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THE OZONE LAYER The ozone layer is a part of the upper atmosphere that protects Earth from the harmful ultraviolet (UV) radiation emitted from the Sun. Ozone is a form of oxygen. When life was first becoming established on Earth there was no oxygen in the atmosphere and therefore there was

no ozone layer. The surface of the planet was continually bathed in a level of UV radiation that would be deadly to almost all living things today. In a strange way this may have been a good thing. The energy from the UV radiation is thought to have been partly responsible for the continual rearranging of the molecules that eventually led to the appearance of life. random change would be harmful. Life developed in shallow waters, around 33 feet (10 meters) deep. Deep enough for the water to filter out the harmful UV rays, but not so deep that light couldn’t get through for photosynthesis. As the oxygen levels increased and the ozone layer began to form, However, as life forms grew more complex, it became more and more likely that any

This fossil shows the imprint of plant leaves from 200 million years ago.

life was gradually able to move into shallower and shallower waters. More and more new forms of life appeared, for example, corals, trilobites and early fishes. It has been estimated that well over a thousand new animals, and probably many times more than that, evolved in the Earth’s seas between 600 and 500 million years ago. There have been life forms that we shall never know about. L I FE ON LAND It was then that the next great step was taken. By this time there was about a tenth of the present-day level of oxygen in the atmosphere. This was enough to provide a shield that would allow life to survive on the land. Evidence has been found that there were land plants by around 440 million years ago. Within a few million years the first land animals had appeared, looking something like present-day scorpions and millipedes.

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