surface layer consumed this oxygen nearly as fast as it was produced. This oxidation process might have been completed more rapidly, but lower solar luminosity (see fig. 6.2) and much higher atmospheric methane levels—at least 100 parts per million by volume[6]—slowed it down. A large quantity of methane in the atmosphere requires a huge abundance of methanogens (methane-producing bacteria) on Earth’s surface. These methanogens work in tandem with photosynthetic life to transform carbon dioxide