|Long, long ago... when nothing but mere matter existed in this world. As countless masses of some unknown things appeared, disappeared, then appeared again... one of them eventually survived. We now know it as 'life.'|
|— Zeke Yeager explains the origin of life|
The "source of all living matter" (有機物の起源 Yūkibutsu no Kigen?) is the tentative name attributed to the anomaly which granted Ymir Fritz the Power of the Titans almost two thousand years ago. It was the source of both the Power of the Titans and the abilities of the Founding Titan.
The "source of all living matter" took on a very insect-like appearance, being compared to a "shining centipede" by Gabi Braun, albeit with no visible, eyes or mouth, though there appeared to be a mass of feelers on the creature's head. The limbs were continuous with its body, having no apparent segments or joints. The anomaly could vary greatly in size, being somewhere around a meter long when joining with Ymir Fritz but growing to massive proportions upon emerging from Eren Yeager's Founding Titan.
While it is unknown whether or not the anomaly was indeed the origin of all life on Earth, it was evidently capable of dramatically affecting organic life in its immediate vicinity. A tree which stood over the anomaly in ancient times grew to immense proportions compared to those surrounding it. Physical contact between Ymir Fritz and the anomaly caused the creation of the Power of the Titans and subsequent connection of living and dead consciousness in the Eldian race through the paths. Zeke Yeager speculated that the nature of the Power of the Titans, the creation of immense and nearly indestructible bodies and a path to a deathless existence, may have resulted from Ymir Fritz's vulnerability and fear of death.
While the anomaly appeared sedentary in nature, it could display great levels of physical initiative when pressured or forced to do so. Two thousand years ago, the anomaly actively reached its limbs towards Ymir as she approached it. In the Battle of Heaven and Earth, the anomaly materialized out of the neck of Eren Yeager's corpse to connect to his severed head on two separate occasions. After failing to reunite with Eren on the second occasion, the anomaly became extremely aggressive, actively attacking the Armored Titan.
Being the source of the Power of the Titans, the anomaly was able to utilize identical abilities to the giants derived from its existence. When it was separated from Eren's Founding Titan at Fort Salta, the anomaly emitted a gas which causes all Subjects of Ymir in its vicinity to turn into Titans and come to its defense similar to how Zeke Yeager's spinal fluid combined with weaponized gas turned the residents of Ragako into Titans.
While fleeing from her Eldian tormentors, Ymir Fritz encountered the life-giving anomaly beneath a colossal tree in the wilderness. Upon physical contact, she was imbued with the Power of the Titans, gaining the immense form of the Founding Titan.
In his journals, Grisha Yeager recounts his conversation with Eren Kruger in which Kruger elaborates on the varying stories surrounding Ymir and the origin of the Titans. While some consider Ymir's power to be of the Devil and others believe her power was a gift from God, Kruger notes that some believe the origin of Ymir's power was the "source of all living matter", though he admits all explanations of such history are ambiguous and ever-changing.
While attempting to come into contact with Zeke Yeager to unlock the power of the Founding Titan, Eren is decapitated by Gabi Braun shooting him in the head with an anti-Titan rifle. Regardless, contact is still achieved and sends the consciousnesses of the two brothers into the paths with Ymir where Eren witnesses the origin of Ymir's power. Upon gaining control of the Founding Titan moments after decapitation, the anomaly manifests from Eren's body and reattaches itself to his head, saving Eren from completely dying.
Gabi Braun witnesses this event and later recounts it to her comrades many days later, comparing the anomaly to a "shining centipede". Suspecting the anomaly to be the true source of the Power of the Titans, Gabi anticipates seeing the anomaly once again if Eren were decapitated a second time. Shortly afterwards, as anticipated, the anomaly manifests out of the spine of Eren's Founding Titan after Jean Kirstein decapitates it with explosives, this time on a much larger scale. The colossal anomaly attempts to enter the Founding Titan's severed head but is stopped by Reiner Braun's Armored Titan. Reiner holds the anomaly in place while Armin Arlert transforms into the Colossus Titan, enveloping both Reiner and the anomaly in its explosive transformation.
The anomaly survives the detonation while Reiner and Armin move toward it in an attempt to kill it. However, another explosion reports from the crater behind them, where Eren reemerges in a new Titan body. Armin moves to face him, leaving Reiner to deal with the "shining centipede." The serpentine being then begins to exude a large amount of gas as it continues writhing, billowing towards the top of Fort Salta. The assembled soldiers quickly determine that this gas is similar to Zeke's spinal fluid cloud, used to turn the inhabitants of Ragako into Titans. Shortly after, the remaining Eldians are transformed into Titans which leap off the mesa, rushing to defend the anomaly. Annie Leonhart and Pieck Finger transform of their own will in order to engage the creature and assist Reiner, but the Warriors are soon overwhelmed by the horde of Titans advancing.
- The "source of all living matter" bears some resemblance to Hallucigenia, an ancient species of aquatic lobopodian worm from the Cambrian period. This resemblance is made more apparent by the appearance of a Hallucigenia sparsa during Zeke Yeager's discussion of the origin of life.
- At the end of the final chapter the tree under which Eren's head was buried is shown to have grown to resemble the one Ymir Fritz stepped inside before being turned into the first Titan. Whether this implies the survival (in some form) of the "source of all living matter" or not is unknown.