Attack On Titan: GENESIS-The Begining(FanFict)

"Several hundred years ago, humans were nearly exterminated by Titans.
Titans are typically several stories tall, seem to have no intelligence,
devour human beings and, worst of all, seem to do it for
the pleasure rather than as a food source.
A small percentage of humanity survived by walling
themselves in a city protected by extremely high walls,
even taller than the biggest Titans."
-Hajime Isayama

"But what causes this existence of Titans?"
"Why there are Titans within the wall?"

"How can a person shift into a Titan?"

"Why does the Ackerman and The Royal Reiss family
are not affected by the memory wipe?"

Here's a fan theory to answer all your question.
Well it might just be theory or might be a prophecy.
Who knows?