Originally were told by Kakashi that the Sharingan originated from the Byakugan and this seemed logical at the time as we were also told that the Byakugan was one of the oldest the clans in Konoha (http://read.mangashare.com/Naruto/ch...8/page013.html
). But as we all know Tobi told us the Uchiha clan originated from the Sage of Six Path's older son, and the Senju clan originated from Sage of Six Path's younger son, the two clans(Senju and Uchiha) then merged and formed Konoha. The Sharingan's origins recently(Chapter 510) have been confirmed once and for all AND more importantly why the Sharingan is the way it is. The So6P via his Rinnegan executed a highly advanced forms of Yin/Yang (dark/light) chakra manipulations and the dark manipulation later became the Sharingan. This was touched upon during the wind resangan training by Yamato at the start of part 2 who ironically is able do the other bloodline limit, but in this case it's a light chakra manipulation as wood jutsu is an example of manipulating chakra into a life source. (http://read.mangashare.com/Naruto/ch...0/page011.html
Sharingan users have physical drawbacks for almost everyone of their techniques, the Sharingan is the extreme use of dark(Yin) manipulation but this is only halve the story as the sharingan can only be complete with the opposite manipulation, light(yang). If your interested in yin/yang chakra manipulation look at the fight with shikamaru vs Tayuya(sound 4 ninja, Orocihmaru's lapdog), in that fight there was a yin(dark) technique used. It was explained because the technique's chakra has a lot more spiritual energy the chakra tries to feed on any body(physical energy) it comes into contact with to balance itself. As we saw with Sasuke all the sharingan techniques had physical draw backs on him and the appearance of the sharingan, the tome symbol of the sharingan shows that its an dark chakra manipulation the same symbol used to describe so6p's dark(Yin) chakra manipulation. As usual Orocihmaru is never to far away when you talk about the sharingan, Orocihmaru is well known of the curse seal he cultivated from Jugo which also has a tome symbol implying it's a dark chakra manipulation as Karin has commented it too yields a dark chakra. Karin mentioned when Sasuke used Sassano his chakra was far darker and colder compared to when he used the curse seal. This would explain how Sasuke survived the 1/10 chance of surviving with the curse seal in his body due to his body being well accustomed to dark chakra manipulation already due to his sharingan bloodline limit. The curse seal eats away the users body which is quite the coincidence considering the curse is a strong dark chakra manipulation trying to find balance too. Is all this a coincidence? I think not, in fact with regards to the Naruto series I don't believe in coincidence anymore. It just too well planned out.
I have dragged on about the sharingan but that's because the sharingan in the end is a chakra manipulation and the series slowly is revealing more and more about light/dark manipulations. A bit more trivia here, Kishi has used a lot of Shintoism within the series such as the Sharingan techniques Tsukiyomi, Ametarasu and Susano'o. Also the legendary weapons;Yata Mirror,Kusanagi and Sword of Totsuka. There are many other references such as certain fights like itachi's sassano vs Orocihmaru's eight headed serpent technique, killer bee(Octopus) vs Kisame(Shark) all of which follow the same result in the mythology and MANY OTHERS. (http://en.wikipedia.org/wiki/Japanese_mythology
The descriptions of Sasuke and Naruto having chakra that feels either dark/cold or bright/warm is well founded now. Sasuke's chakra according to Karin felt colder and darker every time he used a stronger sharingan technique after another due his lineage to the Uchiha clan. And Naruto's chakra in general felt bright and warm which was later shown to have great amounts of life energy due his linage with the senju clan via the Uzumaki clan. I imagine Naruto needed the fox to amply his chakra's life energy to the point we saw Yamato's wood jutsu react, most likely due to the fact that although the Uzumaki clan are related to the senju clan but in the end are simply DISTANT relatives.