Earth originally had a CO2 (carbon dioxide) and nitrogen atmosphere, with a small amount of water vapor and a very small amount of methane.
No oxygen.
The CO2 was mainly emitted from volcanoes.
Plants changed that balance by using CO2 and sunlight to grow, and releasing O2 (oxygen).
O2 oxidized methane into CO2.
Animals also ate the plants, breathed in O2, and exhaled CO2.
Most ancient CO2 is now sequestered as CaCO3 (limestone / coral / shells), such as the white rocks in South England and Normandy in France, and in organic substances such as coal, oil and natural gas.
Much of the fossilized carbon got subducted under continents as the oceans contracted and expanded.
Subduction = the process of one tectonic plate sliding under another, resulting in tensions and faulting in the Earth's crust, with earthquakes and volcanic eruptions
The subducted CO2 will come out of the Earth again when volcanic activity and earthquakes release CO2 into the air.
Without natural CO2 emissions, Earth would run out of CO2, plants would die off, and life as we know it would end.
The oceans are permanently saturated with CO2.
Deep ocean trenches have pools of liquid CO2 that are unable to dissolve in the water.
Australian Geology Professor Ian Plimer said:
"The oceans can only become acidic if the Earth runs out of rocks."
CO2 levels in the air have been higher than today’s CO2 levels for most of Earth's 4.5 billion year history.
Earth has never had runaway greenhouse warming, despite higher CO2 levels in the past.
The atmosphere of Venus is about 95% CO2 -- the surface of the planet is very hot.
The atmosphere of Mars is about 95% CO2 -- the surface of the planet is cold.
The atmosphere of Earth is only about 0.04% CO2 -- most of the surface of our planet is ideal for human and plant life.