User blog comment:San&Ashitaka/TITAN THEORIES/@comment-67.237.237.100-20131014021504

This is what I think,

The Titans were the result of trying to create superhuman soldiers. However, when tested, the subjects lost their human inteligence and began eating humans instinctivley. And not only was there no way to turn them back, but it spread like a plague and most of humanity were either turned into Titans, or eaten.

The surviving population hid themselves behind the walls and were secretly given a gentic immunization in the food or water, thus ending the threat of the disease. While a new government was formed and an army began investigating the Titans themselves, the surviving researchers hid their involvment with the Titans to avoid being arrested and secretly continued their research to try to perfect their experiment and eliminate the failed Titans.

40 years later, when Ymir stole the serum to transform into a Titan, her human form could still be recovered, but she lost her human inteligence and acted like a normal Titan.

100 years later, Grisha Jaeger is part of this group. Bertholdt, Reiner and Annie are test subjects on for the new, almost perfect serum. While Bertholdt retained his intelligence, his size was far too big and slow to fight. Annie's size was tuned, but her lack of skin made it too easy to sheer the muscle and put her at a disadvantage. While Reiner's armor gave him an advantage, his exposed, skinless joints could put him at a similar disadvantage.

When Grisha and the researchers finally completed the ideal Titan form serum, Grisha discovered the true (assumed evil) intentions of the group he was apart of. He took the formula for the serum and ran. After Grisha's former organization put their plans into action, Grisha injected his son with the serum, knowing his stronged-willed son would never betray humanity, and went into hiding.

I know this goes a bit beyond the Titans themselves, but it's what I think.