What was most likely responsible for the fact that organic molecules no longer form spontaneously on earth?

Salutations! 1) A. The primitive earth had a lot of UV radiation. Photosyntesis originated O2 and O2 reacts generating O3 (ozone). The ozone layer prevents a lot of UV radiation to reach inorganic molecules and provide them with a lot of energy. Without that intense UV radiation, inorganic molecules cannot convert into organic molecules (because no other source of energy provides enough energy for that process). Hope this helps you!