What theory explains the formation of the main atmosphere during the Earth's first billion years?

The formation of the Earth's early atmosphere primarily resulted from intense volcanic activity releasing gases. During the planet's formation and into its early history, massive volcanic eruptions occurred, releasing significant amounts of gases such as water vapor, carbon dioxide, sulfur dioxide, and nitrogen into the atmosphere.

As the Earth cooled over time, the water vapor condensed to form oceans, and other gases contributed to the atmosphere's composition. This volcanic outgassing was crucial because it provided the necessary elements to create a thick atmosphere, which was essential for trapping heat and enabling the planet to sustain liquid water.

While oceans cooling played a role in the development of the planet’s surface and atmosphere, it was the volcanic activity that primarily supplied the initial gases. Impacts from celestial bodies did contribute to the formation of the Earth and may have influenced early atmospheric conditions, but volcanic activity was the main source of the gases that formed the early atmosphere. Photosynthesis by ancient plants came much later and was responsible for altering the atmosphere by producing oxygen, but not for its initial formation.