There have been 5 major extinction events in history, the largest being the Permian-Triassic extinction about 250 million years ago where approx 96% of all species on earth were wiped out. A cause of this is believed to be major volcanic activity in Siberia and release of mass quantities of CO2 and other gases which induced a greenhouse effect and caused the oceans to become more acidic. Life eventually rebounded but it was a close one.
Asteroids and comets aren't the only harbingers of death and change...