more accessible backbone, at the start of life (6, 10-16). A pre-RNA world would have come first, during which some substance of this type carried out the genetic functions later taken over by RNA. In the great majority of these theories, Watson-Crick pairing of A with U and of G with C is retained as the basis of genetic template recognition.
These suggestions still presume that the bases adenine, cytosine, guanine, and uracil were readily available on early Earth. I have argued that this presumption is not supported by the existing knowledge of the basic chemistry of these substances (4, 17). If the availability of the Watson-Crick pairs at the start of life appears implausible, then more attention must be given to theories that employ a very different replicator or no replicator at all.
To provide a firm basis for this conclusion, I have undertaken a series of reviews in which I consider in detail the chemical evidence for the availability of the Watson-Crick bases at the start of life. In a previous paper, however, I concluded that current information concerning the availability and chemical properties of adenine did not support the idea that it was used in a replicator at the start of life (17). In this publication, I wish to consider the prebiotic syntheses and the stability of cytosine.