A Hypothesis on the Origin of Ghouls an the Washuu Clan

Based on the rather limited information the manga provides I will share my speculations and personal headcanon about the origin and history of ghouls and how the Washuu clan ties into it.

The first thing to keep in mind is that, unlike in the real world, in the Tokyo Ghoul Universe there exists RC-Cells. The origin of RC Cells is not explained in the manga, neither is their function within the human body. We know that RC Cells act basically like super stem cells, meaning they can become any other cell found in a human or ghoul body (This is the reason why ghouls can regenerate).
Given a certain number (1000 or above) of RC Cells in the human body, these RC Cells seem to mutate (rewrite) DNA, leading to two possible outcomes:
If the mutation is successful the DNA in the human body is rewritten to produce a new organ, the Kakuhou and the beginning of a RC Cell pathway system. At this point the human has turned into a ghoul, a species that genetically is already different enough from a human to make interbreeding very difficult. Additionally a ghoul now possesses increased reflexes, athleticism, strength and durability. The manga gives no explanation on how these traits are achieved, arguable it is the result of more DNA mutation caused by the RC Cells.
If the initial mutation is unsuccessful and no Kakuhou can be formed the person stays human but now suffers from RC Cell Oversecration Disease (ROS Disease, like Shirazu's sister). ROS Disease acts a lot like cancer, creating various RC Cell tumors on and inside the human body. Unlike cancer ROS Disease also affects the mental state of the patient, having them regress to a child like state. Untreated ROS Disease eventually leads to death.

