RNA world and prebiotic chemistry

Intuition Beginner

Life today runs on a partnership between two kinds of molecule. DNA stores the instructions, and proteins do the work, including the work of copying DNA. This sets up a puzzle: DNA cannot reproduce without proteins, and proteins cannot exist without DNA to encode them. Which came first?

RNA offers a way out. Like DNA, RNA is a four-letter information polymer: a sequence of bases (A, U, G, C) can carry a message. Like proteins, RNA folds into three-dimensional shapes, and some of those shapes speed up chemical reactions. An RNA molecule that catalyses a reaction is called a ribozyme. RNA can be both the recipe and the cook.

This unit goes deep on two questions the overview 19.15.01 only sketches. First, what evidence do we have that early life really did run on RNA catalysts? Second, could the chemical pieces of RNA, its sugar, its four bases, and its phosphate backbone, have assembled themselves on the young Earth, before any enzyme existed to help? That second question is prebiotic chemistry.

Visual Beginner

The RNA world sits in the middle of a two-sided problem: how were the nucleotides made, and how did biology escape RNA for the modern DNA-RNA-protein system?

The diagram captures the central claim: RNA links the prebiotic chemistry on the left to the living cell on the right, because it alone can act as both information and catalyst.

Worked example Beginner

In the original 1953 Miller-Urey experiment, roughly 2% of the carbon atoms from methane (CH${}_4$) became amino acids, with glycine the most abundant product. If the starting flask held 0.10 mol of methane, how much glycine could form?

The 0.10 mol of CH${}_4$ carries 0.10 mol of carbon atoms. Two percent of that carbon ends up in amino acids: $0.02\times 0.10=0.002$ mol of carbon atoms. Glycine (NH${}_2$CH${}_2$COOH) holds two carbon atoms per molecule, so the moles of glycine are $0.002÷2=0.001$ mol.

With a molecular weight of 75 g/mol, the mass is $0.001\times 75=0.075$ g, or 75 mg of glycine.

What this tells us: from a modest amount of simple gas and a few days of electrical sparks, tens of milligrams of a genuine amino acid appear. The bottleneck for life is not making amino acids. It is assembling such building blocks into controlled, self-copying chemistry, and for that the RNA world is the leading candidate.

Check your understanding Beginner

Formal definition Intermediate+

The Oparin-Haldane framework and the Miller-Urey experiment

The modern study of abiogenesis begins with the Oparin-Haldane hypothesis (Oparin 1924, Haldane 1929): life on the early Earth arose from organic molecules synthesised in a reducing atmosphere under the action of UV light and electrical discharge, accumulating in a "primordial soup". The hypothesis was made experimental by Miller and Urey in 1953 ^{[Miller 1953]}. A mixture of methane, ammonia, hydrogen, and water was subjected to spark discharge; within days the broth contained glycine, alanine, and other amino acids.

Modern geochemistry revises the assumption of a strongly reducing atmosphere, favouring a weakly reducing CO${}_2$-N${}_2$ atmosphere with trace H${}_2$. Under such conditions atmospheric amino-acid synthesis is less efficient, but spark-discharge experiments still yield organics, and synthesis at hydrothermal vents, on mineral surfaces, and in the parent bodies of meteorites remains favourable. The Miller-Urey result is robust in its qualitative claim (abiotic amino-acid synthesis is easy under many early-Earth conditions) even as the quantitative atmosphere is revised. The richer question this unit addresses is not whether amino acids form but whether the nucleotide components of RNA form, since RNA is the molecule an RNA world requires.

The RNA world, formally

An RNA world is a hypothesised stage of early life in which RNA, or a close chemical analogue, serves simultaneously as the informational polymer (storing heredity in its base sequence) and as the dominant catalyst (accelerating the reactions of metabolism and of its own replication). A ribozyme is an RNA molecule that catalyses a chemical reaction. A ribonucleotide is the monomer of RNA: the five-carbon sugar ribose linked at C1-prime to one of four nucleobases (adenine, guanine, cytosine, uracil) and at C5-prime to a phosphate. Successive ribonucleotides are joined by phosphodiester bonds linking the 3-prime hydroxyl of one ribose to the 5-prime phosphate of the next. The distinguishing feature of RNA against DNA is the 2-prime hydroxyl on ribose: it enables RNA's rich catalytic chemistry (proton transfer, cleavage) and, at the same time, makes RNA chemically fragile.

