Genes in the Making
BETWEEN THE APPEARANCE of the first peptide, haphazardly assembled by interacting RNA molecules, and the inauguration of a fully integrated translation apparatus, complete with an unambiguous genetic code and a reliable set of functional RNAs and enzymes for enforcing the code, emerging life went through a long succession of tiny evolutionary jumps separated by more or less extended periods of random groping. An image that comes to mind is that of a surface of water slowly spreading over an irregular terrain. Fingerlets extend here and there, as local attractive forces battle with surface tension, until a minor breakthrough suddenly occurs in a given direction and all the pressure momentarily concentrates on a single rivulet. After that, groping soon resumes, sending out feelers until the next breakthrough.
Evolution did its groping by means of chance mutations resulting in the synthesis of altered peptide molecules; it achieved its directional jumps through the occasional emergence of an altered peptide product that conferred a selective advantage on the protocell concerned. As with spreading water, the outcome of such a process depends on the structure of the terrain. Without better knowledge of the prebiotic terrain, we cannot reconstruct this phase of evolution in any detail, but we can guess its eventual outcome with some measure of confidence. By the time this phase came to an end, most, if not all, of the twenty proteinogenic amino acids had been recruited for peptide synthesis, the genetic code had reached its present structure, except for possible minor adjustments, and translation of RNA messages into peptides was essentially unequivocal and reliable. What, then, were the next steps?
Most likely, at this stage, genes still consisted of RNA. Those early RNA genes were short, no more than seventy to one hundred nucleotides long (the length of