9 An RNA-Amino Acid Affinity

The origin of the genetic code is obscure, but this event draws continuing comment because it was a critical early informational innovation. How-ever, the first such evolutionary innovation was the appearance of replication. Reasonably accurate replication made possible the transmis-sion of information. Consequently, selection among genetic alternatives, and therefore Darwinian evolution, became possible. Before replication, history was determined by the most probable simple chemical events. Afterward, at an increasing tempo, first replication itself, then other protobiological qualities, could be refined by selection. The previously improbable processes and molecules that characterize biology could emerge from the vast manifold of the possible, ultimately dominating the planet. Among these more complex, accelerating outcomes was the ge-netic code, allowing early replicators access to a versatile new class of peptide catalysts. The molecules that participated in the codes origin can be loosely specified. The most appealing hypothesis is that the informational molecule was an ancient analog of modern RNA (Gilbert 1986). An evolutionary precursor that resembled RNA, but was more resistant to the disruptive effects of chemically heterogeneous monomers (Joyce et al. 1987), may be plausible. However, it seems likely that the first pep-tidyl transferase was RNA-like. Several highly deproteinized modern 50S ribosomal RNAs can carry out a model peptide synthesis reaction at a rate virtually the same as the ribosomal particle (Noller et al. 1992). An essential activity like the peptidyl transferase is strongly constrained by the principle of continuity (Orgel 1968). Thus, the present peptidyl trans-ferase has evolved from the primordial one by transformations that do not disrupt catalysis. Assuming that the present-day catalyst is ribosomal RNA, not the 5 % peptide that survives protease and protein denaturants (Noller et al. 1992), it is likely that RNA at the time of the appearance of