4.5 billion years ago
- When the Earth first formed, its surface was molten.
- As it cooled surface rocks formed.
- An atmosphere formed from volcanic gases: carbon dioxide, steam, ammonia and methane.
3 billion years ago
- The Earth cooled enough for the steam to condense and form the oceans.
- The main gas in the atmosphere was carbon dioxide.
- Simple plants evolved.
2 billion years ago
- The first plant produced oxygen, this killed off many other life forms.
1 billion years ago
- The excess oxygen in the atmosphere reacted in the presence of UV light to form ozone.
- This ozone filtered out most of the damaging UV light and allowed more complicated life forms to evolve.
The present time
- The atmosphere is approximately 1/5 oxygen and 4/5 nitrogen.
- carbon dioxide is absorbed by plants, shellfish and dissolved in the oceans.
- It is given out by burning fuels, the decay of organic waste and released from volcanoes.
- ammonia in the air reacted with oxygen to form nitrogen. More nitrogen was formed by bacteria in the soil.