WARNING: The following contains spoilers for Chapter #137 of Attack on Titan, "Titans," by Hajime Isayama, Dezi Sienty and Alex Ko Ransom, now available in English from Kodansha.
Ever since the Colossal Titan first reared its head above Paradis Island's walls, the question of what Attack on Titan's titular monsters really are has fascinated and perplexed the series' characters and fans alike. 137 chapters later and Hajime Isayama's manga has provided the most solid explanation yet. In fact, given we're just two installments away from the story's end, many will assume this newly revealed origin is a definitive one.
Because Chapter #137, "Titans," also brings the Rumbling to an end -- a world-crushing catastrophe set in motion by Eren's Founding Titan -- it's almost easy to overlook this equally explosive information concerning the true nature of Titans. But though the science behind it sort of makes sense, the very fact that it is science blows huge swathes of the previous explanation -- based entirely on the supernatural -- out of the water.
This new origin story is narrated by Zeke as he and Armin sit together, trapped in the strange dimension between life and death known as the Path. The realm also connects all Eldians together and, before Eren liberated her to enact the Rumbling, it was the prison of the Founder, Ymir (the first Founding Titan) for 2,000 years. Zeke explains to Armin that there once a time when nothing else existed on Earth except for "mere matter [...] As countless masses of some things appeared and disappeared, then appeared again, one of them eventually survived. We now know it as 'life.' [...] Life never stops in its frantic quest to multiply. Death and extermination of the species runs counter to the goal of multiplying. That is why we face the punishment known as fear. And why that child [Ymir] desperately sought to avoid such pain. Something stronger. Something larger. She gave birth to an undying body. And then she escaped to a world that was free of even death."
The story that Zeke is retelling is how Ymir Fritz came to become the world's first Titan. The slave girl was unjustly hunted to death, thousands of years ago, by order of the then-Eldian King, resulting in her body falling down inside what appeared to be a well inside an ancient tree. Illustrations in Eldian history books purport that Ymir made a deal with the Devil of All Earth for power, while the manga depicts the girl's body connecting with what looks like a floating spine. Titan serum comes from spinal fluid and Titans are vulnerable at the nape, so it makes (weird) sense that the Founder was formed from a spinal cord.
Chapter #137 slots the big missing piece of the puzzle into place: what was that strange spine-like thing that Ymir fused with? Until now, we could only assume, based solely on the supernatural clues provided, that it was an act of magic -- whether satanic or godly has been up for interpretation. Zeke's explanation takes religion out of the equation entirely: what Ymir earned her second life from appears to have been some kind of inhuman lifeform lying dormant in that tree's primordial soup. The being latched onto her and created what can only be described as a new, hybrid creature: Titans. Ymir's children ate her body, inheriting the parasite, altering the genetics of all who came after her to continue this hybrid species.
Cold, hard evidence comes straight out of the Founding Titan's own spine when it's separated from Eren's head -- twice now, something inside this spine has acted with a will of its own to reattach body to brain, leaving us to wonder whether or not Eren or the parasitic entity inside him is really at the wheel.
This isn't to say, however, that all of Attack on Titan's lore can now be attributed to science-fiction rather than fantasy. The Path's existence -- other than being some kind of shared, mass hallucination -- can't be so easily debunked. The idea of memory inheritance, too, which all Titan Shifters possess is also something science can't really explain, either. Not to mention smaller things like the Beast and Founding Titans' abilities to verbally command their own kind. Then again, magic and religion are often stand-ins for things science has yet to explain so, who knows -- maybe Attack on Titan has been a sci-fi epic in a pre-technological age all along.
As for the series' roots in real-life Greek and Norse mythology, these have long provided clues as to how Isayama's story will end -- especially things like Ragnarok. But really, in the same way that Neon Genesis Evangelion borrows influence from theological sources in image alone, Attack on Titan's references to these mythologies have little to do with the series' core themes, which are political and existential. They're part of the story's architecture but not necessarily its content.
Much like the differing interpretations of Ymir's supposed deal with the Devil, readers can divine their own explanations and meaning in Attack on Titan. Zeke's Thanos-aping coldness about life and its flaws could make him an unreliable narrator, despite supporting evidence to the contrary. And with Eren's fate up in the air, there may still be more to uncover.