RNA world

The RNA world is a hypothetical phase of the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins.

The concept of the RNA world was first proposed in 1962 by Alexander Rich, and the term was coined by Walter Gilbert in 1986. Alternative chemical paths to life have been proposed, and RNA-based life may not have been the first life to exist. Even so, the evidence for an RNA world is strong enough that the hypothesis has gained wide acceptance.

Like DNA, RNA can store and replicate genetic information; like enyzmes, it can catalyze (start or accelerate) chemical reactions that are critical for life. Many critical components of cells are composed of RNA, and RNA is essential for protein biosynthesis. Similarities between various coenzymes might evidence a shared evolutionary origin in ribozymes during an RNA epoch. Although RNA is fragile, some ancient RNAs may have evolved the ability to methylate other RNAs to protect them.

If the RNA world existed, it was probably followed by an age characterized by the evolution of ribonucleoproteins, which in turn ushered in the era of DNA and longer proteins. The reason why DNA became the predominant storage molecule may be because it is more stable and durable than RNA. Protein enzymes may have come to replace RNA-based ribozymes as biocatalysts because their greater abundance and diversity of monomers makes them more versatile.

One of the challenges in studying abiogenesis is that the system of reproduction and metabolism utilized by all extant life involves three distinct types of interdependent macromolecules (DNA, RNA, and protein). This suggests that life could not have arisen in its current form, and mechanisms have then been sought whereby the current system might have arisen from a simpler precursor system. The concept of RNA as a primordial molecule can be found in papers by Francis Crick and Leslie Orgel, as well as in Carl Woese's 1967 book The Genetic Code. In 1962, the molecular biologist Alexander Rich, of the Massachusetts Institute of Technology, had posited much the same idea in an article he contributed to a volume issued in honor of Nobel-laureate physiologist Albert Szent-Györgyi.Hans Kuhn in 1972 laid out a possible process by which the modern genetic system might have arisen from a nucleotide-based precursor, and this led Harold White in 1976 to observe that many of the cofactors essential for enzymatic function are either nucleotides or could have been derived from nucleotides. He proposed that these nucleotide cofactors represent "fossils of nucleic acid enzymes". The phrase "RNA World" was first used by Nobel laureate Walter Gilbert in 1986, in a commentary on how recent observations of the catalytic properties of various forms of RNA fit with this hypothesis.

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