1 Setting the Stage: The History, Chemistry, and Geobiology behind RNA

Abstract

TWO RNA WORLD HYPOTHESES
Because they are organic molecules, ribonucleic acids can be understood using the language of organic chemistry. Those who accept the premise in the title of this book, however, regard RNA as very special organic molecules. Today on earth, given appropriate collections of catalysts from modern life, RNA can direct its own replication, where the replicates are imperfectly made, and where the imperfections are themselves replicatable.

Such a combination of properties allows an organic system to support Darwinian evolution, the only process known to naturally generate functional behavior in chemistry. Only one other class of molecules, DNA, is known to be similarly special. In modern life on earth, DNA and RNA collaborate with a third encoded class of biomolecules, proteins, to support Darwinian evolution. This collaboration, in turn, generates the enormous range of life that today occupies nearly every habitable niche on the planet.

Those interested in the origin of Darwinian evolution have for four decades speculated that RNA might have supported Darwinian evolution without other encoded molecules (Rich 1962), either DNA or proteins. The “RNA World” title has come to be associated with this speculation (Gilbert 1986).

The notion of an RNA World relates to two classes of hypotheses. One class holds that an earlier form of life on earth used RNA as the only genetically encoded component of biological catalysis. Evidence for this is found within the details of the biochemistry of contemporary life on earth and appears to be reasonably well supported (Crick 1968;Visser...