A Walk Through Time

Putting The O's Into Ozone

2100 Million Years Ago

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 sunís destructive ultraviolet (UV) rays.

Long before Earthís 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.

The solar corona, a halo of light around the darkened Sun, visible during a total eclipse. (Photography courtesy National Center for Atmospheric Research)

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