For billions of years, there was no fire on Earth.

There was heat to ignite it, like volcanic eruptions and lighting strikes. But fire lacked two very crucial ingredients: fuel to burn, and oxygen to feed the flames.

Ironically, plants would provide both.

Fire is a chemical reaction where fuel oxidizes to produce heat and light energy, carbon dioxide, water vapor and other gases.

Fire can start when there’s about 13% oxygen in the air, but it won’t stay lit.

It will burn steadily at 16% oxygen, improving to about 23%, and plateauing at 30%.

Land plants evolved 470 million years ago, consuming carbon dioxide to perform photosynthesis and producing oxygen as a by-product.

As they grew, evolved, and covered more of the Earth, plants finally drove Earth’s atmosphere to above 13% oxygen.

And, 420 million years ago, the first fires started.

Plants continued to thrive, and swampy forests eventually covered the globe during the time of dinosaurs. Oxygen levels hit 29% and wildfires became very common.

Around 50 million years ago, oxygen levels stabilized at 21%, as fire and photosynthesis struck a balance: Fire consumes plants and oxygen and makes carbon dioxide. Plants consume carbon dioxide and make oxygen.

A perfect cycle.