They couldn't exsist as long as humans. Otherwise population of humans would drop to fast during any time period Quinkaes couldn't be made. This means some one or some thing made ghouls. It's just like the mysteries of Attack on Titans. PS: For the sake of everyone who has not learned, Don't write spoilers for anything in the comments please. Sorry to be a pain in the butt.
I've always looked at ghouls as a mutation that became wided spread. As far as I know, humans also have rc cells, but they have significantly less. What I wonder is that, what if because humans have less is the reason that they don't need to feed off of eachother to nuture the cells? Maybe the reason why ghouls need to feed and why everything else tastes bad is because that the Rc cells have a connection to the brain. Perhaps if inject a human will enough Rc cells they will become a ghoul. (Kanou, please help me here) Honestly though, this is just a theory that I have, and it probably has some holes that I haven't noticed, so take my words with a grain of salt. I hope this somewhat answers your question.
I think that someone suffered from ROS,but managed to somehow control it,causing a reaction within his body that turned the appendage into a kagune.Soon though, he realized that he only could eat human meat.This all must have happened 300 years ago as the Washuus formed the CCG in the 1800s.So a massive surge of of RC cells must have caused him to form the need of human meat,like when Amon formed a kakuja by a surge of RC cells.Hope this helped!