9.1

• For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today: around 4/5 nitrogen and 1/5 oxygen, with small proportions of various other gases such as CO2, H2O(g), and noble gases.
• The Earth’s early atmosphere was similar to the atmospheres of Mars and Venus today, mainly CO2 with little or no O2.
• Water vapour condensed to form the oceans.
• Volcanoes produced nitrogen (N2) which gradually built up in the atmosphere and there may have been small proportions of methane (CH4) and ammonia (NH3).
• CO2 dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of CO2 in the atmosphere.
• Algae and plants produced the O2 that is now in the atmosphere by photosynthesis: 6CO2 + 6H2O -> C6H12O6 + 6O2.
• 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 % oxygen gradually increased to a level that enabled animals to evolve.
• Algae and plants decreased the % CO2 in the atmosphere by photosynthesis.
• CO2 was also decreased by the formation of sedimentary rocks that contain carbon (e.g. limestone and coal) and by the production of fossil fuels from the remains of dead plants and animals when they decayed.