The RNA world

Between 1981 and 1986, Thomas Cech and Sidney Altman discovered that there are at least two metabolic reactions where the catalysts are not proteins but RNAs. Up until then, it had been accepted that all enzymes are proteins, and normally the discovery of a few exceptions does not undermine a virtually universal rule, but those two examples had an extraordinary implication.
In 1986, Walter Gilbert formulated it explicitly in these terms: “If there are two enzymic activities associated with RNA, there may be more. And if there are activities among these RNA enzymes, or ribozymes, that can catalyse the synthesis of a new RNA molecule from precursors and an RNA template, then there is no need for protein enzymes at the beginning of evolution. One can contemplate an RNA world, containing only RNA molecules that serve to catalyse the synthesis of themselves”.
In reality, the theoretical possibility of an RNA world had already been suggested by Francis Crick with two prophetic statements. In 1966, Crick wrote that “transfer RNA looks like Nature’s attempt to make RNA do the job of a protein”, and in 1968 he added “Possibly the first ‘enzyme’ was an RNA molecule with replicase properties”.
Another precursor of the RNA world was H.B. White (1976), who noticed that “many coenzymes are nucleotides, or use bases derived from nucleotides, and it is possible therefore that coenzymes are molecular fossils of earliest RNA-based enzymes”.
The discoveries of Cech and Altman gave suddenly a concrete basis to these ideas, and had an enormous impact because were falling on a fertile ground, already prepared to accept them. Two examples, however, are not enough to prove an hypothesis that concerns all molecules of a bygone primordial past. More precisely, the RNA world hypothesis requires a confirmation of three great generalizations:
(1) There has been a period in evolution when all genes were ribogenes.
(2) There has been a period in evolution when all enzymes were ribozymes.
(3) Modern RNAs are all that remains of that primitive RNA world, and are therefore the most ancient molecules of the history of life, some of which are still functional while others have become molecular fossils.