The RNA-world hypothesis resolves the chicken-and-egg problem of nucleic acids and proteins: DNA requires protein enzymes to replicate, but proteins require DNA to encode them. If RNA came first, performing both roles, the mutual dependence never arises.

The prebiotic synthesis problem

Prebiotic synthesis is the abiotic formation, under geochemically plausible conditions, of the canonical biological monomers. For the RNA world it poses three sub-problems ^{[Orgel 2004]}:

1. Sugar synthesis. Ribose can form in the formose reaction (Butlerov 1861), a base-catalysed self-condensation of formaldehyde:

$$2\mathrm{HCHO}\stackrel{\mathrm{Ca}(\mathrm{OH}{)}_2}{\to }\mathrm{HOC}{\mathrm{H}}_2\mathrm{CHO}⟶\cdots ⟶\mathrm{ribose}.$$

But ribose is unstable in water (half-life of hours at moderate pH and temperature) and the formose broth is a complex sugar soup of low ribose yield.

2. Nucleobase synthesis. The bases are more tractable. The Oró synthesis (1960) polymerises hydrogen cyanide to adenine:

$$5\mathrm{HCN}⟶\mathrm{adenine}({\mathrm{C}}_5{\mathrm{H}}_5{\mathrm{N}}_5).$$

Uracil and cytosine form from cyanoacetaldehyde and urea; guanine routes exist but with lower yield.

3. Nucleoside and nucleotide assembly. Joining a base to ribose (the N-glycosidic bond) and phosphorylating the 5-prime hydroxyl proved stubbornly inefficient under prebiotic conditions. This third sub-problem was the canonical obstacle to the RNA world until 2009.

Key mechanism Intermediate+

The RNA world rests on the empirical claim that RNA can catalyse chemically interesting reactions, including the central reaction of biology: peptide-bond formation. Three lines of evidence, each discovered experimentally, establish that claim.

Self-splicing introns: the first ribozyme

Cech and colleagues discovered that the intervening sequence (intron) of the 26S ribosomal RNA precursor in Tetrahymena excises itself without any protein ^{[Cech 1982]}. The reaction is two transesterification steps. First, an exogenous guanosine cofactor (GMP, GTP, or free guanosine) attacks the phosphate at the 5-prime splice site, transferring the 5-prime exon to the guanosine and releasing the 3-prime end of the upstream exon. Second, the newly freed 3-prime hydroxyl of the upstream exon attacks the phosphate at the 3-prime splice site, ligating the two exons and releasing the intron as a linear fragment that cyclises. No protein is required; the intron RNA folds into a three-dimensional active site that positions the guanosine and the scissile phosphate. This was the first demonstration that RNA alone can catalyse a complex, multistep reaction.

RNase P: a ribozyme that acts in trans

Altman and colleagues showed that ribonuclease P, the enzyme that cleaves the 5-prime leader of precursor transfer RNAs, is a ribozyme ^{[Altman 1983]}. RNase P contains one RNA subunit (M1 RNA, ~377 nucleotides in E. coli) and one protein subunit. In buffers with high magnesium, the purified M1 RNA alone cleaves precursor tRNA accurately; the protein merely stabilises the complex in vivo. RNase P established that RNA catalysts act on separate substrates (in trans), not only on themselves, generalising the catalytic repertoire beyond self-modification. Cech and Altman shared the 1989 Nobel Prize in Chemistry.

The ribosome is a ribozyme

The most consequential ribozyme is the peptidyl transferase centre (PTC) of the large ribosomal subunit, where peptide bonds are made. The PTC is composed entirely of 23S (prokaryotic) or 28S (eukaryotic) ribosomal RNA: no protein atom approaches the reacting groups closely enough to catalyse the reaction. Noller, Hoffarth, and Zimniak demonstrated that extensive extraction of ribosomal protein left a particle that still catalysed peptide-bond formation (puromycin reaction), and that destruction of the RNA abolished it ^{[Noller 1992]}. High-resolution crystal structures (Steitz, Moore; Ramakrishnan; Yonath, Nobel Prize 2009) confirmed that the active site is pure RNA.

