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So we have a fairly good idea that aging is primarily the result of wear and tear. Of cells slowly losing information each time they divide.

So imagine then a setting where cultivation of stem cells and human tissue is relatively easy.

When you're born, doctors take some of your stem cells and use it to make a tissue bank for you.

You can as a result easily replace or regrow everything below the neck. This society also has some repair symbiotes made from modified amoeba and nematodes that allow for more precise repairs and procedures.

Would this be enough for humans to live on indefinitely? So long as they have the means to replace old flesh with some healthy young flesh?

Edit: This was part was deleted by mistake.

But the brain can be repaired in this scenario to a somewhat limited extent. Plaque can be cleared up. Telomeres can be lengthened. The brain can be kept reasonably young.

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    $\begingroup$ "Replace or regrow everything below the neck": Not really. For example, there is no (known) way to replace or regrow the hyaline cartilage of the joints. Or the nerves. Or the tendons which anchor the muscles to the bones. The truth is that the human body is designed for a limited lifespan, and there are many structures which are just not made to be repairable. $\endgroup$
    – AlexP
    Commented Jun 11, 2023 at 20:15
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    $\begingroup$ "Everything below the neck" - therein lies half the problem, even assuming you premise, as we age, we loose cognitive function. If you compare people who have had mega-successful ideas, most of them happened in their 20s and 30s. Then there are things like Dementia etc. Even if the body was perfectly repaired, the brain would age to the point of death or debilitating. $\endgroup$ Commented Jun 11, 2023 at 20:34
  • $\begingroup$ @TheDemonLord my paragraph about the brain was deleted by accident. I know that replacing brain tissue its own can of worms. But in this scenario the assumption is that the brain can be reasonably maintained against things like senility, cancer, plaque, and damage. $\endgroup$ Commented Jun 11, 2023 at 20:49
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    $\begingroup$ @AlexP all of those can be replaced, you just have to do it whole hog, don't replace the elbow cartilage or tendon replace the entire arm. $\endgroup$
    – John
    Commented Jun 11, 2023 at 23:07
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    $\begingroup$ I'm not sure if your claim about "we have a fairly good idea that aging is primarily the result of wear and tear" is an actual claim, or a hypothesis for your worldbuilding. You may have a faulty premise. Sounds like you've read a paper like this one and presume we have it all figured out. What we understand about aging is complex and nuanced as this paper describes. $\endgroup$
    – Wyck
    Commented Jun 12, 2023 at 14:23

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Not easily, maybe not at all. The problem lies not in the parts but in the whole that is indeed a whole.

The body does not consist of individual parts like an engine that you can disassemble, then take a worn out part out, replace it with a new one, and then reassemble it.

If you replace an organ, you need a surgery, and surgery is an invasive operation, more akin to drilling a hole in an engine. Every such operation is costly in terms of lifespan of the cells that are responsible to make it work again, to heal the wound.

If you figure out a way to overcome this, then maybe. If you cannot, then no. The wounds can accumulate into a scarred body that eventually has undergone too much damage, which is a problem of its own.

See eg.

https://njms.rutgers.edu/departments/molecular_genetics/faculty/herbig/TelemeresinWoundHealing.php

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  • $\begingroup$ Having read Larry Niven's Known Space series, specifically the books set in the "Gil the ARM" era, interesting to find out that the organ banks might not significantly prolong human lifespan. $\endgroup$ Commented Jun 11, 2023 at 23:45
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    $\begingroup$ @KerrAvon2055 I suppose that along with the organ banks they have some means to "install" the organs so that the procedure is not detrimental to the organism as a whole. As of now, cloning organs is completely feasible, but healing after the "installation" is still an enormous question mark. It is even possible that the solution to wound healing is also a solution to the whole problem here, so organ banks might be redundant. $\endgroup$ Commented Jun 12, 2023 at 0:00
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    $\begingroup$ Y'all forgetting the brain. How does one go about replacing one's brain? $\endgroup$ Commented Jun 12, 2023 at 17:57
  • $\begingroup$ @MindwinRememberMonica Yeah, that is a good question. I would try to avoid anything invasive, and instead aim at activating brain's inherent regeneration mechanisms + trying to maintain optimal brain health to slow down accumulation of damage. fiercebiotech.com/research/… $\endgroup$ Commented Jun 12, 2023 at 19:13
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    $\begingroup$ @JaniMiettinen Organ banks would imho come in as a backup option for cancers (aka regeneration gone wrong) or if some organs are destroyed in their entirety $\endgroup$
    – Hobbamok
    Commented Jun 12, 2023 at 20:00
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I, a simple skeep, have already posted an answer. Now for the answer that I find more interesting. If you want biological immortality, look no further than Turritopsis dohrnii. When faced with death or other hazardous circumstances, this little critter curls up and turns into a little baby. Like when the waiter gets your order wrong, but you've already maxed out your number of daily interactions with strangers.

So rather than dying by natural causes, Turritopsis reverts from its adult, sexually mature stage back to its polyp stage. From there, it grows up all over again, only to revert when again faced with old age. Of course, they can still die from disease or being eaten. The same is true for people.

