Source: “The Bacteria That Changed the World.” Accessed July 29, 2020. https://evolution.berkeley.edu/evolibrary/news/170503_cyanobacteria. 2.4 billion years ago, anaerobic cyanobacteria created the Great Oxygenation Event, where they poisoned the planet with their waste product - oxygen. There is only one process that can produce so much oxygen - photosynthesis. At that time, Earth was populated exclusively by single-celled organisms (Archaea and Bacteria) and only cyanobacteria could perform photosynthesis. Their blue-green colour comes from chlorophyll. Photosynthesis doesn't always produce oxygen, but in the case of cyanobacteria it does. The closest known relatives of cyanobacteria - Melainabacteria (discovered 2013) and Sericytochromatia (discovered 2017) - cannot photosynthesize. They don't have traces of the genetic machinery necessary for this, so it's likely that they never had the ability. The conclusion is that the most recent common ancestor of the three lineages could not photosynthesize. How did cyanobacteria acquire this ability? All the bacteria that can photosynthesize are genetically distant from each other. This leaves horizontal transfer as the method by which cyanobacteria acquired photosynthesis. Scientists still don't know which bacteria lineage was the first to evolve photosynthesis. Humans and most multi-cellular organisms are limited to vertical transfer. However, horizontal transfer is common among single-celled organisms. This is partly why bacteria can evolve so quickly to become resistant to antibiotics. It is thought that cyanobacteria acquired photosynthesis through horizontal transfer, and then later evolved to produce oxygen as part of the process. "Once Earth's atmosphere was full of oxygen, the stage was set for the evolution of aerobic respiration - the process that uses oxygen to convert food into usable energy."" [[The stage is a metaphor for a static space that sits in the background]] - is this a misleading metaphor in this case? Aerobic respiration spread via horizontal transfer to cyanobacteria, melainabacteria and sericytochromatia, and other bacteria, including the lineage that gave rise to our mitochondria, which was swallowed up by our ancestor, the first eukaryote. Another eukaryote engulfed cyanobacteria and evolved into a choloroplast-bearing lineage, the ancestor of plants.