Board Thread:Theories and Speculation/@comment-30355627-20161218164138/@comment-734582-20161220182934

Elveonora wrote: And Sasuke is the best example of proof that Rinnegan isn't an evolution of Sharingan... but people simply refuse to listen and instead go posing an in-universe hypothesis as fact here on wiki, even though it's obviously wrong. The problem with the whole "evolution" thing is that it's not really purely a devolution or evolution. It's a branch power. At least, the Purple rinnegan is. What the Red Rinnegan does specifically i don't know.

So yea, the Rinnegan is part of the Sharingan line, but it's a branch power, a mutation if you will. A Dojutsu that clearly sacrifices higher Sharingan abilities like the Mangekyou for powers over life and death.