The ozone layer absorbs much of the incoming solar ultraviolet radiation (UVR) and thus offers substantial protection from this radiation to all organisms living at, or near to, Earth's surface. Intriguingly, atmospheric ozone is not part of the planet's original system but a product of life on Earth, which began around 3.5 billion years ago. Until a half billion years ago, living organisms could not inhabit the land surface. Life was confined to the world's oceans and waterways, relatively protected from the intense unfiltered solar ultraviolet radiation. About 2 billion years ago as photosynthesizing organism's emitted oxygen (O2), a waste gas (ozone–O3) gradually began to form within the atmosphere. From around 400 million years ago aqueous plants were able to migrate onto the now-protected land and evolve into terrestrial plants, followed by animal life that ate the plants. So the succession has evolved, via several evolutionary paths, through herbivorous and carnivorous dinosaurs, mammals and omnivorous humans. Today, terrestrial species are shielded by Earth's recently acquired mantle of ozone in the stratosphere that absorbs much of the solar ultraviolet.
Unintentionally, the human species has now reversed some of that stratospheric ozone accumulation. Surprisingly, various industrial halogenated chemicals such as the chlorofluorocarbons (CFCs, used in refrigeration, insulated packaging and spray-can propellants), inert at ambient temperatures, react with ozone in the extreme cold of the polar stratospheric late winter and early spring. This time of year combines cold stratospheric temperatures with the "polar dawn", as solar ultraviolet radiation begins to reach the polar stratosphere, where it causes photolytic destruction of human-made gases in the stratosphere, such as the CFCs, methyl bromide and nitrous oxide. This, in turn, generates reactive "free radicals" that destroy stratospheric ozone.
