Land-dwelling plants had a particularly large impact on the atmosphere, as they inhaled carbon dioxide and exhaled oxygen. Levels of atmospheric oxygen rose fast after the Ordovician period, increasing from about 5 to 10 percent of the atmosphere to levels much higher than they are today, perhaps to 35 percent, before stabilizing. Since about 370 million years ago, oxygen levels have mostly remained between 17 percent and 30 percent of the atmosphere.11 We know this because over this entire period researchers see evidence of spontaneous fires, and fires cannot ignite if oxygen levels fall much
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