Practice question

Free oxygen appeared much later after evolution of oxygenic photosynthesis by cyanobacteria. Initially atmosphere remained reducing because oxygen would be quickly consumed by ferrous iron and reduced gases. Absence of oxygen meant no ozone shield, permitting intense ultraviolet flux, and allowed reduced compounds like CH4, NH3, H2 to persist. This chemical milieu favored synthesis of organic monomers rather than their oxidation. The reducing nature itself explains lack of molecular oxygen, linking Reducing atmosphere to prebiotic stability. This concept integrates genetics, ecology and molecular evidence, frequently tested in NEET, GATE and CSIR-NET, highlighting links between genotype,