The early atmosphere
Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.
The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.
It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.
Scientists can’t be sure about the early atmosphere and can only draw evidence from other sources: for example, volcanoes on other planets release high quantities of carbon dioxide or nitrogen and iron-based compounds which are present in very old rocks that could have formed only if there was little or no oxygen.
Changes in the atmosphere
So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?
The proportion of oxygen went up because of…