So here’s me ranting about theories of the Titan’s Origin and things like that. I’ll try to make it as logical as possible.
THIS WILL CONTAIN SPOILERS FOR SnK MANGA, SnK ANIME, SnK ‘BEFORE THE FALL’, and maybe Levis’ Spin-off… MAYBE.
Assumption 1: Titans were all humans to begin with. This is probably the most obvious theory out there right now. Although the Titan Shifters have a definitely different look to them which discerns them from ‘normal’ Titans and even abnormal types (with the exception of Ymir’s Titan form which looks much more like a ‘Normal Type’), the fact there are people that can switch back and forth seems to tell us that humans have something to do with it, indirectly or not. The grotesque resemblance between Titans and Humans also points that way.
The first ‘direct’ clue we get to explain this is in Chapter 38 of the Manga when Connie returns to his village to find it empty. The Recon corps members accompanying them find it strange not to find a single trace of blood despite the obvious signs of destruction and of Titan Activity. The fact there is no trace of escape from the humans living there either clues us in to the fact they may have never ‘left’ at all.
Along with the discovery there really was NO HOLE in the wall this time, I believe the people in Connie's Village became titans, attacking the inside of Wall Rose as a means of distraction for the Ape Titan to do his thing.
The screenshot below is when the Titan that 'crushed Connie's house' speaks to him, welcoming home. This backs up the possibility the entire village was indeed turned into Titans and since the tranformation did not work fully on Connie's mom, she didn't fully form her Titan Body and was rendered unable to move from that spot.
This brings up to the topic of Titan Shifters; If they can shift back and forth, why can't all titans do that? Is there a remedy to this Titan Curse? And so on. The partial answer to this comes from Chapter 46-47 of the manga when Reiner, Bertholdt, Ymir, and Eren are escaping Wall Rose.
At this point, we know all these 4 lovely people are Titans. The 'how' of Reiner and Bertholdt is still unclear, much like Annie's past. We only know that there is some kind of homeland where they come from and possibly houses other humans and even Titan Shifters. In Ymir's case however, we are given valuable information about 'changing into Titans'.
Ymir was a Titan for about 60 years before she shifted back to human form. This said, she doesn't remember much of what happen during that time (or so she says) and the details of her origins are still mysterious. What we do learn from this chapter however is that the 'Titan Gift' is not something that can be acquired by all Titans. In other words, most humans, when turned to titans, cannot shift bak and forth freely. They are stuck in their Titan forms. By eating Reiner and Bert's friend, Berik, Ymir was able to absorb this gift and change back into her human form, not looking a day older than the one when she changed into a titan for the first time.
So only a select few have the ability to do this and are born with it.
BONUS: Before the Fall.
The Prequel manga, happening 70 years prior to the Fall of Wall Maria tells of another invasion of the Titans presumably in the Shingashina District. An even more extreme branch of the 'Wall Cult' demands the opening of the gates and publically execute a representative of the royalty in order to do this. Once the Wall is open, a few titans come through, eat people and get out (I mean seriously, they just pulled a Pyramid Head).
The interesting part is the leader of this mass-scale mob to open the door: the pregnant widow of a Commancer of the Recon Corps.
She -obviously- gets eaten by a titan, BUT DOES THIS WILLINGLY SO. When the Recon people find the spat-out remains of the Titan, they find the widow dead, BUT HER CHILD IS STILL ALIVE AND KICKING INSIDE HER BELLY. Smart as they are, teh Recon dudes, cut her open and save the child. The chapter ends with '13 years later' where the 'Titan's Child' is emprisoned and we only see his silhouette.
Now, this could possibly be the way First-Generation Titan Shifters are born into the world. The fact an unborn child is exposed to whatever the titans are made of and infected with can possibily alter them thus gain the ability to shift back and forth between Human and Titan forms. Afterwards, the gift could be passed down to their children and so on.
This said, the unamed, emprisoned child could possibly be one of Eren's ancestors (possibly Grand-Father) which is the reason why he can shift. Also, this would mean Grisha was also a shifter. Maybe this would explain Eren's new powers since Eren is both a 3rd generation shifter AND his dad experiemented on him (NOT with injections, those were just to make him forget).
This could also explain the reason behind Annie's Father's guilt and remorse. If she is a First-Generation Shifter, this would mean her father killer their mother by feeding her to a titan in order to give his daughter powers.
AND THIS CONCLUDES THE FIRST OF MY THEORIES.
PLEASE REPLY AND COMMENT. I MIGHT BE COMPLETELY WRONG FOO.