The Once and Future Gas: Methane's Multifunctional Roles in Earth's Evolution and Potential as a Biosignature

Methane (CH 4 ) is a simple molecule that, due to its radiative forcing, wields an outsized impact on planetary heat balance. Methane is formed by diverse abiotic pathways across a range of pressures and temperatures. Biological methanogenesis for anaerobic respiration uses a terminal nickel-containing enzyme and is limited to the archaeal domain of life. Methane can also be produced in aerobic microbes during bacterial methylphosphonate and methylamine degradation and via nonenzymatic reactions during oxidative stress. Abiotic CH 4 is produced via thermogenic reactions and during serpentinization reactions in the presence of metal catalysts. Reconstructions of CH 4 cycling over geologic time are largely inferential. Throughout Earth's history, CH 4 has probably been the second most important climate-forcing greenhouse gas after carbon dioxide. Biological methanogenesis has likely dominated CH 4 flux to Earth's atmosphere for the past ∼3.5 billion years, during which time CH 4 is thought to have generally declined as atmospheric oxygen has risen. Here we review the evolution of the CH 4 cycle over Earth's history, showcasing the multifunctional roles CH 4 has played in Earth's climate, prebiotic chemistry, and microbial metabolisms. We also discuss the future of Earth's atmospheric CH 4 , the cycling of CH 4 on other planetary bodies in the Solar System (with special emphasis on Titan), and the potential of CH 4 as a biosignature on Earth-like extrasolar planets. ▪ Before life arose on Earth, abundant atmospheric CH 4 in Earth's early atmosphere was likely key for establishment of habitable conditions and production of organic molecules for prebiotic chemistry. ▪ Biological methanogenesis for anaerobic respiration is only known to exist in some groups of anaerobic archaea, but CH 4 can also be produced via enzymatic and nonenzymatic biological pathways that are not directly coupled to energy conservation. The relative importance of each of these pathways to the global CH 4 cycle is a topic of active research, but archaeal methanogenesis dominates all other biological pathways for CH 4 generation. ▪ As atmospheric O 2 rose over Earth history, models suggest that atmospheric CH 4 declined; in the distant deoxygenated future, atmospheric CH 4 is predicted to rise again. ▪ Future missions to Titan will aid in understanding the complex organic chemistry on the only other planetary body in our Solar System with an active CH 4 cycle.