The Earth in its initial phases of formation was a hot object with a thin layer of atmosphere comprising

The evolution of the Earth's atmosphere occurred in three distinct stages. In the initial phase, the primordial atmosphere consisted primarily of hydrogen and helium. This early atmosphere is believed to have been stripped away from the terrestrial planets, including Earth, due to the impact of intense solar winds during the early stages of the solar system's formation.

The subsequent stage, known as degassing, involved the release of gases like water vapor, nitrogen, carbon dioxide, methane, and ammonia from the Earth's interior through volcanic activity. The final stage involved the modification of the atmosphere by the living world through photosynthesis, which led to the accumulation of free oxygen. Thus, the very first thin layer of atmosphere was composed of hydrogen and helium.