Do Impacts Really Cause Mass Extinctions?

For decades, many scientists have believed that asteroid bombardments, such as that which is thought to have killed the dinosaurs, occur on our planet at regular intervals of about every 26 million years. 

A popular theory was that a dim companion star called Nemesis would approach our Sun at 26 million year intervals, inevitably causing mass extinctions on Earth.

However, a new scientific study confirms that such bombardments, if they do occur, are random.

But a more fundamental question is, are impacts from space really the cause of extinction level events on our planet?