The build-up of three oxygen created our ozone layer around two billion years ago.
<u>Explanation:
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The ozone layer is a part of the stratosphere which is located right after the troposphere in the atmosphere of the Earth.
The ozone gas is made up of the abundance of three oxygen molecules combining with the ultraviolet radiation emitted by the Sun.
The development of the ozone layer happened around two billion years ago when the oxygen concentration levels on the planet began to rise slowly and gradually.
