Editing RNA world (section)

== History ==
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|proteins]]), none of which can function and reproduce without the others, the classic [[Chicken or the egg|chicken-and-egg paradox]]. This suggests that life could not have arisen in its current form, which has led researchers to hypothesize mechanisms whereby the current system might have arisen from a simpler precursor system.<ref>{{cite journal | vauthors = Orgel LE | title = Prebiotic chemistry and the origin of the RNA world | journal = Critical Reviews in Biochemistry and Molecular Biology | volume = 39 | issue = 2 | pages = 99–123 | date = 2004 | pmid = 15217990 | doi = 10.1080/10409230490460765 | s2cid = 4939632 }}</ref> American molecular biologist [[Alexander Rich]] was the first to posit a coherent hypothesis on the origin of nucleotides as precursors of life.<ref>{{cite journal | vauthors = Lehman N | title = The RNA World: 4,000,000,050 years old | journal = Life | volume = 5 | issue = 4 | pages = 1583–1586 | date = October 2015 | pmid = 26791312 | pmc = 4695837 | doi = 10.3390/life5041583 | bibcode = 2015Life....5.1583L | doi-access = free }}</ref> In an article he contributed to a volume issued in honor of Nobel-laureate physiologist [[Albert Szent-Györgyi]], he explained that the primitive Earth's environment could have produced RNA molecules (polynucleotide monomers) that eventually acquired enzymatic and self-replicating functions.<ref>{{Cite book|last=Rich|first=Alexander|url=https://books.google.com/books?id=WCq2AAAAIAAJ|title=Horizons in Biochemistry: Albert Szent-Györgyi Dedicatory Volume|date=1962|publisher=Academic Press|isbn=978-0-12-400450-4|editor-last=Kasha|editor-first=Michael|pages=103–126|language=en|chapter=On the problems of evolution and biochemical information transfer|editor-last2=Pullman|editor-first2=Bernard}}</ref>

Other mentions of RNA as a primordial molecule can be found in papers by [[Francis Crick]]<ref>{{cite journal | vauthors = Crick FH | title = The origin of the genetic code | journal = Journal of Molecular Biology | volume = 38 | issue = 3 | pages = 367–379 | date = December 1968 | pmid = 4887876 | doi = 10.1016/0022-2836(68)90392-6 | s2cid = 4144681 }}</ref> and [[Leslie Orgel]],<ref>{{cite journal | vauthors = Orgel LE | title = Evolution of the genetic apparatus | journal = Journal of Molecular Biology | volume = 38 | issue = 3 | pages = 381–393 | date = December 1968 | pmid = 5718557 | doi = 10.1016/0022-2836(68)90393-8 }}</ref> as well as in [[Carl Woese]]'s 1967 book ''The Genetic Code''.<ref>{{cite book |last1=Woese |first1=C.R. |title=The genetic code: The molecular basis for genetic expression |date=1967 |publisher=Harper & Row |page=186}}</ref> [[Hans Kuhn (chemist)|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 a scenario whereby the critical electrochemistry of enzymatic reactions would have necessitated retention of the specific nucleotide [[Moiety (chemistry)|moieties]] of the original RNA-based enzymes carrying out the reactions, while the remaining structural elements of the enzymes were gradually replaced by protein, until all that remained of the original RNAs were these nucleotide cofactors, "fossils of nucleic acid enzymes".<ref name="hbwhite"/>