Cyanobacteria are credited for
Cyanobacteria possess photosystems I and II, enabling water photolysis releasing oxygen as byproduct. Accumulating over billions of years from 2.4 billion years ago, biogenic oxygen oxidized iron forming banded iron formations, then accumulated in atmosphere. This allowed ozone layer formation shielding ultraviolet radiation and enabling terrestrial life. Isotopic and fossil evidence links atmospheric transition to cyanobacterial blooms. Therefore Oxygenation of atmosphere marks their pivotal planetary impact transforming reducing to oxidizing world. This concept integrates genetics, ecology and molecular evidence, frequently tested in NEET, GATE and CSIR-NET, highlighting links between genotype, phenotype and environment.
Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 1: Origin of Life.