Behind its distinct adorable animation style, the deceiving anime Made in Abyss hides horrible deformities and vicious creatures. Riko, a young girl, and Reg, a robot companion, go with us as they enter the Abyss, a large, monster-infested, mystical abyss. The monsters get more vicious the farther they travel into the Netherworld. Despite this, a curse emanating from the pit that affects those seeking to escape forces them to plunge even further into the monster-filled jaws.
It is frequently stated that the monsters of the pit are primordial or extraterrestrial.. These monsters range drastically in variety, both in terms of how dangerous they are and how different they appear to be from normal earthly animals. While the creatures in the beginning sections of the abyss tend to look more like normal animals, they start to warp into more unique and monster-like the further down into the depth the duo travels. This theory could explain the creatures' true origin story.
The Abyssian Curse
The curse of the abyss, which affects any humans attempting to leave the pit, is triggered by ascending toward the surface. The further into the abyss a human gets, the more drastic the curse's effects become. The effects in the higher levels of the pit are more mild symptoms, like dizziness and nausea. The lower depths, however, can trigger more drastic changes to a human’s psyche and even their physical body.
Through White Whistle Bondrewd’s experiments, Made in Abyss shows viewers that humans attempting to ascend from the depths of the sixth layer will be heavily disfigured as a result of the curse on that layer. His experiments focused on rapid ascension from that point, thus the victims of those experiments turned into writhing blobs of flesh, crying out in agony. However, if an individual attempts to ascend at a slower pace, it appears as though the curse will affect them slower as well.
The Monsters of the Netherworld Might Have Been Originally Human
The idea that the curse affects people at a similar speed as they ascend is evident during the beginning of Riko and Reg’s journey. While Reg is unconscious due to using his incinerator, Riko must drag his body to safety. During this specific time, she is forced to ascend uphill. Her ascension is slow due to carrying the weight of her companion. The symptoms begin to appear slowly to match her speed: first with the dizziness and nausea, then progressively she begins to hallucinate.
Under the assumption that the disfigurement of the sixth layer’s curse could also affect someone slowly -- and with the knowledge that hollows like Nanachi can take on an animal-like appearance -- one could theorize that the Netherworld's monsters may not have originally been monsters at all. With the combined effects of the fifth and sixth layers being disfigurement (with the possibility of manifesting animal-like characteristics) and a loss of one's sense of self, it could be that the supernatural creatures inhabiting the chasm of Made in Abyss' Netherworld were once human.
The Modern Creatures May Be the Ancient Abyss People's Offspring
The entirety of the Abyss is covered in relics, called Artifacts, of long-forgotten native people who called the pit their home. While it is unlikely that many of the current creatures threatening divers on the upper layers were once divers themselves, it is not too much of a stretch to theorize they could in fact be descendants of the ancient natives who inhabited the Netherworld before cave raiders began to descend its depths. Perhaps as much as the current-day cave raiders wish to see what lies at the bottom of the abyss, the ancient people within had wanted to see what was at the top.
An additional plausibility to this theory is that some of the monsters could be humans who were originally sacrificed to the Abyss by the native tribes surrounding the pit's rim of the pit before the city of Orth was founded. While Made in Abyss' timeline seems a little strange, it is explained through Ozen in the inverted forest that time seemed to flow more slowly the deeper into the Abyss a diver got.
This is also supported by meeting the people of the Golden City within the Abyss' Sixth layer, some of whom had met the long-gone native people of the rim. It is possible that sacrifices from these tribes could have been desperate to return to the surface and lost their bodies and minds to the curse of ascension, with their only remaining instinct being to go home.