within mixture reducing oxidizing gasses set stage primitive ecosystems evolve turn atmosphere also evolved ref name wills leaf primary site photosynthesis plants throughout history earth 's atmosphere biogeochemical cycles dynamic equilibrium planetary ecosystems history characterized periods significant transformation followed millions years stability ref name goldblatt evolution earliest organisms likely anaerobic methanogen microbes started process converting atmospheric hydrogen methane h sub sub co sub sub ch sub sub h sub sub anoxygenic photosynthesis reduced hydrogen
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