Theory: As we know, jinchuuriki get physical traits and special abilities from their beast when they become jinchuuriki (like Bee's ability to spit ink, Gaara's sand manipulation, Mito's negative emotions sensing and Roushi's lava jutsus). We also know that the juubi was depicted with 9 tomoe in its eye in its original depictions before its actual revival. Now, the eternal mangekyou sharingan is the fusion of 2 mangekyou sharingans in one eye socket. 2 sharingan eyes would have a total of 6 tomoe. Since 6 is 2/3 of 9 it's possible that an EMS is 2/3 the power of the Juubi's eye (with the eye being the yin energy). We all know how the juubi looks like a giant plant and is basically a big mass of natural energy. Naruto and Kurama even said this. Natural energy is what sage chakara is. Now, what exactly does sage chakara do for you? It dramatically enhances your physical strength, your speed, your ninjutsu, your taijutsu and your genjutsu. In sage mode, Naruto was physically strong enough to throw a rhino and slam Kurama. Interestingly enough, being a senju does these same things for you. Tsunade's physical strength is through the roof. Tobirama Senju and Darui can both use water encmpment wall, but Tobirama's version practically creates an ocean unlike Darui's. What I'm getting at is, Senjus are basically sages that just aren't actually in a mode. If you're a senju and you have sage mode (like Hashirama) then you're basically just an overkill of sage chakara. Having established that the enhanced qualities of a senju are basically just inherited senjutsu, I should remind you once again that the juubi is just a big mass of sage chakara. Now you may be wondering what this has to do with the stuff about jinchuuriki and the dojutsus I was talking about earlier. Well, the so6p was the juubi jinchuuriki and he was said to be both Uchiha and Senju in one body. I don't think he was born like this however. When the sage became the jinchuuriki he probably got 2/3 of the yin energy (the equivalent of an EMS) thus giving him his Uchiha quality, and an unspecified amount of the yang energy (the sage chakara) which gave him his senju quality. As we know, EMS + Senju DNA = Rinnegan. This is probably how the sage got the rinnegan. Therefore, it wasn't the sage that was originally Uchiha and Senju, but it was the Juubi! The sage only got these qualities from the juubi. This theory is further backed up by the fact that the juubi saw a pair of rinnegan inside Naruto along with the other tailed beasts, thus further establishing a connection between the juubi itself and rinnegan rather than the so6p and rinnegan. Finally, there is no proof or indication in the manga that the sage had rinnegan before getting the juubi and it is much more likely based on the evidence presented here that the rinnegan came from the juubi than the so6p somehow being born with some freakishly powerful eyes that only he had (because he was stated to be the first wielder of the rinnegan) or getting some lucky mutation. If the rinnegan was just a mutation I have 1 thing to say. I want that mutation!