Scientists have discovered many reasons for mass extinctions, depending on when the extinctions occurred. Several theories for such extinction include the disease theory (that species died out because of some disease); overpopulation (that could spread disease and increase competition for food); impact theory (that a comet or asteroid struck the Earth, changing the climate); volcano theory (volcanic eruptions that changed the climate); changes in the Sun’s output (again, changing the climate); and even a killer cloud theory (that the Earth encountered a cosmic cloud that increased species’ exposure to radiation). A human theory also exists, but most mass extinctions occurred before what we call “humans” were even around on the planet.