The answer to the question posed in the title of this communication depends on what is meant by ‘protein’. If we restrict this term to polypeptides assembled on ribosomes from a set of twenty tRNA-linked L-amino acids according to an mRNA sequence, then we may safely assume that RNA preceded protein in the development of life, since all the main components of the protein-synthesizing machinery are RNA molecules. Such is the view embodied in the now widely accepted model of an ‘RNA world’ (Crick, 1968; Gilbert, 1986). On the other hand, if we enlarge the definition of protein to include any kind of polypeptide, then there is a good possibility that protein may have preceded RNA, since amino acids were probably among the most abundant biogenic building blocks available on the prebiotic Earth (Miller, 1992), and their spontaneous polymerization, although not readily accounted for in an aqueous medium, is at least easier to visualize than the spontaneous assembly of RNA molecules. Let us consider first proteins stricto sensu. How did such molecules come into being?

According to the most reasonable hypothesis (Orgel, 1989; de Duve, 1991; 1995), primary interactions between amino acids and RNA molecules led to the progressive assembly of a primitive, as yet uninformed peptidesynthesizing machinery. Subsequent evolution of this system saw the gradual development of translation and of the genetic code. This long evolutionary process must have been driven first by the enhanced replicatability/stability of the RNA molecules involved. Later, as translation fidelity improved, advantageous properties of the synthesized peptides became increasingly important. Eventually, the properties of the peptides dominated the evolutionary process.