The Earth's early atmosphere formed billions of years ago from gases given out by intense volcanic activity. These gases were mainly carbon dioxide, with water vapour and nitrogen, and small amounts of methane and ammonia. There was little or no oxygen.
As the Earth cooled, the water vapour condensed and fell as rain, forming the oceans. Today oxygen makes up about a fifth of the air, which is very different from the early atmosphere.
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Specification: Edexcel Combined Science (1SC0), GCSE. Topic: Exchange and transport in animals.
All 26 lessons in Exchange and transport in animals · All Edexcel GCSE Combined Science topics
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