There are many theories made by Sir Obito but to sum them up the sage was an Uzumaki decendent and when he obtained the Juubi the hate and evil manifested into creating the Rinnegan. The Uchiha and Senju are the eyes and body of the sage, the eyes are manifested from the Juubi's eye. If you look at a picture of the Juubi before the sage had it captured it is shown with 9 tomoe:
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Now has 6 tomoe:
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The tomoe were moved from the Juubi to create the Uchiha who are recognised for the eye powere A.K.A sharingan.
The body is inherited from the Uzumaki bloodline of the sage hence the Senju's great physical strength.
(I may be wrong about the Senju part since there is nothing physically given from the Juubi known at this point.)
There are many theories made by Sir Obito but to sum them up the sage was an Uzumaki decendent and when he obtained the Juubi the hate and evil manifested into creating the Rinnegan. The Uchiha and Senju are the eyes and body of the sage, the eyes are manifested from the Juubi's eye. If you look at a picture of the Juubi before the sage had it captured it is shown with 9 tomoe:
You must be registered for see images
Now has 6 tomoe:
You must be registered for see images
The tomoe were moved from the Juubi to create the Uchiha who are recognised for the eye powere A.K.A sharingan.
The body is inherited from the Uzumaki bloodline of the sage hence the Senju's great physical strength.
(I may be wrong about the Senju part since there is nothing physically given from the Juubi known at this point.)