How Earth's Air Came to Be

If you could step into a time machine and travel back about 3 billion years, you’d have a big problem. No, it wouldn’t be meat-eating dinosaurs — they didn’t show up until much, much later. Your problem would be the air. You wouldn’t be able to breathe. In fact, the air would be like poison to you.

When Earth formed, it didn’t have the kind of air we breathe. Our atmosphere today is a mixture of gasses, including nitrogen and oxygen. Your body is made to use oxygen. But 3 billion years ago, the atmosphere was full of carbon dioxide and methane. Those are not gasses that today’s people or animals can breathe.

There also wasn’t much land back then. In fact, some scientists believe there might not have been any dry land at all. It’s thought that the ancient oceans had two to three times more water than they do today. The continents were down too deep to rise above the sea.

What kind of life could survive a water world without breathable air? Bacteria. They were simple life forms, but they did something really important. They completely changed Earth’s air.

The kind of bacteria that lived in the ancient oceans are called cyanobacteria (sai-a-now-bak-TEE-ree-uh). They were the main life form on Earth for more than 1.5 billion years. Like all living things, they needed food to live. They used sunlight to change water and carbon dioxide into sugars. Then they released oxygen as waste. It’s a lot like the way today’s plants take in sunlight and carbon dioxide and release oxygen.

The planet’s oceans were full of cyanobacteria, and they all released oxygen. There was so much oxygen in the water that some escaped into the air. This happened over and over for millions of years. Finally, somewhere around 2 billion years ago, there was enough oxygen in the air that it became a major part of Earth’s atmosphere.

That was bad news for cyanobacteria. All that extra oxygen was deadly to them. They began to die off. Sad for them, but good for us. It took another 1.7 billion years, but life forms that could breathe the new kind of air developed. Eventually, that led to animals — like the dinosaurs — and finally, us.

Cyanobacteria didn’t die off completely, though. Today’s cyanobacteria are sometimes called blue-green algae. (That name isn’t quite right, because they’re bacteria, not algae.) Too many of them in a body of water sometimes end up as the slimy stuff known as pond scum.

We can also see some ancient cyanobacteria today, in layered rocks called stromatolites. Cyanobacteria often clump together. Their stickiness traps bits of minerals, rocks, and soil into layers. The layers build up and up. Over time, the layers turn to rock. A stack of cyanobacteria layers is called a stromatolite. It can take a long time for one to form. It may take 2,000 to 3,000 years to make one 3-foot (1 m) tall stromatolite.

If you visit the shallow waters of Hamlin Pool in Western Australia, you can see stromatolites. Some of the rounded, rocky stacks sit underwater. Others have grown tall enough that their tops are above the water’s surface. Some are thought to be 3.5 billion years old! Even though the bottom layers are ancient, there are still living cyanobacteria on top. The Hamlin Pool stromatolites are considered to be living fossils.

Life as we know it today wouldn’t exist without those long-ago bacteria. The next time you take a breath, thank the cyanobacteria that started it all.