Researchers from the University of Washington have declared that it is highly unlikely that comets caused mass extinctions on Earth. With new data about the Oort Cloud—a region in space approximately 93 billion miles from the sun where the majority of Earth’s comets originate from–scientists used a computer model to map the statistical likelihood of a comet crossing through Earth’s orbit. They concluded that no more than two or three comets could have passed through Earth’s orbit in the last 500 million years. Even then, they say the comet showers would have not been large enough to cause a mass extinction.
Comets a "highly unlikely" cause of mass extinction
New study maps comet region in space, concludes comets are only responsible for minor extinctions