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Unformatted text preview: Nitrogen produced by organisms including denitrifying bacteria Micro-organisms that cannot tolerate oxygen killed off 2 billion years ago Oxygen results in ozone layer Ozone layer filters out harmful ultra violet light 1 billion years ago This allows evolution of new organisms Atmosphere for the past 200 million years: 20% Oxygen Small amounts of other gases including water vapour, carbon dioxide, noble gases 80% Nitrogen...
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- Fall '11