https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Great_Oxidation_Event This is what I am talking about. This perhaps predates

https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Great_Oxidation_Event This is what I am talking about. This perhaps predates even symbiogenesis or perhaps drove it. Anaerobic lineages died out in the oxygen rich environment that the first forms of photosynthetic life created. Cellular respiration using oxygen, as animals do, probably evolved after the atmosphere was already oxygen rich. There is a huge fitness advantage if you cannot make use of light energy, but can absorb cells that can and maintain them as photosynthetic eukaryotes do today through symbiosis of the cyanobacteria that became cloroplasts. What I like about this example are the parallels to modern times. The great oxidation event was an extinction event brought on not by external factors, but by a life form producing gas that changes the composition of the atmosphere and leads to dramatic effects across the planet. Not unlike us humans today with our CO2 production.