Imagine Earth 4 billion ago. There was no air, no plants, no animals—just a rocky, boiling planet. Our atmosphere as we know it didn’t even exist yet! No oxygen, no nitrogen, and definitely no blue sky.
Then, tiny single-celled life forms appeared. These early living things didn’t need oxygen like we do; instead, they “breathed” carbon dioxide (CO₂) and gave off oxygen (O₂) as a byproduct. With these tiny beings working day after day, millions of years went by, and guess what? The oxygen they produced started to fill the atmosphere.

The First Great Oxygen Catastrophe

So much oxygen was filling up the air that it created the first environmental catastrophe! Many of the early life forms were poisoned by all this oxygen because they weren’t used to it. Some died out, but others evolved to live with oxygen—these were the ancestors of the plants and animals we know today.

By then, Earth’s atmosphere had transformed into what we call an “oxygen-rich” atmosphere, and over billions of years, this oxygen cycle was born. There are still bacteria alive today—often in hot springs or deep under the ground—that “breathe” out oxygen, carrying on the tradition of those ancient life forms!

So next time you take a deep breath, remember: you’re part of an ancient, incredible cycle that connects you with all life on Earth!

• Close your eyes and imagine: What was the Earth like before the First Great Oxygen Catastrophe?
• Is it good or bad that the atmosphere shifted to having oxygen?
• What would happen to the air if all plants disappeared from Earth?
• Why was oxygen toxic to many of the first organisms?
• What do you think Earth would look like today without oxygen?
• Do you think humans could survive in the ancient atmosphere?
• If plants give us oxygen, what do they “breathe in” to survive?