How did humans go extinct?

One day many, many years from now, a young Noorfblook child named Plib goes on a class trip to his favourite place - the Natural History Museum! Some kids love the exhibits on Outer Space. Others love the Evolution Room. Could Noorfblooks really have descended from frogs? But Plib's favourite displays are of human beings. Plib loves humans. All his favourite books and movies are about them, and he never goes anywhere without his trusted stuffy, Frank. He'll never get to meet a human being, however, for one very simple reason: they went extinct ten million years ago! But, how did humans go extinct?