Take a deep breath. Hold it for a moment, let the air fill your chest, and then let it out.

It is the most automatic thing you will do today. We treat our atmosphere as a permanent fixture, a factory setting of our planet. But it wasn’t always this way. In fact, for the first half of our planet’s 4.5-billion-year history, taking a deep breath would have suffocated you instantly. The story of how our air became breathable is not a gentle tale of nature’s harmony. It is a story of biological warfare, global catastrophe, and the ultimate recycling project.

If you were to time travel back 2.5 billion years, you wouldn’t recognize your home. The sun was fainter than it is today, yet the planet was hot. The sky was likely not blue, but a hazy, alien orange, choked with methane and carbon dioxide. The oceans were green, heavy with dissolved iron, and ruled by single-celled microbes that thrived in the dark, oxygen-free waters. Then everything changed.

A microscopic organism known as cyanobacteria – often called blue-green algae – stumbled upon a new way to eat. These simple, mound-building colonies, known as........

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