Quote (TobiRikudou)
The Rinnegan doesnt let anyone copy techs, it allows them to master them after witnessing them. That makes the Rinnegan superior to the Sharingan.
The Sharingan originates from the Rinnegan.
And the Rikudou Sennin could use Izanagi without any penalties, and to much greater effect that what we've seen so far.
The man gave life to things with Izanagi
This is the answer. Although Uchihas need Senju cells to use the Izanagi it is a jutsu in their Sharingan which relates it directly to the Rinnegan.