In your world, rather than growing entire organs and swapping them out NASCAR-pitstop-style, your medical scientists use their magical (sorry, engineered) amoebas and nematodes to distribute stem cells throughout the patient's body. The patient then receives hormones that trigger the de-aging process, and they spend the next few decades reverting to childhood.

Now everyone in your world is a Benjamin Button. On your 65th birthday, you get to begin de-aging. It used to be earlier, but the demand keeps increasing with the population. After 60 years, you are chronologically 125 but biologically 5. Then you stop taking your de-aging hormones, and you start growing up again.

When you meet someone new, you ask for their age and whether they're going up or down. It's rude to ask for their chronological age, but you can tell when it's the year 2450 and this kid still remembers YouTube.

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  • $\begingroup$ Where does all the extra body mass go? $\endgroup$
    – RonJohn
    Commented Jun 12, 2023 at 12:38
  • $\begingroup$ @RonJohn Presumably poop. You normally excrete some cellular detritus ( thehealthboard.com/… ) and it's pretty rare for people to gain or lose weight anywhere close to the three-to-five pounds-per-day of food that they eat (precisionnutrition.com/what-are-your-4-lbs) and mostly excrete. $\endgroup$ Commented Jun 12, 2023 at 13:12
  • $\begingroup$ @SarahMesser feces are undigested food, not "stuff extracted from the body". That would be urine; you'd need bigger ones, though, and to drink a lot of water, so that kidney stones don't form. $\endgroup$
    – RonJohn
    Commented Jun 12, 2023 at 13:20
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    $\begingroup$ @RonJohn Never fear, the amoebas and nematodes will properly de-grow the bones instead of letting them osteoporize. You can tell which people are on the growing-down cycle based on the jugs of water the lug around and how often they're running to the bathroom. $\endgroup$
    – skeep
    Commented Jun 12, 2023 at 16:31
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    $\begingroup$ @PaulSmith I don't know about you, but I peaked in 3rd grade. Each time I hit puberty, I start counting down the 120 years until I'm back in my prime. $\endgroup$
    – skeep
    Commented Jun 13, 2023 at 16:24
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Um, Cancer, anyone?

You will need to overcome the biggest problem for your aging - cancer. Cancer cells are able to grow and spread rapidly, are genetically and immunogenically you, and metastasize to numerous organs throughout the body - including the brain. People who have cancer are not considered candidates for giving organs, because any of their organs might have cancer cells in them.

Given enough time, everyone will get cancer eventually. All it takes is for a few cancer cells to turn metastatic and get stuck in the brain (a common enough occurrence) and even replacing the ENTIRE body at once is not longer effective. Because the symbiotes would need to attack "self" (cancer) cells to ward off cancer, the risk of these critters eating their host alive in a sort of pseudo-auto-immune reaction becomes very real.

So excluding and/or handwaving all other reasons, cancer will make your regeneration a handy way of fixing damage and extending health/life, but immortality will be out of reach unless the brain is placed into a copy-capable form (like a computer) that allows transfer of mind without exposure to the body.

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  • $\begingroup$ The largest risk factor for cancer is age itself - a lifetime of mutations accumulate over time and eventually results in cancer. Eliminate aging, and you will also eliminate the vast majority of cancers. Annually refreshing all your cells to a "clean slate" genome will result in a cancer risk equivalent to the risk of an infant developing non-congenital cancer in the first year of life - very, very unlikely. I suppose you'd still have brain cancers if you can't refresh the head, though. $\endgroup$ Commented Jun 12, 2023 at 19:06
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    $\begingroup$ @NuclearHoagie Actually, the metabolic slowdown of aging may be a mechanism in part to slow cancers. You stop renewing rapidly growing cells, and the multiple kinds of damage needed to cause a cancer are less likely to add up to growth. The cancers of old people are slow and relatively treatable, while the cancers of young people tend to kill them quickly and spread before being noticed. $\endgroup$
    – DWKraus
    Commented Jun 12, 2023 at 19:15
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No You can drastically increase life span but that's it

You can't replace the brain, and the brain does decay. Even if your tech allows for full cerebrospinal transplants eventually the brain/spine will fail, most likely the connection between new and old spinal nerves will fail first, nerves can be rejoined but not without wear and tear to the nerves.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596698/

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You want to grant people immortality by "[replacing] old flesh with some health young flesh". I assume you're not writing a horror novel, so I won't dwell on just how creepy that sentence sounds.

As far as we know, neurons can live for a very long time. With sufficient maintenance, perhaps the brain could survive indefinitely. You ask for hard science, but being able to "easily replace or regrow everything below the neck" is still very much science fiction, even with your amoeba surgeons and their nematode residents. So if you think replacing the rest of the body is a-okay in your world, taking the old noggin in for an oil change every 6 months/5000 miles should suffice to keep her humming along.

Unfortunately, there are other parts of the head that will suffer from aging. Male pattern baldness will continue to plague mankind, so low-grade late night TV will still be peppered with infomercials for creams, lasers, and crystals that promise to bring your locks back to their glory days.

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