12.2.1 Ultraviolet radiation on an ancient earth

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Oxygen-liberating photosynthetic organisms appeared on earth about 2500 million years ago, representing a pivotal event for the evolution of multicellular life. Accumulation of O2 in the earth’s early atmosphere enabled evolution of aerobic respiration (highly efficient for generation of ATP), paving the way for evolution of eucaryotes and larger, more complex organisms including vascular plants. An O2-containing atmosphere also led to a dramatic change in the radiation environment of ancient earth. Molecular oxygen absorbs UV radiation, producing ozone. That product (O3) also absorbs strongly in the UV waveband. Resultant filtering of UV radiation by the combined actions of O2 + O3 most probably facilitated diversification of life onto land (Caldwell 1979).

Nowadays, atmospheric ozone is largely concentrated in a thin layer in the stratosphere and continues to shield the earth’s surface from most of the solar UV radiation. However, certain industrial gases can lead to ozone depletion. As a consequence, atmospheric transmission of UV radiation increases, and is cause for concern.

 

 

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