The Lowdown on Noses
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چکیده
The Earth’s early environment was relatively poor in oxygen, and observations and modeling indicate that the rise in oxygen levels did not occur until long after oxygen-enriching bacterial photosynthesis had developed. Catling et al. (p. 839; see the Perspective by Kasting) explain the apparent disconnect between oxygenation and the start of photosynthesis by adding methanogenesis to the process. Photosynthesis split water into O2 and H2, and then methanogenesis used H2 to generate CH4. Finally, ultraviolet photolysis converted CH4 and O2 into CO2, O2, and H2. The escape of H2 into space allowed the buildup of O2.
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