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This is a little bit hard to explain concept, practically some like a kind of symbiosis in which two organisms evolve together one get transport, almost free nutrients and relative security and the other gets the possibility to suplement lost limbs by the body of an specific part of the body from the "living prosthetic".

I think the prosthetic creature specie will get the most benefit as you would be assured of food, transportation, and safety most of the time unless the "holder" has lost a limb and the rest of the time it just rests somewhere inside the holder or wrapped around the body.

But this would require that prosthetic creature would be able to seal a possible wound, adapt to the shape of the limb that the "holder" require and to be correctly used to suplement the lost limb, I meant the moves can be controlled by the "holder".

But I don't about the viability of this and how the different implied mechanisms would work.

The mentioned features are the basics of the concept for this question, the "prosthetic" and the "holder" should be a symbiosis which is almost like be a single creature. The prosthetic not necesarily should have the exactly shape of the previous limb, the "holder" could have lost a an arm but the "prosthetic" turns to a tentacle and in results useful for the "holder" can be kept.

Optional extra point: What kind of scenario would make this viable if is not?

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  • $\begingroup$ today living prosthetics exist, they are called limb transplant donations $\endgroup$
    – user88653
    Aug 21 '21 at 8:34
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    $\begingroup$ This is a pretty cool concept! Question - What year do you hail from , timetraveler? :O $\endgroup$
    – Vladimir
    Aug 21 '21 at 8:50
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    $\begingroup$ This isn't a complete answer, so going to leave it as a comment: I would suggest you research bone screws. A lot of R&D is being do to develop bio compatible surgical screws that dissolve in the body and are replaced by natural tissue. Check out this PopSci article to get you started if you're interested: popsci.com/science/article/2010-03/… $\endgroup$
    – Vladimir
    Aug 21 '21 at 9:13
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    $\begingroup$ This "species" could be a derivative of human stem cells that can differentiate into any organ $\endgroup$
    – Alex bries
    Aug 21 '21 at 11:43
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    $\begingroup$ Hear me out: genetically engineered tongue eating louses that can replicate humanoid limbs instead of fish tongues (also getting such a creature to evolve naturally might be nearly impossible, my best bet would be to at least stimulate artificial selection on similarly parasitic creatures). Also does this little parasitic critter help with explaining your concept? $\endgroup$ Aug 21 '21 at 13:10
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Symbiotic Pseudo-Stem Cells:

For this to work, it would need to be a little slower than what you are imagining. I did something similar with nanites. I imagine a symbiote that has additional functions in the body of an organism related to healing. There is a brain-like center to the symbiote that memorizes the physical layout and tissues of it's host and somehow is able to copy all of it's tissues. Growth could be complicated, and it might not memorize the host until the host reaches adulthood. Whenever the host is injured, the symbiote jumps in and tries to fill in the missing tissue by growing a replacement recapitulating the host. The replacement would grow more like a mass of stem cells migrating into an extracellular matrix. Only in this case, the symbiote would need to provide its own extracellular matrix in the form of memory.

This would likely be a life-long relationship, so the host doesn't recognize the symbiote as foreign and reject the symbiote cells. The actual role of the symbiote might not even be visible until a serious injury happened to the host, or the symbiote could have been filling in lost skin and so on for years. The exact form is up to you.

So when the host lost a hand, the symbiote would migrate large numbers of pseudo-stem cells to the location, and some kind of feedback would be needed to guide the growth of those cells to grow a new hand in the location of the lost one. Functionally, it would look more like the person was regenerating than getting a prosthetic. Since the functionality would be exactly mimicking the host, nerves could match up, etc.

If the symbiote was EXACTLY copying the host (mimicking down to a DNA level), you wouldn't even be able to distinguish the new limb from the old. If the symbiote was simply copying the functionality of the host, the new limb would be a different color, texture, etc.

