Once upon a time, and by that, I mean 1.9 billion years ago, the atmosphere was filled with carbon dioxide and methane, and the first inklings of life had only just begun. Volcanic activity in the early oceans, and erosion off the few continents, enriched the water with iron and silica. Cyanobacteria bloomed in those mineral-rich seas, and they also produced at least one type of toxin: oxygen.