Early in the Earth 's history carbon dioxide and methane likely produced a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. During this time, Earth 's earliest life appeared. These first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water. Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth 's history. With no oxygen, methane stayed in the atmosphere longer and at higher concentrations than it does today.
which gas most likely was not part of earth's early atmosphere
Oxygen oxygen