A prebiotic scenario is proposed, based on the recent "domain hypothesis" model (Lahav, 1989, J. molec. Evol. 29, 475-479.), suggested for domain propagation of RNA-like molecules in a fluctuating environment. The same system is suggested now not only for the evolution of ribozymes, but also for the evolution of directed peptide synthesis, as follows:
Short, self-structured strands (termed prebioectons), each possessing a templatable domain which is chargeable by an amino acid, are the predecessors of tRNA (proto-tRNA). Complementary domains are formed on these prebioectons during an environmental cycle such as wetting-drying, followed by their dissociation from their template domain and ligation, to form the predecessor of mRNA (proto-mRNA).
The evolution of directed peptide synthesis is suggested to be based on the ability of the charged prebioectons to attach preferentially to their complementary domains on the proto-mRNA. Two stages of this process are envisioned, namely: (a) Template-directed, random peptide synthesis taking place when non-specifically-charged prebioectons are sequentially attached each to its complementary domain on the proto-mRNA, followed by peptide bond formation. (b) Template-and-sequencedirected peptide synthesis, which can be realized after the "invention" of a catalytic molecule capable of specifically charging a proto-tRNA by an amino acid; this is the crucial evolutionary stage, where a crude genetic code becomes functional.
Gradually, catalytic peptides and ribozymes are selected for their functions and evolve, while being encoded in the primitive "memory" of the emerging system. Thus, rather than the RNA monopoly postulated by the RNA World hypothesis, an early co-evolution of primitive enzymes and ribozymes is suggested. The evolution of a more sophisticated translation machinery is suggested to consist of a third stage, namely, the evolution of the ribosome. This latter stage may correspond to the "breakthrough organism" suggested by