The early atmosphere and oceans
The earth's early atmosphere was formed from the gases that escaped from valcanoes. Main gases back then- water vapour and carbon dioxide because these gases are still produced by modern volcanoes. Rocks contain a record of the gases that were in the atomsphere at the time they formed. Oxygen is really reactive- reactions from the gas can be things such as -rusting metals -maintenance of fire. We can see the increase of oxygen by looking at rocks as more recent rocks contain iron oxide, but in older rocks the iron is not oxidised. This is evidence for the increase of oxygen levels. Also nitrogen has increased and carbon dioxide has dramatically decreased.
The early oceans was formed when all the water vapour in the atmosphere condensed and fell as rain to make oceans. The carbon dioxide dissolved into the oceans, causing the perecentage to decrease. Early plant life used this to photosynthesis which then released oxygen into the atmosphere, and also some rocks were formed. Then early animal life started to form as the oxygen levels increased and when they got up to 21% we was able to evolve.
Evolution of life caused major changes to the atmosphere.