Minor outgas products from the Earth included molecular nitrogen (N₂) and ammonia (NH₃). Before oxygen became a major constituent of the Earth's atmosphere, ammonia absorbed some of the solar ultraviolet radiation, broke up into component atoms, and recombined into other compounds, including molecular nitrogen.

After lifeforms evolved, anaerobic bacteria reacted organic compounds with nitrates occurring in the environment and within their bodies. This chemical process is called denitrification and was the source of most of the atmosphere's molecular nitrogen.

Because molecular nitrogen is fairly unreactive in the Earth's atmosphere, so it has a very long lifetime, and the source rate of nitrogen from denitrification was greater than the removal rate of nitrogen in the early days, the nitrogen content of the atmosphere built up to a very high mixing ratio over the next few billion years.

This plot shows the overall evolution of the major gas concentrations over the time of the Earth's existence.