If the replicators in a pre-RNA world were simplified forms of RNA, then the changeover to an RNA world would probably have been gradual, with RNA initially acting solely as the genetic material before also developing catalytic abilities and then taking over.
A more aggressive changeover would have happened if the original replicators were based on a completely different system to RNA. In this instance, a pre-existing self-replicating system would sow the seeds of its own destruction by evolving, initially for its own selective advantage, the mechanism for synthesising and polymerising the components of a completely different genetic system, eventually being taken over by it.
A whole host of organic compounds not found in RNA or DNA have been proposed as the basis for possible early replicators, including hydroxy acids, amino acids, phosphomonoesters of polyhydric alcohols, aminoaldehydes, thioesters and molecules containing two sulfhydryl groups. Replication amongst inorganic compounds has also been suggested (see Box 2).
It has even been proposed that systems of high complexity can develop without any need for a distinct store of genetic information. In this scenario, a set of simple small molecules, such as enzymes and amino acid residues, carrying out cycles of mutually reinforcing reactions gradually evolve into greater complexity. This is the model for the origin of life to which Shapiro adheres.
However, the existence and make-up of the pre-RNA world will be even harder to prove and define than that of the RNA world. As Joyce says: 'There is no direct evidence for a pre-RNA world. This is a conjecture based on what seems to be overwhelming difficulties with the prebiotic synthesis and replication of polynucleotides. Unlike the case for an RNA world, there are no 'molecular fossils' within biology to support the existence of a pre-RNA world'.
Nevertheless, despite all the difficulties with the prebiotic synthesis of RNA and the practical impossibility of determining what a pre-RNA world might have been like, proponents of the RNA world hypothesis argue that it is still the most credible theory for the origin of life. Ferris claims that experimental proof of the hypothesis will eventually be forthcoming: 'We just haven't done enough experiments yet to show how to get plausible [chemical] pathways', he says.
Joyce agrees; however, he believes that proof will come from the opposite direction, from the 'molecular fossils' in modern-day cells. 'The RNA world hypothesis is on the verge of receiving a huge boost as the crystal structure of the ribosome emerges', he says. 'Thomas Steitz and colleagues at Yale have a 2.7Å resolution structure of the large subunit, which includes the "peptidyl transfer site", where peptide bond formation occurs. That site is composed entirely of RNA. Thus, even today, the translation apparatus is an RNA machine.' He does admit, however, that 'the problem remains as to how the RNA world arose'.
So, despite the obvious advantages of the RNA world hypothesis, the chemistry of how it all began remains its Achilles' heel. Geologists believe that life only took 600m years to arise on the Earth; let's hope that an explanation for how that happened arises even faster.
Further reading
A. C. Cairns-Smith, Genetic takeover and the mineral origins of life. Cambridge: CUP, 1982.
Raymond F. Gesteland, Thomas R. Cech and John F. Atkins (eds), The RNA world, 2nd Edn. New York: Cold Spring Harbour Laboratory Press, 1999.
Robert Shapiro, Planetary dreams: the quest to discover life beyond Earth. New York: Wiley, 1999.
Source: Chembytes E-zine
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