O 2 not conducive to spontaneous formation of molecules. O 2 atmosphere did not emerge until evolution of photosynthetic process. Organic building blocks were assembled into polymers 3. Formation of organic molecules. CO 2 is much less reactive, much more difficult to generate organics using Miller's apparatus.
Alternative suggestions are that meteors delivered organics.
Early earth was heavily bombarded by meteors, and meteors have been shown to contain carbonaceous compounds. Difficulty is imagining stability during heat of entry. Clays stabilize molecules, allowing them to condense and polymerize and stay polymerized. Self replication - Heredity. Spores from a source outside our solar system - would require sheielding.