The hydrogen, methane and ammonia biosphere on early Earth

Current paradigms for the origin and evolutions of life and the environment on early Earth have been based on the premises that carbon dioxide (CO₂) and nitrogen gas (N₂) were the principal constituents of the atmosphere and the sources of carbon and nitrogen for organisms today. Based on thermodynamic analyses of the redox state of the Hadean (4.56-4.0 Ga) mantle, the atmospheric compositions during the magma-ocean stage, and the effects of submarine hydrothermal fluids on the atmosphere of the ocean-covered Late-Hadean Earth, we suggest that life evolved sometime during 4.50-3.9 Ga under a reducing atmosphere, rich in hydrogen (H₂), methane (CH₄), and ammonia (NH₃), but very poor in CO₂, which was similar to today's atmosphere on Jupiter. The ocean water was alkaline (pH = 10 ± 1) and poor in Fe^2- and S^2-. CH₄ and NH₃ were the principal sources for carbon nitrogen and energy of the first organisms, greenhouse gases, and the UV shield on early Earth. The first organisms on Earth were possibly aerobic phototrophic methanotrophs, either oxygenic and/or anoxygenic. They evolved in micro-aerobic environments, which were created by the photodissociation of H₂O on the surfaces of photocatalytic minerals (e.g., rutile (TiO₂), pyrrhotite (FeS)) that accumulated in shallow water bodies on tropical islands. The enzymes for oxygenic Photosystem II (PSII) and PSI may also have evolved in the Late-Hadean microaerobic environments, facilitating the appearance of cyanobacteria. The symbiotic relationships between CO₂-generating methanotrophs and the CH₄-generating, H₂-oxidizing methanogens and cyanobacteria characterized the first biosphere. Plate tectonics has played a key role in the transformation of the H₂-CH₄-NH₃-rich to CO₂-N₂-rich atmosphere and biosphere by ~ 3.9 Ga because of the increased oxidation of the mantle, caused by the continuous subduction of the oxidized and hydrated oceanic crust into the mantle. Our study suggests that the search for life in the universe should be directed toward planets with a H₂-CH₄--NH₃-rich atmosphere, as well as to those with a CO₂-N₂-rich atmosphere.