Development of the Earth’s atmosphere

• Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years.
• One theory suggests that during the first billion years of the Earth’s existence there was intense volcanic activity, which 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 mainly of carbon dioxide with little or no oxygen gas.
• Volcanoes also produced nitrogen, which gradually built up in the atmosphere, and there may have been small proportions of methane and ammonia.
• When the oceans formed, carbon dioxide dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of carbon dioxide in the atmosphere.
• Algae and plants produced the oxygen that is now in the atmosphere by photosynthesis.
• Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.
• Photosynthesis by algae and plants also decreased the percentage of carbon dioxide in the atmosphere. Carbon dioxide was also used up in the formation of sedimentary rocks, such as limestone, and fossil fuels such as coal, natural gas and oil.