Criticism and Discussion of the RNA World

The RNA world has its critics as well as its proponents. A third group is formed by the soft critics , who warn of too much optimism one of these is Leslie Orgel, an expert on the problems involved, who made it quite clear that the RNA world is a hypothesis and nothing more. 

Shapiro (2000) draws our attention to another type of problem the homopolymer problem in biogenesis. Many biogenesis hypotheses presuppose the spontaneous formation of polymeric organic replicators which were formed from a mixture of inorganic compounds these replicators consist of subunits of a known chemical species. Their structure involves a backbone which is bonded to information-transmitting residues. As we shall discuss in Sect. 6.7, not only RNA and DNA, but also proteins and peptide nucleic acids (PNA) have been suggested as possible information transmitters. Shapiro rightly considers that until now, not 

In prebiotic simulation experiments, the problems referred to were avoided by keeping competing substances out of the polymerisation mixtures, i.e., the conditions were idealized. The formation of an information-transmitting homopolymer from a complex mixture of substances cannot be excluded, but it is extremely unlikely  

How could such difficulties be avoided Shapiro lists minerals which could have served either as the first replicators or as highly selective polymerases. He also considers a further possibility life may have begun as a metabolic network of reactions which involved monomers the replicators may have evolved in a later evolutionary phase. The misgivings mentioned, and the open questions referred to in earlier chapters, indicate that a de novo synthesis of RNA under the conditions present on the young Earth was almost impossible. Thus, models were and are being looked for which could bypass as many as possible of the problems referred to. 

Shapiro remained true to his role of critical observer at the ISSOL conference in 2002 in Mexico there he expressed the opinion that the beginnings of life did not involve polymers at all (be they nucleic acids or proteins, or their hypothetical precursors pre-nucleic acids or pre-proteins), but initially involved interactions between monomers, the polymeric biomolecules being formed in later phases of molecular evolution. In this monomer world , reactions were supported by small biocatalysts (Shapiro, 2002). 

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