Which real-life constraints must be considered when adding immortals living alongside humans on Earth?

Question

Question

Assuming all things in the fictional world are the same to today's world, which real life constraints must be considered in order to increase the credibility in an immortal narrative/story.

Scope

To be clear - I mean which biological, ecological, sociological or anthropological constraints would ruin the credibility of an immortal story because the audience suspension of disbelief would be interrupted. Example : Malthusian Catastrophe.

Many thanks to those individuals who have helped to shape this question already in the comments due to an unclear original question.

Information to Consider

Information to aid answerers:

Immortality is to be interpreted as per the accepted definition. A being which can live exceptionally longer than normal or realistic biological standards to the point that normal humans consider them living forever.

Immortality is not affected by disease.

Immortals may end each other's existence through a single mechanism which ensures immortality is destroyed. In Highlander it was spinal severance, in Vampirism is it aortic puncture with a stake etc. For the sake of argument lets just refer to the method of dispatch as Method X.

Immortals pose no real threat to humans other than that which might the result of anger of other normal emotional responses. Immortals do not feed on or otherwise harvest humans.

If two immortals breed then the offspring can grow to teach the age of the father at the conception.

Immortals can engage in intercourse with humans but an immortal cannot be produced although the human offspring may posses stronger biological abilities - stronger immune system etc although not wildly so.

Should the ecosystem be destroyed through a calamitous event (nuclear explosion, catastrophic solar storm, volcano eruption) then it can be realistically assumed the immortal would also perish as regenerative abilities would be overwhelmed.

Immortals heal almost instantly from breaks, standard burns, blood loss etc unless Method X (discussed above) was the mechanism which ensures death.

Examples

Examples of real-life constraints discussed this far;

Malthusian Calamity

Brain Capacity for Memory is Finite

No true nutrition violates certain laws of Physics since Immortals could not regenerate without nutrition.

Answer 1

Population

Without a check on their population, immortals will replace all human life and then populate the world until they reach the maximum population they can.The primary thing you'll need to do to avoid this is limit the number of immortals. This could be done in a number of different ways.

The easiest way is to give them no method of reproduction. A mortal can, through exceptional circumstance, become immortal, but if immortals can have children, all of those children are mortal. This is seen to some degree in Greek mythology, where many of the gods will bear mortal children who are heroic beings, but mortal.

Healing

Barring the use of magic, healing to the degree you describe strains credibility, specifically due to the 'method X' capacity for death. It may be more reasonable to describe certain wounds as being fatal, and say that most other things won't kill an immortal.

For example, if you say that only severing the spinal column can kill an immortal, what if they get shot in the head? Does their brain have a method of growing back with all of their previous memories, or do they effectively become a new person? What if they get most of their head blown off with a shotgun?

It will make things a bit more believable if any sufficiently grievous wound is fatal. A quick test for a wound could be whether or not it's instantly fatal. If it is, then the immortal dies. If it's not, it's recoverable. It may also be the case, however, that regenerating from horrible wounds takes energy, and sufficient damage overwhelms whatever regenerative capabilities the immortal has.

If the immortals are magic, you can ignore this point. Vampires can turn into dust or clouds when badly wounded and escape that way, which disconnects the link between body integrity and survival.

Answer 2

Apart from the issues with population, I see issues with the brain.

1) Since memories are stored in the brain, somehow, and the brain is finite, there will be an issue with that at some point. I do not have any scientific mean to say how much memories may be held in a brain, but the rule of thumb is that if an human may live at most 130 years, I would not expect memory capacity to be longer than that (which evolutionary benefit would the matter/energy cost of extra memory cells bring, it they were not used). To be in the safe side, say that your human brain can hold 200 years of memories, and after that either will progressively lose old memories or have difficulties forming new ones.$^1$

2) As people mature, brains lose flexibility. Over time society/technology change, but usually not so much so, up to retirement age, you usually keep up with it. To put that in other words, right now I learn way slower than when I was a teenager. This is mostly ok, because the world has not changed that much since then, and most of what I learn there is still of utility. But if my teen years had been in the Pharaoh's time, now I would have to work harder to keep myself updated, because anything valuable would have been learnt at the slower, adult rythm.

Of these, I think 1) is the real constraint. 2) could be just waived with "due to the nature of inmortals, they retain forever the mental flexibility of their younger years" or something like that.

$^1$ In fact I would bet for way less, since anyone over 70 is highly unlikely to be able to father (and sustain while they grow up) descendence. Evolution wants to keep you fit enough for producing the next generation, but after that it forgets about you.