A paradigm for the origin of life

The theory that life began in an “RNA world” postulates that the first chemical system to appear on Earth with the capability to replicate itself was a set of RNA molecules. Replication of sequence information is an implicit and critical concept here, rather than mere “reproduction,” since only a system that quite faithfully reproduces the properties of its parents is capable of Darwinian evolution. The arguments in favor of the theory that life originated with RNA are both chemical and biological in nature (Joyce 1989). The chemical properties that support such a potential will be considered briefly later in connection with the template-directed oligomerization of mononucleotides. The central role of RNA in the expression of genetic information is probably the most persuasive of the biological lines of evidence. The most impressive demonstration of the evolutionary potential of RNA is the fact that selection experiments reveal a growing catalogue of new catalytic functions of RNA molecules (see Chapter 13). Once self-replicating RNA molecules appear on the scene, it becomes possible to understand how evolution could produce increasingly efficient self-replicating systems, although the origin of encoded protein synthesis still presents serious theoretical difficulties. However, the task set for this chapter is to look backward from RNA and ask how the RNA world got its start.