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what was most the atmosphere formed of before plants started to evolve

what did the water vapor do when the earth cooled

what process did the plants produce us to produce oxygen

when the plants died they were buried under layers of sediment, what did the carbon that the plants took up turn into

what did the build up of oxygen in the atmosphere kill off

how was the build up of oxygen in the atmosphere a good thing as they were killing off early organisms. what were the other organisms called