There have been five mass extinctions that our planet witnessed in the last half a billion years, not just one. And we can be nearing the sixth one, where one of the species to possibly disappear is humans. The previous extinctions happened for several reasons: changes in temperature, volcano activity, changes in the sea level, and a lack of oxygen in the oceans... The good thing is, mass extinctions always happened over a span of several million years, not like shown in movies where disasters happen overnight. So we might still have time to recognize it, prepare, and counteract it. 

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