3) Up to about 3000 million years ago, there was no oxygen in the Earth's
atmosphere. Today, about one fifth of the atmosphere is oxygen gas.
Use the information on page 6 to suggest what caused this change.
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20
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3000 million years ago the first photosynthesis appear
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11
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So because of more plants that use photosynthesis and give out oxygen, so the ratio of oxygen is increased
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