Iron–sulfur world hypothesis
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 from extant biochemistry in conjunction with chemical experiments.
Origin of lifeEdit
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.
The water gas shift reaction (CO + H2O → CO2 + H2) occurs in volcanic fluids with diverse catalysts or without catalysts. The combination of ferrous sulfide and hydrogen sulfide as reducing agents in conjunction with pyrite formation – FeS + H2S → FeS2 + 2H+ + 2e− (or H2 instead of 2H+ + 2e−) – has been demonstrated under mild volcanic conditions. This key result has been disputed. Nitrogen fixation has been demonstrated for the isotope 15N2 in conjunction with pyrite formation. Ammonia forms from nitrate with FeS/H2S as reductant. Methylmercaptan [CH3-SH] and carbon oxysulfide [COS] form from CO2 and FeS/H2S, or from CO and H2 in the presence of NiS.
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. They also serve for energy coupling with endergonic reactions, notably the formation of (phospho)anhydride compounds. 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 proved 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 proved experimentally 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 begins with the formation of primitive lipids (e.g. fatty acids or isoprenoid acids) 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. 
William Martin and Michael Russell suggest that the first cellular life forms may have evolved inside alkaline hydrothermal vents at seafloor spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membraneous metal sulfide walls. Therefore, these structures would resolve several critical points germane to Wächtershäuser's suggestions at once:
- the micro-caverns provide a means of concentrating newly synthesised molecules, thereby increasing the chance of forming oligomers;
- the steep temperature gradients inside the hydrothermal vent allow for establishing "optimum zones" of partial reactions in different regions of the vent (e.g. monomer synthesis in the hotter, oligomerisation in the cooler parts);
- the flow of hydrothermal water through the structure provides a constant source of building blocks and energy (chemical disequilibrium between hydrothermal hydrogen and marine carbon dioxide);
- the model allows for a succession of different steps of cellular evolution (prebiotic chemistry, monomer and oligomer synthesis, peptide and protein synthesis, RNA world, ribonucleoprotein assembly and DNA world) in a single structure, facilitating exchange between all developmental stages;
- synthesis of lipids as a means of "closing" the cells against the environment is not necessary, until basically all cellular functions are developed.
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 to bona 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 of lipids as directed by genetically encoded peptides is consistent with the presence of completely different types of membrane lipids in archaea and bacteria (plus eukaryotes). The kind of vent at the foreground of their suggestion is chemically more similar to the warm (ca. 100 °C) off ridge vents such as Lost City than to the more familiar black smoker type vents (ca. 350 °C).
In an abiotic world, a thermocline of temperatures and a chemocline in concentration is associated with the pre-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 which acetic acid production and its eventual oxidization 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 developing cell 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 increase metabolic complexity within the cell membrane, whilst leading to greater environmental simplicity in the external environment. In principle, this could lead to the development of complex catalytic sets capable of self-maintenance.
Russell adds a significant factor to these ideas, by pointing out that semi-permeable mackinawite (an iron sulfide mineral) and silicate membranes could naturally develop under these conditions and electrochemically link reactions separated in space, if not in time.
- Wächtershäuser, Günter (1988-12-01). "Before enzymes and templates: theory of surface metabolism". Microbiol. Mol. Biol. Rev. 52 (4): 452–84. doi:10.1128/MMBR.52.4.452-484.1988. PMC 373159. PMID 3070320.
- Wächtershäuser, G (January 1990). "Evolution of the first metabolic cycles". Proceedings of the National Academy of Sciences of the United States of America. 87 (1): 200–04. Bibcode:1990PNAS...87..200W. doi:10.1073/pnas.87.1.200. PMC 53229. PMID 2296579.
- Günter Wächtershäuser, G (1992). "Groundworks for an evolutionary biochemistry: The iron-sulphur world". Progress in Biophysics and Molecular Biology. 58 (2): 85–201. doi:10.1016/0079-6107(92)90022-X. PMID 1509092.
