More than 90% of all organisms that have ever lived on Earth are extinct. As new species evolve to fit ever changing ecological niches, older species fade away.
But the rate of extinction is far from constant. At least a handful of times in the last 500 million years, 50+ percent of all species on Earth have disappeared in a geological blink of the eye.
Though these mass extinctions are deadly events, they open up the planet for new life-forms to emerge. Dinosaurs appeared after one of the biggest mass extinction events on Earth, the Permian Triassic extinction about 250 million years ago.
One of the most studied mass extinction, between the Cretaceous and Paleogene periods about 65 million years ago, killed off the dinosaurs and made room for mammals to rapidly evolve.
The causes of these mass extinctions are unsolved mysteries, though volcanic eruptions and the impacts of large asteroids or comets are prime suspects in most cases. Both would eject tons of debris into the atmosphere, darkening the skies for at least months on end. Starved of sunlight, plants and plant-eating animals would quickly die.
Space rocks and volcanoes could also unleash toxic and heat trapping gases that once the dust settled enable runaway global warming.