The mechanism is an acid-base-catalysed aminolysis of an ester. The growing peptide is attached by an ester bond to the 3-prime hydroxyl of the terminal adenosine (A76) of the P-site tRNA. The free amino group of the aminoacyl-tRNA in the A site attacks that ester carbonyl, forming a new peptide bond and transferring the peptide to the A-site tRNA:

$${\text{peptidyl-CO-O-tRNA}}_P+{\text{H}}_2{\text{N-aa-tRNA}}_A⟶{\text{peptidyl-aa-NH-tRNA}}_A+{\text{tRNA}}_P.$$

The 2-prime hydroxyl of A76 (the P-site tRNA) functions as a general acid-base proton shuttle, stabilising the oxyanion transition state. Because the catalyst is RNA, the PTC is interpreted as a molecular fossil of the RNA world: the universal machine that makes proteins was already an RNA enzyme before proteins existed.

The discovery that the cell's central catalytic activity is RNA, not protein, is the strongest single piece of evidence that an RNA-based biology preceded the modern DNA-RNA-protein system. The fact that all three of these ribozymes (group I intron, RNase P, the ribosome) survive in modern cells, and that the ribosome is universal, places RNA catalysis at the base of the tree of life.

Exercises Intermediate+

The ribozyme repertoire and the limits of RNA catalysis Master

Natural ribozymes sort into two structural classes. Small nucleolytic ribozymes (hammerhead, hairpin, hepatitis delta virus, glmS, twister, pistol, hatchet) are 30-150 nucleotides and catalyse site-specific cleavage or ligation of a phosphodiester backbone by a 2-prime hydroxyl attack, forming a 2-prime,3-prime cyclic phosphate. The rate enhancements reach $10^9$- to $10^{11}$-fold, comparable to protein nucleases. Large ribozymes catalyse more chemically diverse reactions: the group I and group II self-splicing introns (transesterification), RNase P (hydrolysis of precursor tRNA), the spliceosome's RNA core (also group-II-like transesterification), and the ribosome (peptidyl transfer, an aminolysis of an ester). Across the repertoire, RNA catalyses phosphoryl transfer and ester chemistry but performs no redox chemistry and no radical chemistry on its own; RNA lacks the thiol of cysteine, the imidazole of histidine (though some ribozymes recruit adenosine or metal ions as surrogates), and the carboxylate of aspartate and glutamate.

In vitro selection (SELEX) has expanded the known catalytic repertoire of RNA dramatically. Starting from random-sequence pools, laboratories have selected ribozymes for RNA ligation, cleavage, nucleotide synthesis, peptide bond formation, N-glycosidic bond formation, alkylation, acyl transfer, Diels-Alder cycloaddition, and RNA-templated RNA polymerisation ^{[Joyce 2002]}. The RNA polymerase ribozyme developed in Joyce's laboratory (Wochner, Attwater, et al., 2011; improved through 2016 by the Joyce and Chaput groups) can copy RNA templates of more than 100 nucleotides and, in the most advanced variants, synthesise a complementary strand of sufficient length to be itself a template, approaching the threshold for self-replication. These selected ribozymes demonstrate that the catalytic potential of RNA far exceeds what survives in modern cells, consistent with an RNA world in which ribozymes performed many reactions later taken over by more efficient protein enzymes.

Two limits bound the RNA world and motivate the transition to proteins. First, RNA catalysis is generally slower than protein catalysis: the ribosome makes peptide bonds at about 2-20 per second, whereas protein proteases catalyse hydrolysis at $10^6$-$10^7$ per second. Second, RNA's chemical versatility is narrower: four side-chain-equivalent groups (the bases) plus the 2-prime hydroxyl, against the twenty amino-acid side chains (including thiols, imidazoles, carboxylates, and metal-binding ligands). The ribosome's peptidyl transferase, the most sophisticated natural ribozyme, catalyses only one reaction. A cell that needed dozens of distinct metabolic reactions could not staff them all with RNA at modern rates. The evolutionary solution is recorded in the ribosome itself: an RNA core carries the catalytic centre (the fossil of the RNA world), while proteins decorate the periphery and improve rate and fidelity (the legacy of the transition). The same hybrid pattern is seen in the spliceosome and in RNase P across different lineages, marking the gradual handoff from RNA to protein catalysis.

Prebiotic synthesis pathways for the nucleotides Master

Three prebiotic routes to the nucleobases and the nucleotides dominate the modern literature. They are complementary rather than exclusive, and a synthesis of all three now underpins "prebiotic systems chemistry".

