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Hypothetical scenario for the origin of life
The iron–sulfur world hypothesis is a set of proposals for the origin of life and the early evolution of life advanced in a series of articles between 1988 and 1992 by Günter Wächtershäuser, a Munich patent lawyer with a degree in chemistry, who had been encouraged and supported by philosopher Karl R. Popper to publish his ideas. The hypothesis proposes that early life may have formed on the surface of iron sulfide minerals, hence the name. It was developed by retrodiction (making a "prediction" about the past) from extant biochemistry (non extinct, surviving biochemistry) in conjunction with chemical experiments.

Wächtershäuser proposes that the earliest form of life, termed the "pioneer organism", originated in a volcanic hydrothermal flow at high pressure and high (100 °C) temperature. It had a composite structure of a mineral base with catalytic transition metal centers (predominantly iron and nickel, but also perhaps cobalt, manganese, tungsten and zinc). The catalytic centers catalyzed autotrophic carbon fixation pathways generating small molecule (non polymer) organic compounds from inorganic gases (e. g. carbon monoxide, carbon dioxide, hydrogen cyanide and hydrogen sulfide). These organic compounds were retained on or in the mineral base as organic ligands of the transition metal centers with a flow retention time in correspondence with their mineral bonding strength thereby defining an autocatalytic "surface metabolism". The catalytic transition metal centers became autocatalytic by being accelerated by their organic products turned ligands. The carbon fixation metabolism became autocatalytic by forming a metabolic cycle in the form of a primitive sulfur dependent version of the reductive citric acid cycle. Accelerated catalysts expanded the metabolism and new metabolic products further accelerated the catalysts. The idea is that once such a primitive autocatalytic metabolism was established, its intrinsically synthetic chemistry began to produce ever more complex organic compounds, ever more complex pathways and ever more complex catalytic centers.

Reaction of carbon monoxide (CO), hydrogen sulfide (H2S) and methanethiol CH3SH in the presence of nickel sulfide and iron sulfide generates the methyl thioester of acetic acid [CH3 CO SCH3] and presumably thioacetic acid (CH3 CO SH) as the simplest activated acetic acid analogues of acetyl CoA. These activated acetic acid derivatives serve as starting materials for subsequent exergonic synthetic steps. However, Huber and Wächtershäuser reported low 0.5% acetate yields based on the input of CH3SH (methanethiol) (8 mM) in the presence of 350 mM CO. This is about 500 times and 3700 times the highest CH3SH and CO concentrations respectively measured to date in a natural hydrothermal vent fluid. Reaction of nickel hydroxide with hydrogen cyanide (HCN) (in the presence or absence of ferrous hydroxide, hydrogen sulfide or methyl mercaptan) generates nickel cyanide, which reacts with carbon monoxide (CO) to generate pairs of α hydroxy and α amino acids: e. g. glycolate/glycine, lactate/alanine, glycerate/serine; as well as pyruvic acid in significant quantities. Pyruvic acid is also formed at high pressure and high temperature from CO, H2O, FeS in the presence of nonyl mercaptan. Reaction of pyruvic acid or other α keto acids with ammonia in the presence of ferrous hydroxide or in the presence of ferrous sulfide and hydrogen sulfide generates alanine or other α amino acids. Reaction of α amino acids in aqueous solution with COS or with CO and H2S generates a peptide cycle wherein dipeptides, tripeptides etc. are formed and subsequently degraded via N terminal hydantoin moieties and N terminal urea moieties and subsequent cleavage of the N terminal amino acid unit. Proposed reaction mechanism for reduction of CO2 on FeS: Ying et al. (2007) have shown that direct transformation of mackinawite (FeS) to pyrite (FeS2) on reaction with H2S till 300 °C is not possible without the presence of critical amount of oxidant. In the absence of any oxidant, FeS reacts with H2S up to 300 °C to give pyrrhotite. Farid et al. have experimentally shown that mackinawite (FeS) has ability to reduce CO2 to CO at temperature higher than 300 °C. They reported that the surface of FeS is oxidized, which on reaction with H2S gives pyrite (FeS2). It is expected that CO reacts with H2O in the Drobner experiment to give H2.

Early evolution is defined as beginning with the origin of life and ending with the last universal common ancestor (LUCA). According to the iron–sulfur world theory it covers a coevolution of cellular organization (cellularization), the genetic machinery and enzymatization of the metabolism.

