The life forms that Humans refer to as Mice (singular: Mouse) are, unbeknownst to mankind, the most intelligent species on Earth, and not actually native to the planet. They are in fact hyper-intelligent pan-dimensional creatures whose rodent aspect represents merely a three-dimensional projection of their actual form.
Billions of years ago, they have been responsible for creating the supercomputer known as Deep Thought, and also for hiring the Magratheans to build the planet that would become known as Earth - actually an even more impressive and massive supercomputer designed by Deep Thought.
-Hitchhiker’s Guide to the Galaxy.