- Günter Wächtershäuser, G (2006). "From volcanic origins of chemoautotrophic life to Bacteria, Archaea and Eukarya". Philosophical Transactions of the Royal Society B: Biological Sciences. 361 (1474): 1787–806, discussion 1806–8. doi:10.1098/rstb.2006.1904. PMC 1664677. PMID 17008219.
- Wächtershäuser, Günter (2007). "On the Chemistry and Evolution of the Pioneer Organism". Chemistry & Biodiversity. 4 (4): 584–602. doi:10.1002/cbdv.200790052. PMID 17443873. S2CID 23597542.
- Seewald, Jeffrey S.; Mikhail Yu. Zolotov; Thomas McCollom (January 2006). "Experimental investigation of single carbon compounds under hydrothermal conditions". Geochimica et Cosmochimica Acta. 70 (2): 446–60. Bibcode:2006GeCoA..70..446S. doi:10.1016/j.gca.2005.09.002. hdl:1912/645.
- Taylor, P.; T. E. Rummery; D. G. Owen (1979). "Reactions of iron monosulfide solids with aqueous hydrogen sulfide up to 160°C". Journal of Inorganic and Nuclear Chemistry. 41 (12): 1683–87. doi:10.1016/0022-1902(79)80106-2. Retrieved 2009-05-02.
- Drobner, E.; H. Huber; G. Wächtershäuser; D. Rose; K. O. Stetter (1990). "Pyrite formation linked with hydrogen evolution under anaerobic conditions" (PDF). Nature. 346 (6286): 742–44. Bibcode:1990Natur.346..742D. doi:10.1038/346742a0. S2CID 4238288.
- Cahill, C. L.; L. G. Benning; H. L. Barnes; J. B. Parise (June 2000). "In situ time-resolved X-ray diffraction of iron sulfides during hydrothermal pyrite growth". Chemical Geology. 167 (1–2): 53–63. Bibcode:2000ChGeo.167...53C. doi:10.1016/S0009-2541(99)00199-0.
- Mark Dorr, Mark; Johannes Käßbohrer; Renate Grunert; Günter Kreisel; Willi A. Brand; Roland A. Werner; Heike Geilmann; Christina Apfel; Christian Robl; Wolfgang Weigand (2003). "A Possible Prebiotic Formation of Ammonia from Dinitrogen on Iron Sulfide Surfaces". Angewandte Chemie International Edition. 42 (13): 1540–43. doi:10.1002/anie.200250371. PMID 12698495.
- Blöchl, E; M Keller; G Wächtershäuser; K O Stetter (1992). "Reactions depending on iron sulfide and linking geochemistry with biochemistry". Proceedings of the National Academy of Sciences of the United States of America. 89 (17): 8117–20. Bibcode:1992PNAS...89.8117B. doi:10.1073/pnas.89.17.8117. PMC 49867. PMID 11607321.
- Heinen, Wolfgang; Anne Marie Lauwers (1996-04-01). "Organic sulfur compounds resulting from the interaction of iron sulfide, hydrogen sulfide and carbon dioxide in an anaerobic aqueous environment" (PDF). Origins of Life and Evolution of Biospheres. 26 (2): 131–50. Bibcode:1996OLEB...26..131H. CiteSeerX 10.1.1.967.5285. doi:10.1007/BF01809852. hdl:2066/29485. PMID 11536750. S2CID 9391517.
- Huber, Claudia; Günter Wächtershäuser (1997-04-11). "Activated Acetic Acid by Carbon Fixation on (Fe,Ni)S Under Primordial Conditions". Science. 276 (5310): 245–47. doi:10.1126/science.276.5310.245. PMID 9092471. S2CID 40053445.
