Abiogenesis
The RNA world

The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. Many researchers concur that an RNA world must have preceded the DNA-based life that now dominates. However, RNA-based life may not have been the first to exist. Another model echoes Darwin's "warm little pond" with cycles of wetting and drying.

RNA is central to the translation process. Small RNAs can catalyze all the chemical groups and information transfers required for life. RNA both expresses and maintains genetic information in modern organisms; and the chemical components of RNA are easily synthesized under the conditions that approximated the early Earth, which were very different from those that prevail today. The structure of the ribosome has been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18 Å of the active site that catalyzes peptide bond formation.

The concept of the RNA world was proposed in 1962 by Alexander Rich, and the term was coined by Walter Gilbert in 1986. There were initial difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil. Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals.

RNA replicase can function as both code and catalyst for further RNA replication, i.e. it can be autocatalytic. Jack Szostak has shown that certain catalytic RNAs can join smaller RNA sequences together, creating the potential for self-replication. The RNA replication systems, which include two ribozymes that catalyze each other's synthesis, showed a doubling time of the product of about one hour, and were subject to natural selection under the experimental conditions. If such conditions were present on early Earth, then natural selection would favor the proliferation of such autocatalytic sets, to which further functionalities could be added. Self-assembly of RNA may occur spontaneously in hydrothermal vents. A preliminary form of tRNA could have assembled into such a replicator molecule.

Possible precursors to protein synthesis include the synthesis of short peptide cofactors or the self-catalysing duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.

Eugene Koonin has argued that "no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system."
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