Board Thread:Naruto Discussions/@comment-25298629-20141005021504

This is just a theory but I think that the Byakugan came from the Sharingan, a downgraded version of the Rinnegan. Heres how:

The Byakugan was created when Sharingan users used Izanagi and Izanami to change their fates and lost their vision. When they lost both of their eyes, they struggled to see, so in order to fight they continued to direct their chakra to their eyes. Overtime they developed a Byakugan and the ability to "see through their eyelids" using chakra.

(Notice when Itachi lost his vision his eye turned white like the Byakugan). []

We already know that the Sharingan can develop into a Rinnegan, the most powerful eye, with Senju DNA or Hagaromo chakra and a MS Sharingan. So a Byakugan I think is a different form of a Sharingan, which is a downgraded Rinnegan.  