The reducing environment of primitive earth was transformed into oxidizing environment by

Early earth's atmosphere was reducing atmosphere. It contained actively reducing gases such as hydrogen, carbon monoxide, and hydrogen sulfide. There was no oxygen in the atmosphere. As chemical energy gradually reduced, this was proven to be a positive environment for photoautotrophs like cyanobacteria to evolve the photosynthetic metabolic pathways using sunlight as an energy source. Due to photosynthesis by cyanobacteria, there was gradual increase in the oxygen level of the earth's atmosphere. This changed a reducing atmosphere to an oxidizing atmosphere.