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...the life of the planet began the long, slow process of modulating and regulating the physical conditions of the planet. The oxygen in today's atmosphere is almost entirely the result of photosynthetic living, which had its start with the appearance of blue-green algae among the microorganisms.

Lewis Thomas , in Late Night Thoughts on Listening to Mahler's Ninth Symphony
life earth science atmosphere oxygen microorganisms formation-of-the-earth origin-of-oxygen