The Great Oxygenation Event and its Implications
Before the appearance of cyanobacteria, free oxygen was not prevalent in the atmosphere. However, once cyanobacteria appeared that changed drastically. Cyanobacteria produce their own energy through the process of photosynthesis. As a result, they produced oxygen which led to the death of much life on Earth (who had thrived in an anaerobic environment). Later, cyanobacteria became incorporated into a primitive plant cell via endosymbiosis. Chloroplasts in plants today are the same cyanobacteria that began producing oxygen nearly 3 billion years ago.