For 200 million years the proportions of different gases in the atmosphere have been much the same as they are today: · about four-fifths (80 %) ____; · about one-fifth (20 %) oxygen; · small proportions of various other gases, including carbon dioxide, ____ and ____ vapour. During the first billion years of the Earth's existence there was intense ____ activity. This activity released the gases which then formed the early atmosphere and water vapour which ____ to form the ____. During this period the Earth's atmosphere was probably mainly ____ and there would have been little or no ____ gas (like the atmospheres of Mars and Venus today). There would also have been water vapour and small proportions of methane and ____. When ____ evolved and successfully colonised most of the Earth's surface: · the atmosphere gradually became more and more "polluted" with oxygen. This meant that, gradually, there were fewer habitats suitable for microorganisms which could not tolerate oxygen; · most of the carbon from the carbon dioxide in the air gradually became locked up in ____ rocks as carbonates and ____; · the ____ and ammonia in the atmosphere reacted with the oxygen.

Atmosphere

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