Mass extinctions have occurred many times during the course of Earth's history. Specifically, when most of life is wiped out, it is due to something environmental that occurs that most organisms are unable to adapt to. It could be due to climate change--periods of ice age, or periods of higher temperatures. It could be due to an event that causes other events to occur--example, if a large asteroid hit earth, the dust would darken the skies, photosynthesis would decrease, temperatures too would decrease, the food chains may begin to fail and many organisms could die off. This possibly occurred when dinosaurs died out 65 million years ago.