I dont think it was just one person that started the Uchiha clan. Because it would be kinda weird and illogical, Sharingan isnt a disease that u spread to others. And Uchiha's are not vampires or something like that.
They came from the Hyuuga clan as a variation of the Byakugan. So IMO it's like having very distant cousins that live on the other side of the world so they are still related to you but WAY different. The Hyuuga clan must be pretty friggin old. And probably a whole branch of it started evolving/devolving naturally into Sharingan. So that it's likely to have been one or several families giving birth to these "Sharingan" babies.
I bet you the first sharingan baby must've freaked out everyone lol like "OH MY GOD! WTF IS THIS???!" lol ppl probably think he was a retarded kid or something and made fun of his eyes.
Also it's also possible that Kishimoto given his DBZ influence has Sharingan as something like Super Saiyan. Same with Byakugan.
So if you say that Byakugan is Super Saiyan. Then Sharingan could be seen as being Super Saiyan 2 or 3. Or maybe even Super Saiyan 4 (which is a whole form altogether and would make sense).
Just like how Goten and Trunks were able to naturally become Super Saiyans because Goku and Vegeta were at SSJ level when their mothers got pregnant. Thus maybe because of certain ppl in the Hyuuga clan having achieved higher levels of Byakugan and passing those genes down to their children who also had this natural ability lying dormant and pushing it further and passing it down. Until one day a Byakugan suppdenly went Super Saiyan 4 and said SHARIIIIIIIIIIINGAN! lol
This is interesting because then it's possible for Hinata and Neji to achieve Sharingan!
However this is a whole lot of speculation. What do you guys think?