The Earth hasn’t always had ozone — or even oxygen — in its atmosphere.
Our planet spent more than 2.4 billion years with an oxygen concentration of less than 1 part per million, in fact. While the element is plentiful throughout the universe, what little Earth was born with quickly evaporated away into space.
It wasn’t until the arrival of plants and photosynthesis, which consumes abundant carbon dioxide from the atmosphere and leaves oxygen as a byproduct, that the concentration of oxygen reached a measly few percent.
Around 500 million years ago, those levels shot up again — this time to the roughly 20 percent concentrations that we have today. And with that abundant oxygen came ozone.
