After Cain killed his brother Abel, he was cursed by God to wander the earth forever. On his travels, he met a sympathetic woman named Lilith. They discovered that they had a mutual beef with their creator. Lilith, first wife of Adam, refused to suit to her husband and was cast out of the garden by God. Cain, Adam's first born son, was rejected by his father after his precious sacrifice wasn't accepted by God. The two kindred spirits founded their own race of humans called Evangelions, and set about to bring their former benefactors to their knees.

This race of humans is stronger and faster, growing to be about 8ft tall. Otherwise, they are biologically similar to regular humans. The biggest difference is that they possess a unique organ referred to as a "core". This organ allows for the regeneration of cells, giving Evas the ability to regrow body parts. Arms, legs, and even the brain can be regenerated almost immediately, providing them with a form of biological immortality.

However, this core has a weakness. Although it can regenerate cells and other organs, it is unable to regenerate itself. It cannot repair damage done to it, which reduces the Evas regenerative ability. Once it has taken too much damage, the being dies for good.

How would an organ that regenerated cells be unable to repair itself?

The answer is in Genesis: The Tree of Life.

https://www.biblegateway.com/passage/?search=Genesis+3&version=KJV

3:22 And the Lord God said, Behold, the man is become as one of us, to
know good and evil: and now, lest he put forth his hand, and take also
of the tree of life, and eat, and live for ever:

3:23 Therefore the Lord God sent him forth from the garden of Eden, to
till the ground from whence he was taken.

God expelled Adam and Eve before they could eat from the Tree of Life. But Lilith remembered. Also she knows a lot more about angels than Cain does. She snuck them back in to Eden and got out with a fruit from the Tree of Life.

The fruit regenerates itself. The immortality core in the Evangelons is a bite from this fruit. Each Evangelon takes a bite as a rite of passage - gaining prohibited immortality and scorning the command of God. This bite of fruit integrates itself into the being, granting divine immortality. But it is not the fruit and it will not regenerate itself. If it is damaged somehow or taken from the Evangelon, that being becomes just another son or daughter of Adam.

It would be a basic biologic protection system. The "core" would need to produce a sort of totipotent stem cells and, probably, keep the blood saturated in them. The stimulus to produce such cells would then be the depletion of such cells in the blood.

At the same time the organ has to "retire" the cells grown too old and weak, and implement some kind of regenerative check on all cell lines to prevent random mutations from devolving into cancers (see "integrity check", below). And/Or maybe the Hayflick limit for "normal" or fully grown cells is much lower, so they don't have the time to degenerate but rather go into apoptosis - they live fast, die early and leave a beautiful corpse.

(So another difference of the Evas would need to be that their cells are on average much "younger" than a normal human's).

If we make this work this way, at least two awkward limitations follow.

One: reproduction becomes incredibly complicated, unless the core allows for a "grace period" before going active (maybe just after puberty?) and pregnancy shuts down the core enough to let a foetus grow and mature to term undisturbed (a newborn will have a DNA which isn't the same as the parent's, and the DNA check routine would kill it at a very early stage).

Two: the core itself cannot "self-check". Immature core cells are immature core cells, mature core cells are totipotent stem cells. In some ways, the core is a stabilized tumour, and in some other ways it behaves like a liver. It can have some limited regeneration capabilities, but the reproduction of immature core cells is a very, very touchy business - triggering it too often or too fast is likely to make the whole process go awry in some terrifying way. Most likely, the core goes fully cancerous and starts eating up the rest of the organism.

(It might be possible to have an Evangelion regenerate by pumping him full of some metabolism-slowing drug, and keeping him hypothermic and in a coma, so the core doesn't exceed its design specs. Or transplanting a large enough compatible core chunk).

Hypothetical core development

• at birth: the core begins to grow (e.g. somewhere under the heart).


• age three to fifteen: the core grows but is otherwise nonfunctional. Normal body growth can take place, bones are digested and remade longer, new organs can develop and mature.


• age fifteen to eighteen (more or less): the core activates and takes some roles of a human's Major Histocompatibility Complex, plus some really wicked, DNA-based integrity check system. Some biological effects and possibly a shock occurs. Also, any damaged or mutated tissues are swiftly killed and replaced, possibly triggering some form of septic shock or rhabdomyolisis. Bottom line: not all Evas survive their coming of age.


