So you guys, my first theory.
I kind of have the feeling that the Sharingan could be a locked Rinnegan. Just thinking about this a for a few months and i would like to hear your opinion about it:
i refer to the sons of the sage. One of them had the body and was the good one and the other had the eyes of the sage and was kind of the bad one.
Here is my theory how the sharingan was created. I think the Sage kind of put a seal in his son with the eyes, that when he would do something cruel it would be sealed. Like when he attacked his brother because of jealousy, maybe that seal was activated and the sharingan was created. That the "inner circle" of the rinnegan got the tomoes on it = sharingan. So this could be the point why the uchiha, the descendants of the bad bro have a sharingan and not a rinnegan (although the descendant had some some sort of spiral in his eyes). Maybe another thing that would support my theory is that the tomoes seem to be kind of related to seals. For example as it was on sasukes left neck/shoulder. There were 3 tomoes and this writing around it.
It could be total bullshit but just wondering what are your thoughts !