As cyanobacteria-generated oxygen continues to accumulate in the atmosphere, a protective ozone layer begins to form. In another remarkable twist in the evolutionary story of life, oxygen, once a fatal form of air pollution, becomes a shield for future life against the sun’s destructive ultraviolet (UV) rays.
Long before the Earth's ozone layer formed, bacteria had developed mechanisms to cope with UV light: making sheaths, submerging themselves, digging in. They evolved the ability to repair genes damaged by UV light. Had no UV-absorbing ozone layer formed, life on Earth might have remained the exclusive realm of the microbes.