ANCIENT CYANOBACTERIA became the powerhouses of all green plant cells as chloroplasts, which “perform essential metabolic and biosynthetic functions of global significance,” including oxygenation of our planet, nitrogen fixation, and giant organic fluxes of trillions of metric tons per year. Not only stewards of all things green, but of the planetary cycles of carbon, nitrogen and other key elements.
http://www.sciencedirect.com/science/article/pii/S0960982204000909