According to Oparin, the atmosphere of primitive earth was reducing because $\mathrm{H}$ atoms were most numerous and most reactive. Large quantities of $\mathrm{H}_2, \mathrm{~N}_2$, water vapour $\mathrm{CO}_2, \mathrm{CH}_4$ and $\mathrm{NH}_3$ were present, but free oxygen was not present in significant amount.
Related Theory
Oparin and Haldane suggested that life originated from non-living organic molecules like proteins and RNA. This was followed by the theory of chemical evolution which suggested that atmospheric conditions of earth led to the formation of organic molecules from inorganic molecules.