How do scientists think the Earth and atmosphere formed?
Scientists think that the present atmosphere was formed by volcanic emissions.
Whatever atmosphere was created during the formation of the earth would have been ejected from the earth's atmosphere.
The present atmosphere would have been formed by volcanic activity, emitting water, sulfur and other gases. This means that the atmosphere has always been an oxidizing mixture of gases.
This goes against the Miller Urey experiments that postulated a reducing atmosphere necessary for their theory of a biogenesis .