The layer of atmosphere around Earth is relatively thin, proportionately about the same as the skin around an orange.
This layer is very important, however, because its gases absorb most of the harmful rays of the Sun while letting through sunlight for energy and warmth. Without the atmosphere, living things would be burned by the strong sunshine.
Scientists have suggested that early forms of life developed under about 30 feet (10 meters) of water to avoid being destroyed by the Sun. Green plants in the oceans very gradually added oxygen to the atmosphere.
As the gases in Earth’s atmosphere increased, living things were shielded from some of the Sun’s dangerous rays and eventually could exist on land as well as underwater.