
Originally Posted by
Rlinfamous
There are some who theorize that the Rinnegan is a mutation from the Byakugan. Since the Sharingan came from the Spiritual half of the Rinnegan (the Elder Brother's line), the Sharingan could still be said to come from the Byakugan.
The original Rikudou possessing the Byakugan before achieving the Rinnegan makes some sense; since he was the first to discover the true nature of chakra, it makes sense that he may also have originally possessed the Byakugan, which allows greater understanding of the body's own chakra network.
The Uchiha lack the Physical energies of the Rikudou, which probably means they also never could possess the full Rinnegan. The Rikudou, and every subsequent Rikudou, has possessed Senju/Uzumaki Physical Energies (The First Rikudou simply possessed them from the start, Madara transplanted Hashirama's cells to gain these, and Nagato was an Uzumaki). I think the Rikudou only passed down the Spiritual component of his eyes to the Uchiha, which is why the Sharingan specializes in Genjutsu, space/time and perception of techniques.
The main thing is that I consider the Rinnegan to be representative of the "whole" Rikudou, his physical and spiritual energies. The same could be said for his "perfect" Izanagi, which created the Bijuu permanently and altered reality itself, not just his own personal reality like Danzou's weaker version. The occular power of the Rikudou may have been passed down to the Uchiha, but it was necessarily watered down because their bodies couldn't handle the complete power of the Rinnegan.
That's my understanding/theorycrafting.
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