If the Sharingan was formed from the Byakugan then...

[Bajan]

My theory about the relationship between the eye techniques is a very simple one. The Sharingan was created from the Byakugan (using some kind ninjutsu) probably sometime during one of the great ninja wars or even before the Leaf was settled by the 1st Hokage (more likely) in order to control the tailed beasts. If the look at the history of the ninja world gaining control over the beasts was and still is a huge part of the story.