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Oxygen already killed most of the life on Earth once. The first time it filled the air, around 2.4 billion years ago, it was so poisonous that nearly everything alive died. Scientists call it the Oxygen Catastrophe. Back then the oceans were full of tiny microbes, and none of them used oxygen. Then one kind, an ancestor of the green scum you still see on ponds, started giving off oxygen as a waste gas, the same way you breathe out air you don’t need. Oxygen is a wrecker. It rips apart the delicate machinery inside a living cell, including the DNA, and as it built up in the water and then the sky, it triggered the first mass extinction this planet had ever seen. A few survivors hid in the mud and deep underground where the gas couldn’t reach, and some of their descendants are still down there. But one tiny cell did something nobody else did. It ate a bacterium that had learned to use oxygen rather than die from it, and instead of digesting its meal, it kept it alive inside itself. That trapped bacterium became the mitochondria, the little engines that power your cells right now. Almost every cell you are made of carries hundreds or thousands of them, all descended from that one strange truce with a poison. The trade was worth it because burning food with oxygen releases about 18 times more energy than burning it without. It is the reason anything can swim fast or think hard. Every big, fast-moving animal on Earth, you included, runs on the gas that almost ended life. Oxygen changed the sky too. Some of it floated up high and turned into ozone, a thin layer that blocks most of the sun’s harshest rays. Before that shield existed, raw sunlight was strong enough to fry the DNA of anything out in the open, so life had to stay underwater, where a few feet of sea soaked up the danger. For almost two billion years, nothing lived on land at all. Only once the ozone grew thick enough, a few hundred million years ago, did the first plants and animals crawl out of the water. And the old poison never really left. Every second, the oxygen your cells burn throws off tiny broken bits called free radicals, and they keep nicking your DNA and the proteins around it. The damage adds up, slowly, your whole life. Back in 1956 a scientist named Denham Harman suggested this slow rusting from the inside is a big reason we get old. People still argue about how much it matters, and no antioxidant pill has ever been shown to make anyone live longer, but the basic idea has held up. The gas keeping you alive right now is also quietly wearing you down, year by year. The joke just got the timing wrong. Oxygen really does kill slowly, and billions of years before we showed up, it already proved it can kill fast.