In the history of life on Earth, ozone has played an incredibly important role. For much of the early history of life, the atmosphere contained little oxygen, slowly being replaced by carbon dioxide through photosynthesis. It wasn’t until a critical mass of this CO2 was replaced that animal life could venture onto land. This is due, in large part, to the role of ozone. As oxygen began to fill up the atmosphere, it made its way into the stratosphere. There it encountered ultraviolet radiation, where it underwent a chemical reaction involving a catalyst molecule, and turned into ozone. This is how ozone forms, and it plays an integral role in moderating the greenhouse effect to maintain global temperatures, as well as absorbing ultraviolet light from the sun. Without significant protection from ultraviolet light, animal life on land would have been completely different and much harsher, if it were possible at all.
Owen's Astronomy Class Blog 