Putting The O's Into Ozone

As cyanobacteria-generated oxygen continues to accumulate in the atmosphere, a productive 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 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 by UV light. Had no UV absorbing layer formed, life on Earth might have remained the exclusive realm of microbes.