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Abstract
(2001) |
The
Origin of Life at a submarine alkaline seepage
Michael J. Russell, Allan J. Hall, Laiq Rahman & Dugald Turner Abstract (2001) The Earth agglomerates and heats. Volcanoes evolve carbon oxides, methane and pyrophosphate. Convection cells, in the planetary interior, begin the cooling process. An acidulous Hadean ocean condenses from the carbon oxide sky. Stratospheric smogs absorb a proportion of the Sun's rays. The now cool ocean leaks into the stressed crust and convects. Acid springs of high temperature, coupled to emergent magmatic plumes, emit ferrous iron and other transition metals to the ocean. Solar energy oxidises some iron to the ferric state, generating a dispersed positive terminal. Cooler alkaline waters emanate from the deep ocean floor, bearing hydrogen, methane, ammonia, formaldehyde, cyanide, thiols and hydrosulfide - molecules reduced from water and carbon oxides by reaction with ferrous silicate, residual nickeliferous iron and ferrous sulfide. Where these waters seep into the ocean, mounds, comprising layers of ferrous sulfide and green rust flocculants and films, arise in the ocean darkness. Here the reduced organic molecules are filtered and adsorbed. Concentrated thus they react to form glyceraldehyde, amino acids, and the components of nucleosides. The fluids are frustrated in their further attempt to mix thoroughly with their oceanic source by the spontaneous precipitation of a "biomorphic" barrier of colloidal iron compounds. Nucleotides are then assembled in green rust and refluxed. The thermal potential begins to be dissipated but the chemical potential is dammed. The mounds are negative electrodes. The Earth is a giant photo-electrochemical cell. The ocean and the iron sulfide are the electrolytes. Ferric iron, the potential positive electrode, is eddy-pumped by strong lunar tide to the ocean bottom. Iron sulfide and hydroxide bubbles inflate fitfully at the seepage. Though the hydrothermal solution is constrained, electrons escape from adsorbed hydrogen through the conducting layers of iron monosulfide, drawn to reduce the photolytic ferric iron. The now activated hydrogen takes part in further extempore dynamic combinatorial chemosynthesis on mineral surfaces at around 50°C. And there is spasmodic invasion of the iron sulfide/hydroxide barrier, by protons, pyrophosphate and carbonic acid, through iron sulfide-walled micro-channels. The nucleotides poison the iron sulfide but spell the password to a coevolutionary future, with peptides, "through a molecular passage". The side chains of particular amino acids register to fitting nucleotide triplet clefts. Keyed in, the amino acids are polymerised, through acid-base catalysis, to alpha chains by invading protons. The resulting short protopeptides sequester ready-made iron sulfide clusters to form ferredoxins, ubiquitous proteins with the longest evolutionary pedigree. These take-over the role of catalyst and electron transfer agent from the iron(nickel) sulfides, promote further chemosynthesis and so support the hatchery, the electrochemical reactor, from which they sprang. Peptides, the would-be tenuous outer-most filaments on iron monosulfide, continually peel away from bound RNA, the penultimate band, driven by the proton-motive force. Reactions and interactions fall into step as they negotiate the first biochemical pathways. This hydrothermal circuitry offers a continuous supply of electricity and proticity at a voltage appropriate for the onset of Life in the dark, a kinetic structure emerging in a "gradual instant", yet born to persist and make waste while the sun shines. From these ebullient beginnings, the emergent prokaryotes decouple from the immediate hydrothermal system and individuate. Other electron donors and acceptors are exploited, some made newly available through the geochemical and biochemical evolution of the planet. And other transition metal centres appropriate to particular potentials, are co-opted, though simple structural and dynamic patterns are re-membered. Contention ensues though genes are exchanged. Lower and higher temperature niches are colonised. The chirality war looms. Photosynthesis beckons. Symbionts will be conscripted from complex, well-populated prokaryotic consortia, RNAs imbricated and eukaryotes conceived. Yet springs and seepages remain the refugia from extraterrestrial assaults to the volatisphere, and serve as the forcing frames for evolutionary complexification. Inspiration: Krishnamurthy et al. 1999.
Origins of Life, 29, 139-152. |
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Abstract
(2001) |
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last updated: 18th December 2001 |