Cellularization occurs in several stages. It may have begun with the formation of primitive lipids (e. g. fatty acids or isoprenoids) in the surface metabolism. These lipids accumulate on or in the mineral base. This lipophilizes the outer or inner surfaces of the mineral base, which promotes condensation reactions over hydrolytic reactions by lowering the activity of water and protons. In the next stage lipid membranes are formed. While still anchored to the mineral base they form a semi cell bounded partly by the mineral base and partly by the membrane. Further lipid evolution leads to self supporting lipid membranes and closed cells. The earliest closed cells are pre cells (sensu Kandler) because they allow frequent exchange of genetic material (e. g. by fusions). According to Woese, this frequent exchange of genetic material is the cause for the existence of the common stem in the tree of life and for a very rapid early evolution. Nick Lane and coauthors state that "Non enzymatic equivalents of glycolysis, the pentose phosphate pathway and gluconeogenesis have been identified as well. Multiple syntheses of amino acids from α keto acids by direct reductive amination and by transamination reactions can also take place. Long chain fatty acids can be formed by hydrothermal Fischer Tropsch type synthesis which chemically resembles the process of fatty acid elongation. Recent work suggests that nucleobases might also be formed following the universally conserved biosynthetic pathways, using metal ions as catalysts". Metabolic intermediates in glycolysis and the pentose phosphate pathway such as glucose, pyruvate, ribose 5 phosphate, and erythrose 4 phosphate are spontaneously generated in the presence of Fe(II). Fructose 1,6 biphosphate, a metabolic intermediate in gluconeogenesis, was shown to have been continuously accumulated but only in a frozen solution. The formation of fructose 1,6 biphosphate was accelerated by lysine and glycine which implies the earliest anabolic enzymes were amino acids. It had been reported that 4Fe 4S, 2Fe 2S, and mononuclear iron clusters are spontaneously formed in low concentrations of cysteine and alkaline pH. Methyl thioacetate, a precursor to acetyl CoA can be synthesized in conditions relevant to hydrothermal vents. Phosphorylation of methyl thioacetate leads to the synthesis of thioacetate, a simpler precursor to acetyl CoA. Thioacetate in more cooler and neutral conditions promotes synthesis of acetyl phosphate which is a precursor to adenosine triphosphate and is capable of phosphorylating ribose and nucleosides. This suggests that acetyl phosphate was likely synthesized in thermophoresis and mixing between the acidic seawater and alkaline hydrothermal fluid in interconnected micropores. It is possible that it could promote nucleotide polymerization at mineral surfaces or at low water activity. Thermophoresis at hydrothermal vent pores can concentrate polyribonucleotides, but it remains unknown as to how it could promote coding and metabolic reactions. In mathematical simulations, autocatalytic nucleotide synthesis is proposed to promote protocell growth as nucleotides also catalyze CO2 fixation. Strong nucleotide catalysis of fatty acids and amino acids slow down protocell growth and if competition between catalytic function were to occur, this would disrupt the protocell. Weak or moderate nucleotide catalysis of amino acids via CO2 fixation would favor protocell division and growth. In 2017, a computational simulation of a protocell at an alkaline hydrothermal vent environment showed that "Some hydrophobic amino acids chelate FeS nanocrystals, producing three positive feedbacks: (i) an increase in catalytic surface area; (ii) partitioning of FeS nanocrystals to the membrane; and (iii) a proton motive active site for carbon fixing that mimics the enzyme Ech". Maximal ATP synthesis would have occurred at high water activity in freshwater and high concentrations of Mg2+ and Ca2+ prevented synthesis of ATP, however the concentrations of divalent cations in Hadean oceans were much lower that in modern oceans and alkaline hydrothermal vent concentrations of Mg2+ and Ca2+ are typically lower than in the ocean. Such environments would have generated Fe3+ which would have promoted ADP phosphorylation. The mixture of seawater and alkaline hydrothermal vent fluid can promote cycling between Fe3+ and Fe2+. Experimental research of biomimetic prebiotic reactions such as the reduction of NAD and phosphoryl transfer also support an origin of life occurring at an alkaline hydrothermal vent .

The 6 of the 11 metabolic intermediates in reverse Krebs cycle promoted by Fe, Zn2+, and Cr3+ in acidic conditions imply that protocells possibly emerged in locally metal rich and acidic terrestrial hydrothermal fields. The acidic conditions are seemingly consistent with the stabilization of RNA. These hydrothermal fields would have exhibited cycling of freezing and thawing and a variety of temperature gradients that would promote nonenzymatic reactions of gluconeogenesis, nucleobase synthesis, nonenzymatic polymerization, and RNA replication. Wet dry cycling of hydrothermal fields would polymerize RNA and peptides, protocell aggregation in a moist gel phase during wet dry cycling would allow diffusion of metabolic products across neighboring protocells. Protocell aggregation could be described as a primitive version of horizontal gene transfer. Fatty acid vesicles would be stabilized by polymers in the presence of Mg2+ required for ribozyme activity. These prebiotic processes might have occurred in shaded areas that protect the emergence of early cellular life under ultraviolet irradiation. Long chain alcohols and monocarboxylic acids would have also been synthesized via Fischer–Tropsch synthesis. Hydrothermal fields would also have precipitates of transition metals Geothermal convection could also be a source of energy for the emergence of the proton motive force, phosphoryl group transfer, coupling between oxidation reduction, and active transport. The abundance of subaerial hydrothermal fields would have been rare and offered no protection from either meteorites or ultraviolet irradiation. Clay minerals at subaerial hydrothermal fields would absorb organic reactants. Pyrophosphate has low solubility in water and can't be phosphorylated without a phosphorylating agent.