The evolution of the atmosphere

• Theories about what was in the Earth’s early atmosphere and how the atmosphere was formed have changed and developed over time. Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years.
• 1 - One theory suggests that during the first billion years of the Earth’s existence there was intense volcanic activity that released gases that formed the early atmosphere and water vapour that condensed to form the oceans. At the start of this period the Earth’s atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas.
• 2 - Volcanoes also produced nitrogen which gradually built up in the atmosphere and there may have been small proportions of methane and ammonia.
• 3 - Over time the amount of carbon dioxide in the air was reduced. One way was by the gas dissolving in newly formed oceans, forming carbonates. These then precipitated into sediments, formed fossil fuels, or sea creatures used the dissolved carbon dioxide to form shells made of calcium carbonate.
• 4 - Another way that the levels of carbon dioxide decreased was due to the growth of algae and primitive plants that used this carbon dioxide for photosynthesis, which consequently produced oxygen - gradually increasing the amount of oxygen in the atmosphere.
• Photosynthesis

  carbon dioxide + water → glucose + oxygen

  6CO2 + 6H2O → C6H12O6 + 6O2
• 5 =- Over a few billion years plants evolved and the percentage of oxygen increased to a level that enabled animals to evolve. This nearly killed all other life on the planet, because of how reactive oxygen can be. This is referred to as The Great Oxygenation event.