Theiron–sulfur world hypothesis is a set of proposals for theorigin of life and the early evolution of life advanced in a series of articles between 1988 and 1992 byGünter Wächtershäuser, aMunich patent lawyer with a degree in chemistry, who had been encouraged and supported by philosopherKarl R. Popper to publish his ideas. The hypothesis proposes that early life may have formed on the surface ofiron sulfide minerals, hence the name.[1][2][3][4][5][excessive citations] It was developed byretrodiction (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 (predominantlyiron andnickel, but also perhapscobalt,manganese,tungsten andzinc). 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 andhydrogen 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 reductivecitric 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.
Thewater–gas shift reaction (CO + H2O → CO2 + H2) occurs in volcanic fluids with diverse catalysts or without catalysts.[6] The combination offerrous sulfide (FeS,troilite) andhydrogen sulfide (H
2S) asreducing agents (both reagents are simultaneouslyoxidized in the reaction here under creating thedisulfide bond, S–S) in conjunction withpyrite (FeS
2) formation:
has been demonstrated under mildvolcanic conditions.[7][8] This key result has been disputed.[9]Nitrogen fixation has been demonstrated for theisotope15N2 in conjunction with pyrite formation.[10]Ammonia forms fromnitrate with FeS/H2S asreductant.[11]Methylmercaptan [CH3-SH] andcarbon oxysulfide [COS] form from CO2 and FeS/H2S,[12] or from CO and H2 in the presence ofNiS.[13]
Reaction ofcarbon monoxide (CO),hydrogen sulfide (H2S) andmethanethiol CH3SH in the presence ofnickel sulfide andiron sulfide generates the methylthioester of acetic acid [CH3-CO-SCH3] and presumablythioacetic acid (CH3-CO-SH) as the simplest activated acetic acid analogues ofacetyl-CoA. These activatedacetic acid derivatives serve as starting materials for subsequentexergonic synthetic steps.[13] They also serve for energy coupling withendergonic reactions, notably the formation of(phospho)anhydride compounds.[14] 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[15] the highest CH3SH and CO concentrations respectively measured to date in a naturalhydrothermal vent fluid.[16]
Reaction ofnickel hydroxide withhydrogen cyanide (HCN) (in the presence or absence offerrous hydroxide,hydrogen sulfide ormethyl mercaptan) generates nickelcyanide, which reacts withcarbon monoxide (CO) to generate pairs of α-hydroxy andα-amino acids: e.g.glycolate/glycine,lactate/alanine,glycerate/serine; as well aspyruvic acid in significant quantities.[17]Pyruvic acid is also formed at high pressure and high temperature from CO, H2O, FeS in the presence of nonyl mercaptan.[18] Reaction ofpyruvic acid or other α-keto acids withammonia in the presence offerrous hydroxide or in the presence offerrous sulfide andhydrogen sulfide generatesalanine or otherα-amino acids.[19] Reaction ofα-amino acids in aqueous solution with COS or with CO and H2S generates apeptide cycle whereindipeptides,tripeptides etc. are formed and subsequently degraded via N-terminalhydantoinmoieties and N-terminalurea moieties and subsequent cleavage of the N-terminal amino acid unit.[20][21][22]
Proposed reaction mechanism for reduction of CO2 on FeS: Ying et al. (2007)[citation needed] have shown that direct transformation ofmackinawite (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.[citation needed] 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 theorigin of life and ending with thelast universal common ancestor (LUCA). According to the iron–sulfur world theory it covers acoevolution of cellular organization (cellularization), thegenetic machinery andenzymatization of themetabolism.
Cellularization occurs in several stages. It may have begun with the formation of primitive lipids (e.g.fatty acids orisoprenoids) in the surfacemetabolism. These lipids accumulate on or in the mineral base. Thislipophilizes the outer or inner surfaces of the mineral base, which promotes condensation reactions overhydrolytic reactions by lowering theactivity of water and protons.