- Günter Wächtershäuser; Michael W. W. Adams (1998). "The case for a hyperthermophilic, chemolithoautotrophic origin of life in an iron-sulfur world". In Juergen Wiegel (ed.). Thermophiles: The Keys to Molecular Evolution and the Origin of Life. pp. 47–57. ISBN 9780748407477.
- Chandru, Kuhan; Gilbert, Alexis; Butch, Christopher; Aono, Masashi; Cleaves, Henderson James II (21 July 2016). "The Abiotic Chemistry of Thiolated Acetate Derivatives and the Origin of Life". Scientific Reports. 6: 29883. Bibcode:2016NatSR...629883C. doi:10.1038/srep29883. PMC 4956751. PMID 27443234.
- Reeves, Eoghan P.; McDermott, Jill M.; Seewald, Jeffrey S. (April 15, 2014). "The origin of methanethiol in midocean ridge hydrothermal fluids". Proceedings of the National Academy of Sciences of the United States of America. 111 (15): 5474–79. Bibcode:2014PNAS..111.5474R. doi:10.1073/pnas.1400643111. PMC 3992694. PMID 24706901.
- Huber, Claudia; Günter Wächtershäuser (2006-10-27). "α-Hydroxy and α-Amino Acids Under Possible Hadean, Volcanic Origin-of-Life Conditions". Science. 314 (5799): 630–62. Bibcode:2006Sci...314..630H. doi:10.1126/science.1130895. PMID 17068257. S2CID 94926364.
- Cody, George D.; Nabil Z. Boctor; Timothy R. Filley; Robert M. Hazen; James H. Scott; Anurag Sharma; Hatten S. Yoder (2000-08-25). "Primordial Carbonylated Iron-Sulfur Compounds and the Synthesis of Pyruvate". Science. 289 (5483): 1337–40. Bibcode:2000Sci...289.1337C. doi:10.1126/science.289.5483.1337. PMID 10958777. S2CID 14911449.
- Huber, Claudia; Günter Wächtershäuser (February 2003). "Primordial reductive amination revisited". Tetrahedron Letters. 44 (8): 1695–97. doi:10.1016/S0040-4039(02)02863-0.
- Huber, Claudia; Günter Wächtershäuser (1998-07-31). "Peptides by Activation of Amino Acids with CO on (Ni,Fe)S Surfaces: Implications for the Origin of Life". Science. 281 (5377): 670–72. Bibcode:1998Sci...281..670H. doi:10.1126/science.281.5377.670. PMID 9685253. S2CID 33706837.
- Huber, Claudia; Wolfgang Eisenreich; Stefan Hecht; Günter Wächtershäuser (2003-08-15). "A Possible Primordial Peptide Cycle". Science. 301 (5635): 938–40. Bibcode:2003Sci...301..938H. doi:10.1126/science.1086501. PMID 12920291. S2CID 2761061.
- Wächtershäuser, Günter (2000-08-25). "Origin of Life: Life as We Don't Know It". Science. 289 (5483): 1307–08. doi:10.1126/science.289.5483.1307. PMID 10979855. (requires nonfree AAAS member subscription)
- "Before Enzymes and Templates: Theory of Surface Metabolism" (PDF). Microbiology and Molecular Biology Reviews. December 1998.
- Martin, William; Michael J Russell (2003). "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 358 (1429): 59–83, discussion 83–85. doi:10.1098/rstb.2002.1183. PMC 1693102. PMID 12594918.
- Martin, William; Michael J Russell (2007). "On the origin of biochemistry at an alkaline hydrothermal vent". Philos Trans R Soc Lond B Biol Sci. 362 (1486): 1887–925. doi:10.1098/rstb.2006.1881. PMC 2442388. PMID 17255002.
- Michael Russell, Michael (2006). "First Life". American Scientist. 94 (1): 32. doi:10.1511/2006.1.32.
- Russell, Michael (Ed), (2010), "Origins, Abiogenesis and the Search for Life in the Universe" (Cosmology Science Publications)[ISBN missing]