For as long as infalling planetesimals contained some hydrous and carbonate minerals, Earth's proto-atmosphere had to be formed during accretion and was composed primarily of CO2. H2O was released and incorporated into the proto-atmosphere to form H2O-CO2 supercritical fluid after accretion when the event of a giant Moon-forming impact took place. When Earth's surface cooled down to about 450 - 300°C, the indigenous ocean began to form and it was a hot ocean of either a dense supercritical H2O-CO2 mixture or a fluid H2O-CO2 mixture. The hot ocean interacted both on the surface with CO2-dominated proto-atmosphere and on the bottom with feldspars in the crust. The latter removed CO2 from the ocean to form carbonates and clay minerals on the crust. The interactions on the surface would quickly dissolve CO2 into the indigenous ocean from the atmosphere and would evaporate H2O into the atmosphere. This would effectively remove all CO2 in the proto-atmosphere via the ocean to the crust. The interactions among atmosphere, ocean and crust would exchange not only materials but heat between different bodies. This in turn might have helped Earth cool down more rapidly than its neighbor Venus.