<P> If the RNA world existed, it was probably followed by an age characterized by the evolution of ribonucleoproteins (RNP world), which in turn ushered in the era of DNA and longer proteins . The reason why DNA became the predominant storage molecule may be because it is more stable and durable than RNA . Protein enzymes may have come to replace RNA - based ribozymes as biocatalysts because their greater abundance and diversity of monomers makes them more versatile . As some co-factors contain both nucleotide and amino acid characteristics, it may be that amino acids, peptides and finally proteins initially were co-factors for ribozymes . </P> <P> One of the challenges in studying abiogenesis is that the system of reproduction and metabolism utilized by all extant life involves three distinct types of interdependent macromolecules (DNA, RNA, and protein). This suggests that life could not have arisen in its current form, and mechanisms have then been sought whereby the current system might have arisen from a simpler precursor system . The concept of RNA as a primordial molecule can be found in papers by Francis Crick and Leslie Orgel, as well as in Carl Woese's 1967 book The Genetic Code . In 1962, the molecular biologist Alexander Rich had posited much the same idea in an article he contributed to a volume issued in honor of Nobel - laureate physiologist Albert Szent - Györgyi . Hans Kuhn in 1972 laid out a possible process by which the modern genetic system might have arisen from a nucleotide - based precursor, and this led Harold White in 1976 to observe that many of the cofactors essential for enzymatic function are either nucleotides or could have been derived from nucleotides . He proposed that these nucleotide cofactors represent "fossils of nucleic acid enzymes". The phrase "RNA World" was first used by Nobel laureate Walter Gilbert in 1986, in a commentary on how recent observations of the catalytic properties of various forms of RNA fit with this hypothesis . </P> <P> The properties of RNA make the idea of the RNA world hypothesis conceptually plausible, though its general acceptance as an explanation for the origin of life requires further evidence . RNA is known to form efficient catalysts and its similarity to DNA makes clear its ability to store information . Opinions differ, however, as to whether RNA constituted the first autonomous self - replicating system or was a derivative of a still - earlier system . One version of the hypothesis is that a different type of nucleic acid, termed pre-RNA, was the first one to emerge as a self - reproducing molecule, to be replaced by RNA only later . On the other hand, the recent finding that activated pyrimidine ribonucleotides can be synthesized under plausible prebiotic conditions means that it is premature to dismiss the RNA - first scenarios . Suggestions for' simple' pre-RNA nucleic acids have included peptide nucleic acid (PNA), threose nucleic acid (TNA) or glycol nucleic acid (GNA). Despite their structural simplicity and possession of properties comparable with RNA, the chemically plausible generation of "simpler" nucleic acids under prebiotic conditions has yet to be demonstrated . </P> <P> RNA enzymes, or ribozymes, are found in today's DNA - based life and could be examples of living fossils . Ribozymes play vital roles, such as those in the ribosome, which is vital for protein synthesis . Many other ribozyme functions exist; for example, the hammerhead ribozyme performs self - cleavage and an RNA polymerase ribozyme can synthesize a short RNA strand from a primed RNA template . </P>

In the rna world model for the origin of life the first protocells