Dear Jacob,

When I wake up from a cat nap, I stretch and take a deep breath. It feels good to fill my lungs with oxygen.

But that wasn’t always possible on Earth.

I talked about this with my friend Sean Long. He’s a geologist at Washington State University.

“The cool thing is the answer has to do with life,” Long said. “Early life forms on Earth gave us all the oxygen. They were single-celled bacteria.”

Our planet is about 4.5 billion years old. There’s been life on Earth for 3.5 billion years. The first life forms were made of just one cell. They were bacteria and their cousins called archaea.