The Earth is about 4.6 billion years old, so the evidence for what the early atmosphere was like is limited. Theories about it have changed and developed over time as scientists have gathered new evidence.
One theory says that during the first billion years there was intense volcanic activity. Volcanoes released gases that formed the early atmosphere, and also released water vapour. As the Earth cooled, the water vapour condensed to form the oceans.
At the start of this period the atmosphere may have been like the atmospheres of Mars and Venus today, which are mainly carbon dioxide with little or no oxygen. Volcanoes also produced nitrogen, which gradually built up in the atmosphere. There may also have been small proportions of methane and ammonia.
When the oceans formed, carbon dioxide dissolved in the water. Carbonates were precipitated, making sediments, and this reduced the amount of carbon dioxide in the atmosphere.
