Earth was originally made up off CO2. over a period of time it decreased due to certain events E.G photosynthesis.
Some of the carbon dioxide reacted with other substances in the sea to form insoluble compounds such as calcium carbonate and soluble compounds like calcium hydrogen-carbonate.
These compounds then became concentrated in shells of sea creatures. when these died the remainings formed carbonate rocks, locking the carbon away.
The amount CO2 seemed to balance out int he last 200…