Cytosine is a pyrimidine that is one of five organic bases found in the nucleic acids of cells. In both DNA and RNA, cytosine pairs with guanine. Controversy surrounds its prebiotic origin, together with that of uracil. In 1999, the American chemist Robert Shapiro (1935–2011) pointed out that:1
No reactions have been described thus far that would produce cytosine, even in a specialized local setting, at a rate sufficient to compensate for the decomposition.
This claim is opposed, however, by Stanley Miller and his colleagues who believe that cytosine and uracil could have been manufactured efficiently on the young earth in chemically concentrated environments around the edges of lagoons.
1. Shapiro, R. "Prebiotic Cytosine Synthesis: A Critical Analysis and Implications for the Origin of Life," Proceedings of the National Academy of Science, 96 (8), 4396 (1999).