The atmosphere, then and now

The idea

For about the last 200 million years the atmosphere has been about four-fifths nitrogen and one-fifth oxygen, with small amounts of carbon dioxide, water vapour and noble gases. The early atmosphere probably formed from volcanic gases: mainly carbon dioxide, with little or no oxygen. Water vapour condensed to form the oceans. Algae and plants produced oxygen by photosynthesis, starting about 2.7 billion years ago. Carbon dioxide fell as it dissolved in the oceans, was used in photosynthesis, and was locked into sedimentary rocks and fossil fuels.

1. Describe the early atmosphere.
2. Explain how oxygen increased.
3. Explain how carbon dioxide decreased.

Watch out Evidence about the early atmosphere is limited because it was billions of years ago.