Recent research has shown that a particular algae has recently merged with a cyanobacteria (effectively making it an organelle) to allow it to fix nitrogen. This transition from endosymbiosis to organelle has happened notably a few times in evolutionarily history: 2.2 billion years ago a single-celled organism swallowed up a bacterium that became mitochondria and later some cells absorbed cyanobacteria, resulting in chloroplasts. #biology