Then the world began to change with the emergence of cyanobacteria—blue-green algae that use the sun’s rays to power photosynthesis. This made cyanobacteria much more effective at generating energy, giving them a huge evolutionary advantage. Their numbers boomed. Over a period of several hundred million years, they pumped vast amounts of oxygen—a by-product of photosynthesis—into the oceans and atmosphere. This Great Oxygenation Event transformed the planet.[15]