In the next stagelipid 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 (sensuKandler) because they allow frequent exchange of genetic material (e.g. by fusions). According toWoese, this frequent exchange ofgenetic material is the cause for the existence of the common stem in the tree of life and for a very rapid early evolution.[23] 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".[24]
Metabolic intermediates inglycolysis and thepentose phosphate pathway such as glucose, pyruvate,ribose 5-phosphate, anderythrose-4-phosphate are spontaneously generated in the presence of Fe(II).[25]Fructose 1,6-biphosphate, a metabolic intermediate ingluconeogenesis, was shown to have been continuously accumulated but only in a frozen solution. The formation of fructose 1,6-biphosphate was accelerated bylysine andglycine which implies the earliest anabolic enzymes were amino acids.[26] It had been reported that 4Fe-4S, 2Fe-2S, and mononuclear iron clusters are spontaneously formed in low concentrations of cysteine and alkaline pH.[27] Methyl thioacetate, a precursor toacetyl-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 toadenosine triphosphate and is capable of phosphorylatingribose andnucleosides. 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.[28] Thermophoresis at hydrothermal vent pores can concentrate polyribonucleotides,[29] but it remains unknown as to how it could promote coding and metabolic reactions.[30]
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.[31] 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".[32] 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 than 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+.[33] Experimental research of biomimetic prebiotic reactions such as the reduction of NAD+[34] and phosphoryl transfer[35] also support an origin of life occurring at an alkaline hydrothermal vent .
William Martin andMichael Russell suggest that the firstcellular life forms may have evolved insidealkalinehydrothermal vents atseafloor spreading zones in thedeep sea.[36][37] These structures consist of microscale caverns that are coated by thin membraneousmetal sulfide walls. Therefore, these structures would resolve several critical points germane to Wächtershäuser's suggestions at once:
This model locates the "last universal common ancestor" (LUCA) within the inorganically formed physical confines of an alkaline hydrothermal vent, rather than assuming the existence of a free-living form of LUCA. The last evolutionary step en route tobona fide free-living cells would be the synthesis of a lipid membrane that finally allows the organisms to leave the microcavern system of the vent. This postulated late acquisition of the biosynthesis oflipids as directed by genetically encoded peptides is consistent with the presence of completely different types of membrane lipids inarchaea andbacteria (pluseukaryotes). The kind of vent at the foreground of their suggestion is chemically more similar to the warm (ca. 100 °C) offridge vents such asLost City than to the more familiarblack smoker type vents (ca. 350 °C).
In anabiotic world, athermocline of temperatures and achemocline in concentration is associated with thepre-biotic synthesis of organic molecules, hotter in proximity to the chemically rich vent, cooler but also less chemically rich at greater distances. The migration of synthesized compounds from areas of high concentration to areas of low concentration gives a directionality that provides both source and sink in a self-organizing fashion, enabling a proto-metabolic process by whichacetic acid production and its eventualoxidization can be spatially organized.
In this way many of the individual reactions that are today found in central metabolism could initially have occurred independent of any developingcell membrane. Each vent microcompartment is functionally equivalent to a single cell. Chemical communities having greater structural integrity and resilience to wildly fluctuating conditions are then selected for; their success would lead to local zones of depletion for important precursor chemicals. Progressive incorporation of these precursor components within a cell membrane would gradually increasemetabolic complexity within the cell membrane, whilst leading to greater environmental simplicity in the external environment. In principle, this could lead to the development of complexcatalytic sets capable ofself-maintenance.
Russell adds a significant factor to these ideas, by pointing out thatsemi-permeablemackinawite (an iron sulfide mineral) andsilicate membranes could naturally develop under these conditions and electrochemically link reactions separated in space, if not in time[clarification needed].[38][39]
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.[40] 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.[26] ATP synthesis and oxidation of ferrous iron via photochemical reactions or oxidants such as nitric oxide derived from lightning strikes, meteorite impacts, or volcanic emissions could have also occurred at hydrothermal fields.[41]
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.[42] These prebiotic processes might have occurred in shaded areas that protect the emergence of early cellular life under ultraviolet irradiation.[43] Long chain alcohols and monocarboxylic acids would have also been synthesized viaFischer–Tropsch synthesis.[44] Hydrothermal fields would also have precipitates of transition metals[4] and concentrated many elements includingCHNOPS.[45] 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.[4] It's noted byDavid Deamer and Bruce Damer that these environments seemingly resembleCharles Darwin's idea of a "warm little pond".[42]
The problems with the hypothesis of a subaerial hydrothermal field of abiogenesis is that the proposed chemistry doesn't resemble known biochemical reactions.[46] 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.[44] It doesn't offer explanations for the origin of chemiosmosis and differences between Archaea and Bacteria.[47]