Since there is no explanation given for the origin of RC Cell we have to assume that all humans in the Tokyo Ghoul Universe are born with a certain number of them (between 200 and 500 as stated in Tokyo Ghoul Part 1).
I hypothesize that the existence of humans predates that of ghouls, meaning that until a certain point only humans existed in the Tokyo Ghoul Universe.
Given that the necessary factors to turn from a human into a ghoul are either the amassing of RC Cells (natural ghoul) or the implantation of a Kakuhou (artificial ghoul), assuming that humans predate ghouls leaves only the amassing of RC Cells as starting point for the existence of ghouls. The most common way the manga shows to amass RC Cells is trough cannibalism, since RC Cells are part of the human body and blood.
Cannibalism in a tribalistic setting is well documented and we can assume that some early human tribes in the Tokyo Ghoul Universe also practiced it. Modern science claims that the early human originated in Africa, which leads us to the ancestors of the Washuu clan. The manga states that the ancestors of the Washuu clan originated from the Middle East, not quite the center of Africa but pretty close.
I hypothesize that the ancestors of the Washuu clan were one of these cannibalistic tribes but what made them special was that they not only cannibalized members of other tribes but also each other. A cannibal who consumes other cannibals would have a higher RC Cell count than either humans or regular cannibals (this is paralleled by the way ghouls develop a Kakuja).
I assume that the number of RC Cells in the human body is influenced by genetics and therefore hereditary, which in the environment of the outer and inter clan cannibalism of the Washuu ancestors and over many generations would lead to the birth of humans with higher amounts of RC Cells than usual. Eventually one of these people would have amassed enough RC Cells to trigger the mutation that, if successful, would turn him into a ghoul.
With that we have the birth of the first ghoul in existence and he very likely was a Washuu ancestor. A ghoul would be accepted a lot easier in a society that practices cannibalism already but even if that was not the case, a ghoul would still be able to blend in a lot easier. Over time more and more ghouls arise out of the ancestors of the Washuu clan and given their inter clan cannibalism these ghouls would also start to cannibalize each other.
As seen in the manga it does not take that much to turn from a ghoul into a dragon but given that most of the dragons that appear or are mentioned in the manga have Washuu genes it is possible that this is a trait unique to the Washuu bloodline.
After his fight with Suzuya Kaneki's RC Cell count was pretty much at zero, we do not know the exact number of the Oggai but if there were indeed 101 as Furuta claimed and Kaneki ate them all thus turning him into a dragon we can assume that any ghoul with Washuu genes would turn into a dragon after consuming about 100 other ghouls who also posses Washuu genes.
The manga states that the Underground King who turned into a dragon and who's remains are found by Ayato, was a Washuu. Kaneki and Rize who also became dragons share the same Washuu genes.
Therefore it is safe to assume that the first dragon, who in the manga is mentioned as the possible original source of ghouls in the world, was also a Washuu ancestor.
I have to do a little nitpicking here as to my understanding a dragon can not exist without a ghoul who turned into said dragon. Instead I propose the following hypothesis:
I see the first dragon not as the origin of the first ghoul, but as the explenation how the number of ghouls in the world went up by a massive degree as well as the reason for the ghouls migration out of the Middle East/Africa and into the rest of the world.
Here is what I believe happened, among the Washuu ancestors there existed one ghoul who was more successful the the rest. He basically became the leader of his tribe and due to him still consuming other ghouls of the Washuu bloodline turned into the first dragon.
Now, we know that dragons can release RC Cells in gaseous form, raising the number of RC Cells of anyone in an area of unknown size (at least as big as modern day Tokyo). This would have either killed or turned any remaining human of the Washuu bloodline into ghouls and most likely did the same for the other human tribes who were settled around the area of the Washuu ancestors. I also believe that this massive increase in RC Cells is the reason as to why the members of the Washuu bloodline are special and, as the manga states, are Kakuja from birth. Everyone in this wide area turning into a ghoul would force any ghoul reluctant to become a cannibal into migrating into areas where humans were settling thus beginning the spread of ghouls first throughout Africa and later the entire world.
I believe that the ghouls of the Washuu ancestors who prior to the dragon incident were still humans, were absolutely horrified by these events since it was caused by members of their own tribe. The manga states that the Washuu were trying to find a way to become human again and it would explain their general animosity towards ghouls.
Sui Ishida mentioned in the afterword of the final chapter of Tokyo Ghoul:re that he was raised Christian which makes me think that the dragon incident might be a reference to the fall of man mentioned in the bible.
Among humans the “forbidden fruit” no one was allowed to eat would be human flesh and the dragon incident and resulting spread of ghouls over the earth as a bane for mankind was the punishment for this “sin”. Being a ghoul was basically the same as being marked as a sinner. Also like in the bible not only the direct perpetrators of the incident were punished but also the innocent who failed to prevent it and all future generations as well.
I believe that the Washuu were also trying to redeem themselves by becoming ghoul exterminators, making them both the first ghouls as well as the first to ever rise against them. The manga states that the Washuu ancestors over many generations migrated east until they reached Japan where they finally took the name “Washuu” for themselves and also became the most esteemed family of local ghoul exterminators.
Eventually the other exterminator families rallied around them leading to the establishment of the precursor for the CCG with the Washuu Clan at the center. Always searching for a way to become human again a well esteemed and influential position in society served the Washuu's well to continue their work on their goal which eventually through the discovery of genetics seemed to slowly become graspable.
In order to further and maintain their position the Washuu Clan became corrupt, collaborating with the ghoul organization V to facilitate a situation in which the they would always be needed by society. The Washuu established the Sunlit Garden experimenting with interbreeding ghouls with humans in order to create fully human offspring again. But their experiments failed creating only half-humans with a reduced lifespan, another “cursed” existence.
Ironically these half-humans, who as the Washuu's henchmen slowly replaced the ghouls in the V organization, played a massive part in the downfall of the Washuu clan. The half-humans did not care if they were humans or ghouls they just wanted to have a normal lifespan and that's exactly what Furuta promised them.
The Rize dragon created by Furuta would raise the RC Cell numbers of the half-human V members turning them into ghouls and giving them back a normal life span. And for that promise they turn against the Washuu and lead by Furuta assisted in their annihilation.
Another interesting point is that Eto, the biggest enemy of the Washuu Clan, bites into an apple which is said to be the forbidden fruit from the bible just before having a conversation with Kanae where she brings up the “forbidden fruit” and expresses sympathy for those who would want to eat from it.
This would serve as a nice thematic contrast to the Washuu clan who condemns the eating of human flesh as the forbidden fruit of the real world.