The formamide route (Saladino, Crestini, Costanzo, Di Mauro and colleagues) treats formamide (HCONH${}_2$) as a concentrated, thermally stable reservoir of HCN chemistry ^{[Saladino 2012]}. Formamide forms by hydrolysis of HCN and by reaction of formaldehyde with ammonia; it is liquid across a wide temperature range and dissolves phosphate and many metal ions. Heated formamide in the presence of mineral catalysts (phosphates, titanates, clays, meteoritic minerals) yields all four canonical nucleobases (adenine, cytosine, guanine, uracil), and the same conditions generate amino-acid precursors and acyclonucleosides. The attraction of formamide is its dual role: it is both the solvent and the carbon-nitrogen feedstock, and it drives phosphorylation toward nucleotides.

The meteoritic / exogenous route rests on the analysis of carbonaceous chondrites. The Murchison meteorite (1969) and related falls contain amino acids, sugars, sugar alcohols, and nucleobases; Callahan and colleagues (2011) reported adenine, guanine, xanthine, hypoxanthine, and purine in eleven carbonaceous chondrites and in Antarctic ice. The flux of exogenous organics during the period of heavy bombardment (roughly 4.4 to 3.8 billion years ago) was substantial, with model estimates of $10^5$-$10^7$ kg of organics per year globally. Meteoritic delivery does not solve the origin problem (it relocates the synthesis to the parent body or the interstellar medium), but it establishes that nucleobase chemistry occurs abiotically throughout the solar system and contributes to the terrestrial prebiotic inventory.

The Sutherland cyanosulfidic pathway resolved the central obstacle that Orgel had emphasised ^{[Powner, Gerland and Sutherland 2009]}. Starting from hydrogen cyanide, hydrogen sulfide, and ultraviolet light, and using phosphate as both buffer and reagent, the route proceeds through 2-aminooxazole, condenses with glyceraldehyde to a pentose aminooxazoline, and, via phosphate-mediated crystallisation (which selects the correct ribo-stereochemistry), cyclises with cyanoacetylene to the pyrimidine cytidine nucleotide as its 2-prime,3-prime cyclic phosphate, the activated form ready for polymerisation. The route never releases free ribose, never requires a separate glycosidic-bond-forming step, and produces an activated nucleotide directly. Crucially, the same HCN/H${}_2$S/UV network also produces precursors of amino acids (glycine, alanine) and of lipids, and the photochemistry is driven by the higher ultraviolet flux of the young Sun (Ribas 2010). Sutherland and colleagues extended this into prebiotic systems chemistry (Patel, Percivalle, Ritson, Sutherland 2015): a single reaction network, operating under one set of conditions, generates the monomers for all three information-bearing polymer classes, RNA, peptide, and lipid, simultaneously.

The remaining open problem is the purine ribonucleotide. The Sutherland 2009 route solves the pyrimidines (cytidine, uridine). Purine routes (the Becker wet-dry-cycle pathway, the inosine route of Powner and colleagues, and related chemistry) have made progress but have not yet matched the pyrimidine route's elegance. The prebiotic synthesis of the full nucleotide set, each as an activated polymerisation-ready monomer, from common feedstocks under mutually compatible conditions, is the active frontier of the field.

Lipid-world and systems-chemistry alternatives Master

Not all origin-of-life models place RNA first. A family of lipid-world and systems-chemistry models argue that organisation, compartmentalisation, and network-level catalysis can precede any single information polymer. These do not replace the RNA world; they sit alongside it and increasingly merge with it.

The lipid world (Deamer, Hargreaves, and colleagues) emphasises that amphiphilic molecules self-assemble into bilayer vesicles without any instruction. Fatty acids and related amphiphiles present in the Murchison meteorite form vesicles in aqueous solution; these vesicles grow by incorporating free amphiphiles, compete for membrane components, and divide under shear. A lipid vesicle provides three things that a free replicator cannot: concentration of solutes (raising reaction rates), separation of internal from external chemistry, and a unit of individuality on which selection can act. The lipid world is mechanistically complementary to the RNA world, since RNA encapsulated in fatty-acid vesicles acquires both heredity (from RNA) and individuality (from the membrane). The detailed protocell lifecycle (growth, competition, division, inheritance) is treated in the sibling unit 19.15.01 and is not repeated here.

