Only a small part of it. Respiration by fauna, wildfires, volcanic emissions, etc. produce some of it. Some of it has always been there and for most of this planet's history, far beyond @400ppm.
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Earth’s original atmosphere was rich in
methane,
ammonia,
water vapour, and the noble gas
neon, but it lacked free
oxygen. It is likely that hundreds of millions of years separated the first biological production of oxygen by unicellular organisms and its eventual accumulation in the atmosphere.
The composition of the atmosphere encodes a great deal of information bearing on its origin. Furthermore, the nature and variations of the minor components reveal extensive interactions between the atmosphere, terrestrial
environment, and biota.
evolution of the atmosphere, the development of
Earth’s
atmosphere across
geologic time. The process by which the current atmosphere arose from earlier conditions is complex; however, evidence related to the evolution of Earth’s atmosphere, though indirect, is abundant. Ancient sediments and rocks record past changes in atmospheric
composition due to chemical reactions with Earth’s crust and, in particular, to biochemical processes associated with life.
Earth's early and modern atmospheres compared
Comparison of Earth's prebiotic and modern atmospheres. Before life began on the planet, Earth's atmosphere was largely made up of nitrogen and carbon dioxide gases. After photosynthesizing organisms multiplied on Earth's surface and in the oceans, much of the carbon dioxide was replaced with oxygen.