The ancient atmosphere formed by a process know as degassing, from gases emitting from volcanoes. The early atmosphere had mixtures of sulfur dioxide, carbon dioxide, nitrogen and water vapor, with little oxygen. Condensation played a vital role in the evolution of the atmosphere. As it cooled, water vapor from volcanic emissions condensed and formed oceans.
The absence of oxygen from the atmosphere had a major role in the evolution of life--an oxidizing environment would have precluded the formation of cells.