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.

Earth exists inside a bubble of gases—called its atmosphere. It’s mostly nitrogen and oxygen now. That oxygen is in the air we breathe. Another kind of oxygen in the atmosphere forms the ozone layer. That’s like a shield that protects Earth from some of the sun’s energy.

If you took a time machine back to early Earth, you wouldn’t be able to breathe. The atmosphere was carbon dioxide, methane and water vapor. There was no oxygen to fill your lungs.

There was no ozone layer either. So, the only safe place to live was underwater.

Around 2.5 billion years ago, a new type of bacteria showed up. They were cyanobacteria. Today, cyanobacteria are sometimes called blue-green algae—even though they’re not really algae.

Cyanobacteria lived in warm, shallow oceans. They grew in flat layers that covered the sea floor. They became mat- and mound-shaped fossils called stromatolites.

Cyanobacteria figured out how to do photosynthesis. They could suck in carbon dioxide from the atmosphere and use it—with water and sunlight—to make sugar. At the end of photosynthesis, they spit out leftover oxygen. It’s the same way green plants make energy today.

The oxygen those cyanobacteria spit out began to build up.

“Starting about 2.5 billion years ago, oxygen levels in the atmosphere started to rise and rise and rise because the bacteria were doing so well,” Long said. “By about 1.8 billion years ago, oxygen levels were high enough to create an ozone layer.”

Eventually, the atmosphere became more like the one we have today. Scientists call the change to an atmosphere with lots of oxygen the Great Oxygenation Event.

That change was a big deal for life on Earth. Thanks to oxygen, life on land would eventually be possible. One day there would be life forms made of lots of cells—like you and me.

It’s amazing to think bacteria too tiny to see could change the atmosphere and life on Earth. I guess you’re never too small to make a big difference in the world around you.


Dr. Universe