Evolution

• Phase 1 - the surface was covered in volcanoes that erupted and released lots of gases. The early atmosphere was mostly carbon dioxide, with virtually no oxygen. Volcanic activity also released nitrogen, which built up over time, as well as water vapour and small amounts of methane.
• Phase 2 - When water vapour condensed, it formed oceans. Lots of carbon dioxide was removed from the early atmosphere as it dissolved in the oceans. Green plants and algae evolved and absorbed some of the carbon dioxide so they could carry out photosynthesis.
• Phase 2 - Later, marine animals evolved. Their shells and skeletons contained carbonates from the oceans. Some of the carbon became locked up in rocks and fossil fuels after the organisms died. When plankton and marine animals die, they fall to the seabed and get buried by layers of sediment. They become compressed and form sedimentary rocks, oil and gas. This traps the carbon and reduces the amount in the atmosphere.
• Phase 3 - Green plants and algae produced oxygen by photosynthesis. Algae evolved first, about 2.7 billion years ago. Then over the next billion years or so, green plants evolved. As oxygen levels built up, more complex life (like animals) could evolve. Eventually about 200 million years ago, the atmosphere reached a composition similar to what it is today.
• The atmosphere is made up of around 80 percent nitrogen, 20 percent oxygen and small amounts of other gases.