As a young girl who was cast out from her village in ancient times, Ymir Fritz wandered the forest until she found the giant tree and entered the opening at its roots. There, she came into contact with the centipede-like organism and set events in motion that would define the history of Paradis, Marley, and the entire history of “Attack on Titan.”
Unfortunately, there is no 100% clear answer as to what the source of all living matter is. Like the island in “Lost” or the Black Lodge in “Twin Peaks,” it’s deliberately vague despite its importance to the story of “Attack on Titan.” Still, the moniker that it’s often known by, as well as a few flashbacks, suggests that it was around when the first organisms came to life on Earth and is possibly responsible for every living creature on the planet as a result.
Despite its endless power and mysterious origins, though, the worm is killed after Ymir accepts death and Eren’s head is severed from the power of the Founder. As such, the organism that caused the events of “Attack on Titan” is wiped out, as are all titans, and humanity must return to a more traditional form of warfare, which, unfortunately, it inevitably does. Still, the final scene in the series finale sees a young boy entering a familiar-looking tree — an ending that hints that the whole cycle will once again begin anew.