Evolution of atmosphere is the name of the process by which the Earth's atmosphere has changed over time since the planet was formed, from a mixture of gases escaping from inside the cooling planet, with no oxygen present, to become the breathable air we have today.
The Earth's atmosphere has undergone evolution over time.
Soon after the formation of the Earth, its atmosphere is thought to have consisted mainly of ammonia, methane, water, and hydrogen sulphide.
It is also thought that maybe it included carbon monoxide, carbon dioxide and neon, released from the centre of the cooling planet, and with no oxygen at all.
The development of breathable air came about over millions of years, as the first organisms appeared.
Among these primitive bacteria were some that released oxygen as a by-product of their metabolism.