Perhaps a more revealing way to look at things is to see that the cyanobacteria essentially made the Earth's atmosphere toxic with their excreted metabolic by-product. Life, however, appears almost infinitely adaptable and so eventually organisms emerged that could make use of the toxic by-product by means of incorporating it into their respiratory process. Today we think of oxygen as essential for life but the truth is that animals have simply learned to make use of what was poison, and plants & fungi have learned to tolerate it despite its corrosive effects.

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