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Let's say that you have a person who's, if certainly not immortal in the general sense, biologically immortal, as enumerated in this question - not only do their telomeres never shorten, they're also-near-immune to cancer. This means that, while they're going to die eventually, due to a variety of factors, they'll also live for a very long time if they don't have some kind of accident. They can be physically killed, but their body will never break down.

Anon's answer to that question pointed out that:

The other problem here is that the human body does not retain the ability to replace structures. If your adult teeth are removed or badly damaged, there is no biological mechanism in place to repair or replace them, for example. This will lead to another answer to your question, along the lines of "what life-critical part will wear out first"? Specifically in females who birth children, depleted-bone-calcium is a contender - if that's still a part of your future, population control will be a consideration.

Assume that this hypothetical long-lived individual lives for a thousand years. I'm not sure if that's actually how long they'd live, but let's use it as a benchmark.

So, aside from adult teeth, and bone calcium, what other parts of the body will irreversibly wear out in the span of a thousand years?

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  • $\begingroup$ Might not be right, but in the comment form, every organ as well as various muscles. If they age, that might/probably means that any repair functions die down over time, and so most organs will deteriorate to the point where they might just fail at one point, and so if you can measure the deterioration, directly or as a proxy of how well it's functioning, you might get a rough idea of age and further how long they have left, and if you can chart the data over a longer time period you might get it to be far more precise. $\endgroup$ Nov 30, 2021 at 20:29
  • $\begingroup$ @Madman That's the thing; in the case of this specific immortal, their organs and muscles never age under normal circumstances. Since the telomeres never burn out, the cells are never damaged "all at once"; sure, chemical damage and background radiation might chip away at their genes over time, but that's incremental, meaning that the cells have time to repair the damage before it gets replicated and snowballs into more damage. But that's all irrelevant. This question isn't about aging; this question is about what body structures would irreversibly wear out in someone who lived 1,000 years. $\endgroup$
    – KEY_ABRADE
    Nov 30, 2021 at 20:37
  • $\begingroup$ Could you put that in the description? You said their cells age very slowly, not not at all. $\endgroup$ Nov 30, 2021 at 20:39
  • $\begingroup$ @Madman Yeah, my B. $\endgroup$
    – KEY_ABRADE
    Nov 30, 2021 at 20:39
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    $\begingroup$ @AlexP Fail eventually due to aging? Or fail eventually due to being physically damaged by external causes, in a way that they can't regenerate? $\endgroup$
    – KEY_ABRADE
    Dec 1, 2021 at 0:25

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Eyes

The eyes lack a lot of repair mechanisms. For example, floaters. The fluid in your eyes slowly breaks down over time. Those floaters you see are bits of protein casting shadows. This is not cellular damage, it is the fluid itself degrading. Even in healthy people floaters appear and slowly build over time. Eventually, it will all wear out and your immortals will have very cloudy vision (but not necessarily totally blind).

Additionally, your rods and cones, vision cells, do not regenerate if they die. This means that bright light (sun) damage can eventually blind them. Though, ironically, enough eye floaters will help protect against this type of damage.

It is also worth to note that your eyes and your body are not immuno-compatible. Your body will treat the inner parts of your eyes like foreign attackers if exposed to them.

Ears

Hearing loss is not only age related due to cellular degeneration, it is also age related due to physical structural degeneration. Your hearing is somewhat complicated in how it turns vibrating air pressure into nerve signals. A critical step in the process is the hair cells in your ears, about 15000 of them. Loud noises and physical trauma can damage them, which will reduce hearing capacity over time.

Nail beds

Not quite appropriate to your answer as it doesn't wear out over time. Your nails might grow constantly, but damage to the beds of the nails can cause permanent damage. Tears, cuts, scars, and hard enough blunt hits can cause nails to no longer regrow.

Cartilage

It is considered finite in the human body. Between bones it wears out and eventually have bone on bone rubbing. There is also some in your nose, ears and lungs. It does regrow itself in limited capacities, so it doesn't universally just disappear. However, trauma to the nose and ears would require surgical repair. Most joints will eventually need medical treatment.

