There, now we can see that there really is a theory for the evolution of Neptune. Poor Neptune, nobody thinks you have any importance. Little do the dogmatists know you have a place in the evolution of stars themselves.
Now you do. Notice how nothing is mentioned on Wikipedia's article on Neptune concerning its evolutionary path:
Neptune is an ancient star at the very end of its evolution, it is a tad bit younger than the Earth and is well on its way to becoming a water ocean world. All right!