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If you are talking about a few million years after the Earth was born, then the Earliest Earth Atmosphere was composed of the gases from the solar nebula - which is primarily hydrogen. In addition to that, the atmosphere would also be reach with hydrides and methane, ammonia and some water vapor.

But if you are talking about the atmosphere from the Age of Volcanoes to the Great Bombardment (the time where Earth is showered by thousands of comets per year), then earth's atmosphere by the time would consist mostly of Nitrogen and Carbon dioxide, sulfur dioxide and traces of oxygen and water vapor.