The GARD (Graded Autocatalysis Replication Domain) model of Segre, Lancet, and colleagues formalises lipid-world dynamics. GARD treats a compositional genome, a small set of amphiphilic molecule types in a vesicle, whose composition is maintained by a network of mutual catalytic influences. Autocatalytic closure of the network allows a vesicle's composition to be preserved with some fidelity when it grows and divides, producing a kind of heredity without a sequence polymer. The Kauffman-style autocatalytic-set theorem underlying such networks is treated in 19.15.01. GARD's interest is that it shows compositional information can be inherited before any nucleotide polymer exists, giving a bridge from pure lipid chemistry to the threshold where RNA takes over.

Cairns-Smith's clay crystal hypothesis (1966) proposed an even earlier genetic substrate: clay minerals, such as montmorillonite, whose growth and replication propagates crystal defects that could template organic chemistry. Montmorillonite does catalyse the polymerisation of activated nucleotides into RNA chains of up to 50 nucleotides (Ferris, 1996 and following), so the clay hypothesis connects to the RNA world as a substrate for early RNA synthesis rather than as a permanent replacement. Modern work rarely treats clay as the sole genetic material, but the catalytic role of mineral surfaces is universally accepted.

Prebiotic systems chemistry (Sutherland, Powner, and others) is the contemporary synthesis. Rather than championing a single origin-of-life scenario, it asks whether a single geochemical environment, irradiated HCN and H${}_2$S in the presence of phosphate and sulfite, can produce the monomers for RNA, peptide, and lipid simultaneously. The experimental answer, accumulating since 2009, is largely yes. Systems chemistry dissolves the old competition between RNA-world, metabolism-first, and lipid-world models: each model captures a real requirement (information, energy, compartmentalisation), and a single chemical network can supply all three at once. The remaining disagreements concern the geochemical setting (subaerial geothermal pools versus submarine hydrothermal vents, the role of wet-dry cycles, the ultraviolet flux) rather than the identity of the first information polymer.

LUCA and the RNA fossils of the modern cell Master

The Last Universal Common Ancestor (LUCA) was not the first organism but the most recent population from which all surviving cellular life descends. Its inferred features are reconstructed from features shared across all three domains (Bacteria, Archaea, Eukarya); the full phylogenomic reconstruction of the LUCA genome (Weiss et al., 2016; ~355 protein families, anaerobic, thermophilic, Wood-Ljungdahl carbon fixation) is treated in 19.15.01. What concerns this unit is the RNA content of LUCA, because the RNA machinery of the modern cell is the direct archaeological record of the RNA world.

Three classes of RNA fossil are universal and therefore present in LUCA. First, the ribosome, in particular the peptidyl transferase centre of the large-subunit rRNA and the decoding centre of the small-subunit rRNA, is conserved in every cellular lineage. The catalytic core is RNA; the protein components are later embellishments that vary across domains. Second, transfer RNA and the genetic code are universal (with minor variant codes in mitochondria and some ciliates). The code is not chemically arbitrary at every position: hydrophobic amino acids cluster on codons whose second base is U, and several aminoacyl-tRNA synthetase classes show stereochemical affinities to their cognate codons or anticodons, a pattern consistent with a stereochemical era when RNA directly recognised its amino acids. Third, the nucleotide-derived cofactors, ATP, NAD, NADP, FAD, coenzyme A, S-adenosylmethionine, and the cobalamin and thiamine cofactors, are all ribonucleotide derivatives. The persistence of an adenosine handle on metabolically unrelated cofactors is inexplicable by present-day chemistry and is read as a relic of an RNA-based metabolism in which ribonucleotides were the universal carrier and handle.

The pattern is uniform: where a reaction is universal and ancient, the catalyst is often RNA or the cofactor is a nucleotide. Where a reaction is lineage-specific and recent, the catalyst is a protein. This stratigraphy lets biologists read the RNA world out of modern cells the way geologists read a stratigraphic column: the deepest, most universal layer is RNA, and proteins accrete above it. The most parsimonious reconstruction is that LUCA already possessed a fully fledged ribosome and the genetic code, and so the RNA world predates LUCA, consistent with geological dates that place life at roughly 3.5-3.8 billion years ago and the origin of the RNA world in the preceding hundreds of millions of years.

Connections Master

• Origin of life, mechanistic scenarios 19.15.01 provides the broader framework into which this unit's RNA-world chemistry fits: protocell lifecycle, alkaline-vent and iron-sulfur metabolism-first models, Eigen's error threshold, hypercycles, chiral amplification, and LUCA genome reconstruction. The present unit goes deep on the RNA-world mechanism and prebiotic nucleotide synthesis specifically; 19.15.01 supplies everything else.

