Okay, I have this theory about the Titans and how they came to be, and it's the best explanation I've got. I consider this my headcanon until the manga/anime says so otherwise.

The Titans are humans trapped in an endless nightmare, not able to control their bodies, but Ymir transformed back into a human right? And Bertholt, Reiner and Annie were regular titans as well, as far as I believe. I think after that great war that weakened humanity to the titans, a government began to experiment to create a supersoldier. A Titan. Powered by sunlight, an endless energy source. However this Titan could not control himself and was trapped in his titan body, without a way of turning back. In its madness it began to instinctively eat humans and inadvertendly turn other humans into the same mindless titans as well. Due to its size and only one weakness it spread like wildfire across the world, forcing humanity into retreat. The Titan Shifters, excluding Eren as he wasn't a titan before, are what those supersoldiers were supposed to be, able to direct their bodies against the enemy. I have no idea how the Beast Titan has a role in this theory, but it's the best I can come up with now.

What do you think?