RNA world hypothesis

The RNA world refers to the self-replicating ribonucleic acid (RNA) molecules hypothesised to have been the precursors to all current life on Earth. The hypothesis that current life on Earth descends from an RNA world is widely accepted, and is supported by several lines of evidence including how ephemeral RNA was protected on the early earth. Alternative chemical paths to life have been proposed, and RNA-based life may not have been the first life to exist.

The RNA world would have eventually been replaced by the DNA, RNA and protein world of today, likely through an intermediate stage of ribonucleoprotein enzymes such as the ribosome and ribozymes, since it is argued that proteins large enough to self-fold and have useful activities would only have come about after RNA was available to catalyze peptide ligation or amino acid polymerization. DNA is thought to have taken over the role of data storage due to its increased stability, while proteins, through a greater variety of monomers (amino acids), replaced RNA's role in specialized biocatalysis.

The RNA world hypothesis is supported by many independent lines of evidence, such as the observations that RNA is central to the translation process and that small RNAs can catalyze all of the chemical group and information transfers required for life. The structure of the ribosome has been called the "smoking gun," as it showed that the ribosome is a ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the active site where peptide bond formation is catalyzed. Many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. Also, many critical cofactors (ATP, Acetyl-CoA, NADH, etc.) are either nucleotides or substances clearly related to them. This would mean that the RNA and nucleotide cofactors in modern cells are an evolutionary remnant of an RNA-based enzymatic system that preceded the protein-based one seen in all extant life.

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