They gave off CO2 and a balance of CO2 and O2 was developed by plant and animal metabolism 8. In the high atmosphere oxygen is converted to ozone O3 which blocks out most of the U.V. light and thus protects complex organic molecules from damage. B. James Lovelock developed Gaia hypothesis in 50's and 60's 1. Earth's atmosphere is nearly explosive and yet has remained stable over a very long geologic time. Yet the temperature of the earth has remained fairly stable.
Water has neither all frozen or all vaporized.
Maybe oceans were very prolific from nutrients from land. Glaciers covered enough land that nutrients ran out and decay built up CO2 again and warmed the earth. Methane in the atmosphere reacts with O2 to make CO2 and H2O. 6. Ellsmere island near north pole once had alligator-like animals. It was warmer, CO2 perhaps 5-10X what it is now.