Side Notes:

Prions: A non-genetic or cellular disease. Basically misfolded proteins that cause other proteins to fold incorrectly. 100% fatal upon contraction. Very rare, but a real limiter as it isn't cancerous, genetic, and has no immune response.

Scar tissue is a major concern for those long time periods. Scar tissue will prevent many structures from regenerating normally. Even small cuts and scrapes can scar, so many of your immortals will have a lot of scars to show off. The majority of them are of no concern. Some will prevent hair to regrow in certain spots. Internal ones can cause some problems depending on the trauma that causes it.

You may also be interested in cross-linking proteins.

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  • $\begingroup$ Ears also are dependent on the speed at which nerves transmit information. A lot of old folk's hearing problems are due to the nerves slowing down and thus, making it difficult to make sense out of the sounds. Effects are: inability to hear higher pitches, more difficult to hear speech when there is reverb or noise in the background. $\endgroup$
    – David R
    Dec 1, 2021 at 14:56
  • $\begingroup$ Lungs also lose flexibility and thus older people can't get as much oxygen into the body. $\endgroup$
    – David R
    Dec 1, 2021 at 14:57
  • $\begingroup$ rod and cone cells do regenerate but only from migrated stem cells so very slowly $\endgroup$
    – John
    Aug 22, 2023 at 13:59
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First a minor frame challenge

Bone calcium will not be an issue. Bone calcium depletion is the result of our digestive tract becoming less efficient as a natural part of aging. The acute depletion that happens in women as a side-effect of child rearing is only temporary; so, your immortals may become calcium deficient for a few years while they are of child bearing age, but once they are done having kids, they would naturally recover as long as thier digestive track is "locked-in" at a youngish age.

Menopause and tooth-lose may or may not be issues. The very assumption that a person can live for 1000 years means that thier body has ways of regenerating itself that our bodies do not. A big part of what defines aging is how we become damaged and unable to fully regenerate by our life's events. Old people don't just automatically have sore backs, bad knees, etc. A lot of these things develop as a response to trauma. So, if we assume that they only regenerate as well as a human, then thier bodies will slowly accumulate scar tissue which will lead to decreased physical performance and essentially just translate to "getting old" anyway. So, this means we need to assume some manner of super human regeneration, and this also means they may be able to regenerate missing teeth and eggs in ways that we do not.

The biggest issue will be the brain

All parts of your body eventually get damaged by something; so, genetic immortality assumes your body can perfectly pull up the required genetic blueprints it needs for any repair, however, you are more than just your genetics... your wisdom and experience are a part of you that can never be repaired when damaged. So, every hit to the head or major infection risks loosing just a little bit of your mental faculties. Even if your brain can physically heal itself, overtime it will become such a patchwork of new and old brain that your thoughts could become completely incoherent.

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  • $\begingroup$ ... assuming that your brain doesn't constantly cross-wire itself as part of normal operation. I'm yet to meet anyone whose thoughts are entirely coherent, and every day brings new stimuli to push bits of your brain-structure in different directions. Your past self will be gradually overwritten, but I doubt very much that this would result in incoherence for an otherwise-healthy brain: keep the wetware healthy and the mind patches itself... $\endgroup$
    – Anon
    Dec 1, 2021 at 4:19
  • $\begingroup$ @Anon A person with brain damage does not simply need to re-learn lost skills, he would need to figure out what to do with all the left over fragments of memories that are now just random pattern fragments. Those left over corrupted memories can still fire off, creating vague and confusing thoughts. Imagine how much harder it is for a 50 year old to keep his thoughts straight, than a 25 year old, then multiply that difference by 40. $\endgroup$
    – Nosajimiki
    Dec 1, 2021 at 14:54
  • $\begingroup$ Understood. Not necessarily agreed with, but understood. $\endgroup$
    – Anon
    Dec 2, 2021 at 4:40
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Memory?

Frame challenge: (sort of) while memory is not a technically a body part as referenced in the original question it is an essential function of a body part i.e neurons. And there are physical limitations imposed upon neurons in terms of how well/long they can perform this function. Also memory function is essential for the survival of all but the simplest of organisms.

