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I saw a similar post here, however, the discussion became too based on immortality in general and how memory would be affected over millions of years. Let's say a figure like Bill Gates who is getting up in age decides to extend his life by either cyber enhancement or 3D printing new organs every time one fails. If the brain were the only thing not to be able to be replaced, how long would it theoretically last? I want to avoid points on the brain being uploaded to a new one or a computer.

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  • $\begingroup$ behold cryogenic freezing... it's a miracle... it's fountain of youth... it's elixir... it's about 4 minute. $\endgroup$
    – user6760
    Commented Mar 7 at 2:20
  • $\begingroup$ The mother of a friend of mine is suffering from dementia. It's causing the mother - and the family - a lot of trouble (ever tried to explain to someone with dementia why they shouldn't give a shyster money?). Her mother likely has a good 15-20 years of life left. There's more to the brain than simply keeping the tissue alive. I wouldn't be surprised if it's far more common for the brain to stop functioning well long before death than it is to die with a fully functioning brain. $\endgroup$
    – JBH
    Commented Mar 7 at 3:21
  • $\begingroup$ @JBH I've had family with that, and others that didn't, my grandmother lived to over 105 and her brain was fine, and nope, I'm sure it's more common for death to occur well before the brain stops functioning adequately (even excluding deaths due to violence and accidents), any idea where we find some stats? $\endgroup$
    – Pelinore
    Commented Mar 7 at 8:38
  • $\begingroup$ I remember reading estimates of a "perfect brain lifespan" of 300 years, but I don't have the source. Also apparently brain depends on a lot of stuff operational within the body, recent discoveries also report stem cells being used to repair/resupply the brain's neuron population, thus if "everything else" could be replicated, I'd say 300Y is what you might experience for your characters feasibly. $\endgroup$
    – Vesper
    Commented Mar 7 at 12:11
  • $\begingroup$ "I don't want to live forever, ask me again in 500 years".... Bill Gates :S2 Page 178 $\endgroup$
    – Kilisi
    Commented Mar 8 at 1:30

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No part of the human body is constructed in such a way as to enable it to last much more than the average lifespan. Why would it? Mother Nature is a paragon of parsimony, She does not do overengineering for no purpose.

"The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow". (Psalms 90:10)

The brain is not an exception. Neurons die and cannot be replaced. Cognitive functions dimish with age. The brain of a 90 years old Bill G. is not the brain of a 20 years old Bill G.

The maximum natural lifespan of humans is variously estimated between 115 and 125 years; in real life, no human has lived for more than 122 years. This is the endurance which natural evolution has built the brain for; remember that the brain has no self-repairing functionality -- once a neuron dies, it cannot be replaced; and about one neuron dies every second...

It is imaginable that the endurance of the brain could be extended by living a very sheltered life, devoid of emotion and devoid of mental strain. But then, what would be the purpose of such life?

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    $\begingroup$ "once a neuron dies, it cannot be replaced" [raises a tentative finger] umm, with stem cell therapies that's no longer entirely true, particularly in respect of the likely near future, hasn't been for a long time, several decades if you count experiments with mice and monkeys and the injection of stem cells into their brains which go back as far as the 60s iirc .. and if the brain isn't human (presumably that's what he's thinking of but he hasn't actually said) there are several species that have been noted to replace neurons naturally which adds a whole other wrinkle. $\endgroup$
    – Pelinore
    Commented Mar 7 at 1:07
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    $\begingroup$ Also worth noting that if the brain isn't one subject to things like strokes and dementia it can often be the last thing to go, the cause of death for the oldest among us seems rarely to be the brain but rather one or other of the brains support systems (like the heart) which suggests all else aside the potential working lifespan of a healthy brain divorced from any issues of the body it's in may be longer than most other organs, so somewhat more than the 120 or so years of posited near maximum lifespan for the entire natural organism. $\endgroup$
    – Pelinore
    Commented Mar 7 at 1:21
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    $\begingroup$ "once a neuron dies, it cannot be replaced" True, in humans, but not all animals, for a given value of true > meaning only after about 18 months old (1.5 years), the brain is still adding neurons until then, which suggests a mechanism is turned off, so potentially a mechanism that can conceivably be turned back on if we know how it works and was turned off in the first place. $\endgroup$
    – Pelinore
    Commented Mar 7 at 1:36
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    $\begingroup$ Hmm, actually Harvard edu disagrees with what I said earlier "Over the years, neurogenesis gradually declines, but the process doesn't stop even into older age. That's especially true in the hippocampus" .. so it seems new neurons are produced but at a significantly reduced rate after early childhood .. also the brain doesn't reach it's full adult weight until around 7 years old anyway so it's got to be adding something until then. $\endgroup$
    – Pelinore
    Commented Mar 7 at 1:55
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    $\begingroup$ "...the endurance of the brain could be extended by living a very sheltered life, devoid of emotion and devoid of mental strain.", not really as the brain's activity controls its blood supply, sounds like a recipe for vascular dementia. Sure, physical fitness helps, but see Steven Hawking. I mean before he died needless to say. $\endgroup$ Commented Mar 7 at 2:56
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Since this is "worldbuilding" and since the question includes, "... if properly cared for" then why not create something that will "properly care" for the brain so that it will last indefinitely?