• Cellular respiration 17.04.01 is the modern descendant of the energy metabolism that the RNA world had to fuel. ATP, the universal energy currency, is itself a ribonucleotide, and the chemiosmotic proton gradients used by every modern cell are the same free-energy source that prebiotic vent and surface environments supplied to early replicators. Understanding ATP synthesis is inseparable from understanding why a nucleotide became the energy handle.

• Biochemistry 17.01.01 supplies the molecular building blocks (amino acids, nucleotides, lipids, sugars) whose prebiotic synthesis this unit reconstructs. The phosphodiester bond, the N-glycosidic bond, and the ester chemistry of the peptidyl-transferase reaction are the same reactions treated by biochemistry in the modern cell, run in reverse to recover their abiotic origins.

• Molecular biology 12.04.01 describes the DNA-RNA-protein system that evolved out of the RNA world. The ribosome, transfer RNA, the genetic code, and the nucleotide-derived cofactors are the fossils of that transition; the modern DNA replication machinery, with its proofreading and repair, is the evolutionary answer to the information-threshold problem that the RNA world could not solve on its own.

Historical & philosophical context Master

The Oparin-Haldane hypothesis (Oparin 1924, Haldane 1929) reframed the origin of life from a question of spontaneous generation, refuted for modern conditions by Pasteur in 1859, into a question of slow chemical evolution under early-Earth conditions. Stanley Miller, working in Harold Urey's laboratory, made the hypothesis experimental in 1953 ^{[Miller 1953]}: amino acids formed in a flask from reducing gases and electrical discharge. The same decade, the molecular biology revolution (Watson and Crick, 1953) established DNA as the genetic material and crystallised the chicken-and-egg problem of DNA and proteins.

The RNA world was proposed in the late 1960s by Carl Woese, Francis Crick, and Leslie Orgel, who each noted that the dual chemical personality of RNA (an information polymer that can also fold and bind) made it a plausible first molecule. Walter Gilbert coined the term "RNA world" in a 1986 Nature paper ^{[Gilbert 1986]}. The hypothesis acquired its decisive empirical support independently in 1982-1983: Thomas Cech's discovery that the Tetrahymena group I intron splices itself ^{[Cech 1982]}, and Sidney Altman's demonstration that the RNA subunit of RNase P is the catalyst ^{[Altman 1983]}. Cech and Altman shared the 1989 Nobel Prize in Chemistry.

The ribosome-is-a-ribozyme result hardened the case in the 1990s and 2000s. Noller and colleagues showed in 1992 that the peptidyl transferase activity survived protein removal ^{[Noller 1992]}, and the high-resolution crystal structures of the large ribosomal subunit (Steitz and Moore, 2000; Ramakrishnan; Yonath, Nobel Prize 2009) showed that no protein atom reaches the active site. The prebiotic chemistry of the nucleotides was revolutionised by John Sutherland's laboratory, whose 2009 paper with Powner and Gerland gave the first plausible prebiotic route to activated pyrimidine ribonucleotides ^{[Powner, Gerland and Sutherland 2009]} and whose subsequent "systems chemistry" programme (2015 onward) produced a unified network for RNA, peptide, and lipid monomers from a common feedstock. Joyce surveyed the expanding catalytic repertoire of in vitro selected ribozymes and argued the case for an RNA world in detail ^{[Joyce 2002]}, and Orgel gave the canonical statement of the obstacles that the Sutherland route later overcame ^{[Orgel 2004]}.

Philosophically, the RNA world dissolves the chicken-and-egg problem without dissolving the origin problem. It replaces one hard question (which came first, DNA or protein?) with another (how did a self-replicating ensemble of RNA catalysts arise from prebiotic chemistry?), but the second question is answerable by chemistry rather than by logic, and the Sutherland, formamide, and meteoritic routes show that its key steps are chemically plausible. The contemporary consensus is not that the RNA world is proven but that it is the most chemically and phylogenetically constrained hypothesis on the table, with every modern ribozyme standing as living evidence for an RNA-based stage that preceded the DNA-protein cell.

Bibliography Master

1. Gilbert, W., "Origin of life: The RNA world", Nature 319 (1986), 618.

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10. Patel, B. H., Percivalle, C., Ritson, D. J., Duffy, C. D. & Sutherland, J. D., "Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism", Nat. Chem. 7 (2015), 301-307.

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