WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 21, "From You, 2,000 Years Ago," now streaming on Funimation, Hulu and Crunchyroll.
Attack on Titan has finally answered one of its most burning questions: Where did Titans come from? It has been well known throughout the series that the Original Titan was a girl named Ymir who lived 2,000 years ago. It was initially believed she made a deal with the devil to become the first Titan. However, Attack on Titan Season 4, Episode 21, "From You, 2,000 Years Ago" revealed Ymir's origin is like everything else in AoT -- a tragedy. As it turns out, the birth of Titans was the result of a slave girl being punished for a crime she didn't commit.
Ymir was a slave in the tribe of Eldia 2,000 years ago, and one day, she was blamed for having freed a pig from its pen. As her fellow slaves and villagers chased her through the woods, shooting her with arrows, the wounded young girl stumbled into a cavern beneath an enormous tree. Succumbing to her injuries, Ymir fell into a large pool of water beneath the tree, where a creature resembling a human spine was swimming around. When the creature approached Ymir, there was a blinding light and a large explosion before Ymir emerged from the steam as the world's first Titan.
Despite her new power as the Founding Titan, Ymir remained a slave to Fritz, the leader of the Eldian Tribe. After exploiting her to grow the Eldian empire, using her Titan powers to build roads and bridges and defeat his enemies -- particularly the neighboring Marley tribe -- Fritz gave Ymir the "honor" of bearing his children. Over the next 13 years, Ymir served as King Fritz's weapon as she raised their three daughters, Maria, Rose and Sina. After saving Fritz from a spear during an assassination attempt, Ymir inexplicably died from the spear that pierced her shoulder, almost as though she willed herself to die from such a minor injury to someone with the power of Titans.
King Fritz refused to let Ymir's power vanish, so he instructed his daughters to consume their mother's flesh, thus creating the ritual of passing on Titan powers by eating a Titan Shifter. This practice eventually resulted in the divvying up of Ymir's power, creating the Nine Titans of the series' present-day timeline. Despite seemingly dying, Ymir spent the next 2,000 years inside the Paths, creating each and every Titan out of water and sand until Eren and Zeke found her. For Ymir, Eren's and Zeke's arrival took an eternity and also happened in an instant, as the tortured young girl has existed in a place beyond time for most of her existence.
Ymir seems to have a similar hatred for the world that Eren Jaeger has developed; she has granted him the full power of the Founding Titan, as evidenced by the very same creature that saved her appearing from Eren's decapitated head back in Shiganshina. Eren appears to have been the first person in her entire existence to treat her like a person rather than a slave or a god, and Ymir cried when Eren told her that she could make her own decisions. It's not clear what happened to Ymir after Eren took control of the Founding Titan -- whether she still exists in the Paths or has finally been put to rest is unknown.
It turns out that Eldian people can be transformed into Titans because they are descendants of the small tribe that Ymir originated from 2,000 years ago. In all that time, Ymir has been carrying out the will of the Fritz family bloodline, as well as building and repairing the Titan bodies of every Titan Shifter, the mindless Titans and even the Wall Titans. Unlike Eren, Ymir didn't choose her fate -- she is simply a victim of a cruel man and a cruel tribe. That she and Eren share the same hatred for the world that has been so cruel to them doesn't bode well for the future of that world as Attack on Titan continues.