• age eighteen onwards: the core has the blood saturated in "repair workers". Most large scale cellular damage can be repaired. Actually, large damage to the brain will be repaired but leave the victim with severe mental effects, amnesia being the least.


• during pregnancy, pregnancy hormones shut down the core and floating cells so that they don't "think" it's a good idea to do something for that poor swollen uterus full of fluids and who-knows-what. Downside: a pregnant Eva is much more vulnerable than a human, since it has next to no regenerative powers (they'd be superfluous with a working core). As a stopgap measure, a wounded pregnant Eva might immediately abort the pregnancy and return to "full defense mode".


• normally, the core awakens periodically and tops up the reservoirs at leisure, and it always filters out weakened stem cells from the bloodstream, destroying them and using the raw materials to build new cells.


• a large systemic shock (e.g. loss of a limb) sends the Eva in a regenerative coma, with metabolism reduced to the bare minimum and just enough circulation to keep repairs going. The core goes into low-level, long-term activation and wears out a bit, eating into its own regenerative reservoirs. It's important to keep the Eva well fed, otherwise his chances on recovery depend on body fat and muscle reserves. In ancient times, fat was synonym of health; for Evas it is still so.


• in case of multiple, repeated activations, while the rest of the body gets "iguana-like" regeneration, the core itself has "lizard-like" regeneration, and the mechanism goes awry. As a result, the Evangelion dies, not too horribly since the heart is the first to go.


• lower genetic diversity between Evas allow easier core transplants between them.

LSemi went all around the answer without actually being specific.

To 'regenerate' a cell means the cell has to die. If the cell itself is immortal, it would never die, and thus there would be no need to regenerate it. Otherwise, one risks the inevitable cancerous growth, where regeneration occurs uncontrollably without concurrent cell death. If the cell does die, and needs to be regenerated, then this process must be very tightly controlled for cancer and mutations. That is, every regenerated cell must be identical to the original, and must only be regenerated upon the death of the original cell.

The weakest link in immortality is the possibility of the cells mutating and the DNA degenerating. The original organism may be immortal, but in subsequent regeneration, will this immortality be lost? If there is one immortality organ organ controlling the regeneration of all other organs, then one of the functions of this organ would be to ensure the quality and purity of all of the reproductions, with the ability to terminate them should they deviate from specs, or become cancerous. That is, this organ would ensure that all regenerated cells would be exact duplicates of the original. However, this quality control can not be absolutely assured if the controlling organ itself can be regenerated. What enforces the enforcer? By what mechanism can it be assured that the regeneration of the regenerating organ is never compromised?

By ensuring that the regenerating organ is always the original copy, and that the regenerating organ can not itself be regenerated, immortality can not be lost through mutation or cancer.

Different DNA

These Cruel Angels actually have two sets of DNA. One for the majority of themselves, and a second for the core. The core is only able to replicate the first set of DNA. Due to the complexity of the core's DNA, or maybe as an evolved safety measure against core cancer, the core cannot replicate its own sequence. The core only grows during gestation. It is full sized and stops shortly after the Eva is born.

Or slightly differently, the core can take in other sets of DNA and replicate it, but not its own. Then the core might be valuable to ordinary Humans, or there might be a way for Evas to regenerate each others' cores.

It is a design limitation

The core was included in the design of Evas and was responsible for their long lifespans and regeneration. Explanation of why the Evas have a core and other humans do not is a bit of a mystery.

However, just as normal human cells have a Hayflick Limit limiting cell to around 50-70 division due to the shortening of the telomeres, the DNA in the core likewise has a division limit, based on something similar to regular telemores, but enough different that the core cannot repair its own.

Cells in the core are also limited to 50-70 divisions, but the cells in the core have a longer life cycle than typical cells. This longer-life design is inappropriate for the human overall, as this also results in a comparatively slow metabolism.

Once the core cells reach their own Hayflick limit, the body dies. It turns out that the body is not really immortal, it is just capable of a very long lifespan.

When I completed this, I saw it as actually similar to the answer given by Xavon_Wrentaille