Consider the following argument. The translation mechanism is complex, comprising on the order of 100 large molecular components. It is also highly accurate in its functioning. In making proteins of the size common today (chains of from 100 to 500 amino acids), obtaining a flawless product 90 percent of the time or more requires an error rate of no more than a few parts in 10,000. Moreover, this accuracy must be attained by a mechanism of molecular dimensions. For such a mechanism to have evolved in a single step is clearly impossible. The primitive version of the mechanism must have been far simpler, smaller and less accurate.
Imprecision in translation would have required the synthesis of proteins that were smaller, and therefore less specific in their action, than proteins are to day. (Otherwise the probability of error in making a protein strand would have been too great.) Among the smaller and less specific proteins would have been the enzymes required to process genetic information. If those enzymes were less precise than today's versions are, the cell's mutation rate must necessarily have been higher and the size of its genome correspondingly smaller. The translation process is the link between genotype and phenotype, between information and its expression; as the process evolved to become more precise, the cell itself necessarily passed through a corresponding series of evolutionary refinements. It evolved from an entity having simple properties, imprecise and general functions and a rather small complement of genes to an entity that functioned with many highly specific enzymes and a complex, precise genetic apparatus. To emphasize the primitive genetic and translational mechanisms of the earlier, simpler cells, I call them progenotes.