What we know about titans:
We know that they suddenly appeared 100 years back, and started devouring humans, simply for the pleasure of it. And so it ended up with humans hiding behind a wall. Or three walls.
Now the question is, where did titans appear from?
It is obvious that they werent around before the huge incident 100 years back, and started continuously multiplying. We know that they cant reproduce, having manly organs, all of them, so how did they appear?
I think we get a clue here.
So here comes my theory. A hundred years ago, probably a company/ government/ hired a brilliant scientist who wanted superhuman strength or immortality, made an experiment to try and become superhuman. They wanted regeneration and super strength. The descendants of this company (or atleast the members who hired the scientist) is probably the Wall Cult. They made their soldiers take it, or the scientist poisoned a town, or so forth. In short, it was an experiment gone bad. The humans changed, they became titans, but seemed to lose their reason. The scientist was able to keep control, in his titan form and became the COORDINATE. Soon he wanted fresh people to experiment upon, which the wall cult didnt want, but it was too late. Due to his experiments, the members of the wall cult managed to turn some of his experiments ( the colossal titans) use their hardening ability, to form the basis of the three walls in today's world.
Finally, the scientist got the formula right, and the products were people like Annie, Eren, Berthold etc. The person/ scientist was most probably Eren's father.
This theory does form a simple line as to what we know right now. The facts shown in Chapter 51, led me to form this line of reasoning between three groups, the rogue Titans, The Monkey Titan, and the wall cult.
What do you think?