[M-IS03] アストロバイオロジー

[MIS03-07] The origin and evolution of life on the methane-rich early Earth

*Hiroshi Ohmoto¹ (1.Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802 USA)

キーワード：Evolution of life, Methane, Oxygen

The current paradigm for the evolution of Earth’s biosphere states that, throughout its history, CO₂ from subaerial volcanism has been the principal greenhouse gas responsible for maintaining the liquid ocean and the primary source for biosynthesis. However, prior to approximately four billion years (Ga) ago, Earth was mostly covered by ocean, and thus chemical constituents of the ocean and atmosphere were mostly supplied by submarine hydrothermal fluids, rather than by terrestrial volcanic gases. Results of thermodynamic computations suggest that: (1). The pre-biotic oceans and atmosphere were very reducing: rich in H₂, CH₄, and NH₃; poor in CO₂, CO, N₂, and H₂S; and free of SO₂ and O₂. (2). The Oparin-Urey-Miller’s model for the origin of life by lightning or UV radiation of the mixtures of reducing gases in the atmosphere is valid. (3). CH₄ was the principal greenhouse gas and the primary source for biosynthesis on early Earth. (4). The first organisms on Earth, emerged before ~3.9 Ga ago, were aerobic anoxygenic phototrophic methanotrophs and oxygenic phototrophs (i.e., ancestors of cyanobacteria), which were in symbiosis. A continuous subduction of the hydrated- and Fe³⁺-enriched oceanic crust into the mantle increased the water content and the oxidation state of the mantle, volcanic gases, and submarine hydrothermal fluids, decreased the ocean volume, and increased the subaerial land surface area. These changes transformed the CH₄-rich world to the O₂- and CO₂ rich world, created diverse environments (e.g., local anoxic basins), and promoted the evolution of diverse organisms, including a variety of heterotrophs, chemolithoautotrophs, and anoxygenic photoautotrophs by ~3.8 Ga ago. (6). Life may have evolved on other planets with methane-rich atmospheres.