Oxygen

Only a few survived because of "Large Igneous Provinces", these are volcanoes that don't explode but huge cracks open in the Earth's crust and lava just starts bubbling out, oozing across the landscape, releasing tons of toxic gases. The most dangerous thing about an igneous province is that it can keep up the oozing for centuries. A natural disaster hit us and brought humanity to the verge of extinction. With the wind effect, these gases spread all over the world and changed the atmospheric.
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