User blog comment:MeDeoPal/On the Origin of Titans and Shifters/@comment-108.235.162.223-20150419211544

you know that the manga acually released how the titans were originally created? afew weeks ago acually: this mad scientist named Dr. Kichirou wanted revenge on an individual group of religious thugs who killed his parents, so he created a serium and injected humans through the water supply, turning them into giant mindless beasts. he created this potion specifically for the avenging of his family and creating a new race with no specific religion or belief of god. to avoid being exterminated by his new species, he created a new potion that he gave to himself and afew other individuals that allowed them to shift into much stronger than normal titans. the shape shifters created a base protected by large outer wall to wait for the titans to kill off the humans. this plan allowed the shifters to avoid war with the titans and lose from their already low numbers. this plan did not suceed though, as their hidden base was found by a small human army. to avoid battle, it was agreed to stay permanetly in human form. the shifters could not kill of the human army at the time, as they had very skilled fighters within their ranks. as the times went on, the humans began to think less of the titans and forgot the threat they posed. the shifters decided that it was the best time to attack. Dr. Grisha Jaeger was a shifter himself who fell in love with a non shifter. even though he was a shape shifter himself, he did not share the hatred that his fellow shifters possesed. he proved this bu single handedly curing the plague. after the death of his wife, he injected his son with the serium so he would not die as easily as his wife. after the titans broke into the walls, Dr. Kichirou sent in titan spies such as Reiner Braun, Bertolt Hoover, and Annie Leonhart to study the human fighting tacticts and military numbers. the planned attack on Wall Sina failed due to a handful of titan shifters who changed sides. Dr. Jeager was killed for high treason with the help of humanity and sharing the potion with Eren and the other new shifters. the recipe to the cure was found in Eren Jeagers basement at his old house. Dr. Jeager had created this cure to prevent further mutations before dying at the hand of Dr. Kichirou.

me and my friend found it somewhere on their official website where only afew people saw it. i dont know if they took it down, i havent checked. but im 98% sure that this is the official theory of how the titans were created.