Not an expert by any means but as I understand the science the human brain has a limited capacity to lay down new memories and retain old ones, even assuming it suffers from no physiological deterioration over time. This is not to say there that capacity isn't large or that there aren't mental exercises and techniques that can be used to help retain memories. Just that there are limits.

People tend to strongly retain childhood memories then as they lay down new ones while growing tend to retain those they use and need most frequently i.e memories related to their current circumstances. Over time memories that aren't relied on or which don't get recalled often tend to fade if they are not 'important' for some reason.

In your example where someone has lived potentially thousands of years there would be whole centuries where people, places and events (unless they were dramatic or otherwise important for some reason) would tend to fade away. Memories from previous centuries might not be an issue but there could well have repercussion for day to day existence if memories perhaps only a few decades old or even less are lost. This is especially the case if, as seems likely the person concerned wished to remain anonymous.

I suggest that your character would want to become an accomplished and determined diarist as a result. If they lived long enough he or she could have an extensive library documenting their past experiences.

FYI there's very good, low key 2007 movie called The Man From Earth that addresses some of the issues involved with living forever in a world where others don't including the issue of memory.

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Poor diet can cause issues to build up that were fatal in pre-surgery times.

For example, many gallstones develop due to diet. Some develop due to lower melatonin levels. Obesity, gender (pre-menopausal women seem to get more than men or post-menopausal women) and alcohol consumption seem to be risk factors. Modern treatment is usually surgical removal (this is the 8th most common surgery in the US) of the gallbladder when a "stone" blocks a duct and causes extreme pain.

The gallbladder acts as a storage container for liquids that help the body digest fats. When it is gone, one needs to eat lower fat meals as there is no buffering for consumption of high fat meals.

There are 2 major types of gallstones: cholesterol and pigment. Pigment stones are created by an excess of bilirubin which is the final breakdown step in the body's disposal of waste hemoglobin (the active ingredient in red blood cells). Cholesterol stones are formed by excessive cholesterol precipitating in the gallbladder. While there are drugs that could soften and partially dissolve gallstones, they need to be taken for long periods of time - months to years. If a stone gets stuck in a duct, the pain is the sort where you're curled up in a fetal position from the pain. Some stones have sharp edges and could damage the bile duct leading to digestive juices leaking into the peritoneum, digesting internal organs. That's going to hurt while it kills you.

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Intercellular space

This is another thing that is bad at regenerating. In fact, non-regenerative properties of interecellular spaces is the reason why teeth and cartilages don't regenerate. Or why our skin age. Or why eyes get worn out by age. In fact, majority of problems described in other answers have their roots in the fact that intercelullar space has issues with regeneration. So even if you had immortal cells you would still have that issue and would thus age.

The reason being that proteins and other macromolecules degrade with time (if for no other reason then because of entrophy). And since those macromolecules are outside of cells there is an issue of replacing them. After we stop growing those mechanisms stop working (or slow down drastically). Restarting those processes wouldn't work either, as we have no mechanisms to remove degraded macromolecules from intercellular space, which means that we would get oversaturated by those macromolecules if we keep on adding to intercellular space. That would lead to serious (and lethal) health issues.

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A biologically immortal being by default has perfect regeneration. Perfect regeneration by default means that every single part of the body can regenerate perfectly regardless of time. Immortal does not mean invulnerable. They can still bleed starve dehydrate suffocate physical trauma etc. If ANY body part wears out over time then it's not biological immortality.

To downvoters grow up. I stated the facts. This is the only accurate answer. If you want something different then ask about longevity without biological immortality.

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  • $\begingroup$ This belongs as a comment not an answer. $\endgroup$
    – John
    Aug 22, 2023 at 14:00
  • $\begingroup$ I disagree. I stated the facts. If a being has body parts that wear out then they are not biologically immortal. $\endgroup$ Aug 23, 2023 at 21:41
  • $\begingroup$ which is why should be a comment, explaining so the OP can correct their terminology or adjust the answer. Stating a fact is not the same as answering the question. $\endgroup$
    – John
    Aug 24, 2023 at 0:04
  • $\begingroup$ Again, I disagree. $\endgroup$ Aug 24, 2023 at 0:28

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