An oxidizing atmosphere (as Earth and Mars are currently) would actually make it much harder for life to emerge in the first place. Oxidation makes it very hard for complex molecules to remain stable enough for life to emerge in the first place. Before photosynthesis, Earth had a reducing environment. The advent of oxygen is often termed the "Oxygen Catastrophe."
Young terrestrial planets should tend towards a reducing atmosphere, due to all the rocks and such oxidizing. It took ages for life on Earth to produce sufficient oxygen to switch to an oxidizing atmosphere. We have evidence that Mars did start reducing and switch at some point. That Mars became oxidizing is certainly curious, considering the reason that happened on Earth.