For example, a biological creature that corrects problems as they arise. This could be something like an ant colony... but instead of ants, we have microbes... instead of managing aphids or fungus (as some ant colonies do), these microbes manage neurons -- healthy neurons that are happily functioning in a human brain... with or without the benefit of a body... but for now.. perhaps without.

For fun, it might be interesting to imagine why such a (colony-) creature would exist... but that can't be too hard... just need to think of a benefit to the creature(s).... maybe they just do this sort of thing for pure pleasure... like an old widow who does nothing but look after a the flowers in a garden. Like she -- and others like her --, these creature(s) are capable of taking care of themselves but they have extra capacity and they enjoy taking care of other cell-colonies -- such as a bunch of human neurons in a brain that has been isolated from its body. it's a hobby!

Why not? It's happening somewhere.

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    $\begingroup$ Reasoning for microbial existence that care for human brain a "hobby"? Ouch. Yet, engineering such a colony can happen, and making artificial microbes is already within our capacity. So, give em a symbiotic colony and have the brain live in peace. Still, the brain itself cannot grow new neurons (or so current science says), thus this brain would at least start losing storage over time, and might eventually disorganize, this would cause death of the individuum but not death of brain itself, and the question looks like it seeks the lifespan of a mind, not just the brain. $\endgroup$
    – Vesper
    Commented Mar 7 at 12:19
  • $\begingroup$ @Vesper "the brain itself cannot grow new neurons (or so current science says)" seemingly untrue as a statement, that appears to be some sort of viral 'old wives tale' when you actually do a search for "neurogenesis" which continues throughout life, if at incrementally reduced rates as we age. $\endgroup$
    – Pelinore
    Commented Mar 7 at 18:22
  • $\begingroup$ @Pelinore last time I tried a research on neurogenesis, I've found that stem cells are involved, so neurogenesis is not the function of the brain as an organ, the new neurons are sort of external to the brain. Need to find proofs tho, as this statement is pretty hard. $\endgroup$
    – Vesper
    Commented Mar 7 at 18:51
  • $\begingroup$ @Vesper You only ever found stem cell therapy references on the topic? Really? .. Harvard edu would beg to differ apparently "Over the years, neurogenesis gradually declines, but the process doesn't stop even into older age" .. the brain must continue producing new cells until around 7 years old anyway as we don't get a full adult weight brain until then, but according to Harvard & others it never stops entirely. $\endgroup$
    – Pelinore
    Commented Mar 7 at 19:19
  • $\begingroup$ @Pelinore well no, I've found more than stem cell therapy on neurogenesis. But this one pubmed.ncbi.nlm.nih.gov/32729779 apparently appeared after I did the search last time, this states that there's an area in the brain that holds those neural stem cells which also behave like "normal" stem cells, meaning they can activate and produce an MPSC (and another NSC) which would then migrate and become a neuron in place. OK, that thing is false, but the same paper states the supply of NSCs is still limited. $\endgroup$
    – Vesper
    Commented Mar 7 at 19:24

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