Theory | Rikudō Sennin, Uzumaki, The Elder & Younger Sons and Jūbi..!
Rikudō Sennin, from what we understand had the Rinnegan - whether he was born with it or not is unknown but we know he did have it. He also defeated the Juubi and sealed it in himself and had two sons.
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The Elder Son was the Uchiha Clan Ancestor and inherited the eyes - the Sage's powerful chakra and spiritual energy.
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On the other hand, The Younger Son was the Senju Clan Ancestor who inherited the body - the Sage's life force and physical energy.
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I believe that the Sage inherited the Rinnegan once he sealed the Juubi inside him - The Uzumaki have been implied to be superior in life energy and physical energy compared to there Senju counterparts so the Uzumaki are like the perfected form of the Yang side of the scale. The Juubi is like the perfected form of the Ying side of the scale and the Uchiha counterparts are like a less perfect for of the Juubi.
Juubi -> Uchiha
Uzumaki -> Senju
With this theory, that means the children must have been born after the sealing because, how would the Elder Son have got the Sharingan Ancestor without the Juubi and if the Elder Son was concepted before the Younger Son then the Younger was also born after the sealing.
But is this really the case? How could one son have been born with so many of the traits of the Jubi but the next one was born with absolutely none.
Let logic prevail - the Senju Clan Ancestor was actually Elder Son who was born before the sealing of the Jubi.
Now another problem with this theory is that the Elder Son believed power will bring peace. If you reverse both that'll be stupid and unlikely but I won't switch this one. Is it simply coincidence that the descendant of the Elder Son, Hashirama was in favour of a village system or in other words power..? Nope.
In Simplification:
Rikudō Sennin was born.
The Elder Son (Senju Clan Ancestor) was born to Rikudō Sennin.
Rikudō Sennin fights and seals the Jubi inside himself.
The Younger Son (Uchiha Clan Ancestor) was born to the Rikudō Sennin.
The reason why the Uchiha Clan Ancestor was born with so many similarites and traits of the Juubi is because the seal that was placed on the Rikudō Sennin was beginning to weaken and the Juubi was beginning to mix with the Sage. The Ying over powered the Yang and the son was born with more Juubi than Rikudō Sennin.
What do you think of my theory? Enjoy and no flamming alright?
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