The Earth is about 4.6 billion years old, so there is very little evidence about its early atmosphere. Scientists' theories about how the atmosphere formed have changed and developed over time, and to evaluate a theory we judge how well the evidence supports it.
One theory says that during the first billion years there was intense volcanic activity. Volcanoes released the gases that formed the early atmosphere, and also 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 those of Mars and Venus today, mainly carbon dioxide with little or no oxygen. Volcanoes also produced nitrogen, which gradually built up in the atmosphere, and there may have been small proportions of methane and ammonia.
When the oceans formed, carbon dioxide dissolved in the water and carbonates were precipitated, producing sediments. This reduced the amount of carbon dioxide in the atmosphere.