RNA world

The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins.^[1] The term also refers to the hypothesis that posits the existence of this stage.

Alexander Rich first proposed the concept of the RNA world in 1962,^[2] and Walter Gilbert coined the term in 1986.^[3] Alternative chemical paths to life have been proposed,^[4] and RNA-based life may not have been the first life to exist.^[3][5] Even so, the RNA world hypothesis seems to be the most favored abiogenesis paradigm, but even proponents agree it still has not reached conclusive evidence to completely falsify other paradigms and hypotheses.^[2][6][7] The concurrent formation of all four RNA building blocks further strengthened the hypothesis.^[8] Regardless of its plausibility in a prebiotic scenario, the RNA world can serve as a model system for studying the origin of life.^[9]

• Like DNA, RNA can store and replicate genetic information.
• Like protein enzymes, RNA enzymes (ribozymes) can catalyze (start or accelerate) chemical reactions that are critical for life.^[10]

One of the most critical components of cells, the ribosome, is composed primarily of RNA. Ribonucleotide moieties in many coenzymes, such as acetyl-CoA, NADH, FADH, and F420, may be surviving remnants of covalently bound coenzymes in an RNA world.^[11]

Although RNA is fragile, some ancient RNAs may have evolved the ability to methylate other RNAs to protect them.^[12]

If the RNA world existed, it was probably followed by an age characterized by the evolution of ribonucleoproteins (RNP world),^[3] which in turn ushered in the era of DNA and longer proteins. DNA has greater stability and durability than RNA; this may explain why it became the predominant information storage molecule.^[13] 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 cofactors contain both nucleotide and amino-acid characteristics, it may be that amino acids, peptides and finally proteins initially were cofactors for ribozymes.^[11]