  • NOTE: This might also be a cure for aging. As the host gets older and older, the parts on the host that wear out are slowly replaced by symbiote. Eventually, it would be hard to determine where host ended and symbiote began.
  • It also is a fair approximation of how a realistic "Invasion of the Body Snatchers" pod-people might work. Something to think about.
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    $\begingroup$ Perhaps as an alternative: rather than the symbiont being inside the body, it is an external creature. Upon the loss of a limb it is likely it can find some stemcells in the remainder of the bone marrow, or it could have invaded the host earlier and extracted some. If it can extract nutrients at the wound, by keeping it open for example and feeding off of the blood as it flows into the wound area, then it could basically become a bio-3D printer on the wound instead. I would imagine a snail/octopus type creature using the body as support for itself and its parts to create a good wound seal. $\endgroup$
    – Demigan
    Aug 21 '21 at 16:20
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    $\begingroup$ @Demigan That could work if the external creature could disable the host's immune system. Then it would only work for parts of the body that could be mirrored with the missing piece on the other side (it is able to understand bilateral symmetry) since it would not remember how the host looked before. $\endgroup$
    – DWKraus
    Aug 21 '21 at 16:54
  • $\begingroup$ I think the external creature could have more information on the body. DNA does not directly make a bodyplan, rather it semi-randomly activates and deactivates sequences. Chickens create a tail-section in their fetal stage, then one sequence kicks in and makes the tail re-absorb. Stop that sequence and a tail remains. This makes it hard to truly know the bodyplan unless you have a brain to map it directly, like an external symbiont. Maybe the symbiont can use the stemcells to copy receptors that mark the symbiont as from the body to avoid an immune response. $\endgroup$
    – Demigan
    Aug 21 '21 at 17:27
  • $\begingroup$ This is a very interesting answer, makes remember a Venom's comic. $\endgroup$
    – Drakio-X
    Aug 22 '21 at 1:53
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Wall of text, but necessary I think. For a prosthetic to work from the moment a limb is taken off you need a few systems to make the prosthetic work:

  1. it needs to be able to identify and envelope the wound.
  2. it needs to be able to identify what limb it needs to be.
  3. it needs to be able to anchor itself on the limb without causing rejection from the body
  4. it needs to be able to recognize signals for movement.
  5. it needs to be able to feed itself prior to attatching.
  6. it needs to be able to feed itself once it is attached.
  7. it needs to be able to adapt its musculo-skeletal structure to become whichever limb it needs to be (assuming there are no specific prosthetics for different limbs).
  8. it needs to be able to create a seal around the limb it is attached to.
  9. it needs to be able to grow/shrink to the correct size once attached.
  10. bonus: be able to signal things like heat, pressure, nociception, stresses in bone and muscle positions to the host body for proper proprioception.
  11. Bonus: be able to encourage the body to grow and replace the lost tissue.

That is a big list of things it needs to accomplish. I'll try to go down them one by one, assuming the living creature can borrow any animal characteristic to accomplish its task.

  1. Your Living Prosthetic (LP) should not accidentally identify a large scrape that takes the top of the skin off as a torn limb. The easiest solution would be a rudimentary eye that can find the wound and a "mouth" that can taste the wound. If it identifies things like blood, bone particulates, sinovial fluid and other markers that combined meab a bone or joint has been hit, it will attach itself.
  2. The LP needs to have a template of the creature it attaches to in its mind, and a way to feel or see the entirety of its host. I would suggest a tentacle that hoists itself across your body using the body itself as support. This also means the LP can identify if it attached to a severed limb or that something took a deep chunk out of the body. Then it can adapt what it will do and prevent people walking around with a 3rd leg.
  3. A massive struggle in modern prosthetics. You need to circumvent the host's immune system but still attach something. Perhaps it could be possible by creating a bone protrusion into the host body and growing that into the bone of the host (or the opposite end of the joint). Then stimulate the host body to create a thick facia (tendon-like membrane) inbetween the host and the LP in order to seperate the two with as little rejection possibility as possible.
  4. If the LP can already recognize what limb it is replacing, it can also try to find the nerves responsible for movement. Using the ability of sharks to identify electromagnetic disturbances of nerve activity a tentacle could identify what movement the host body might want. Another (group of) tentacle(s) could move all the way to the other limb and check how electromagnetic changes move that limb as a template for itself. It is wise for the LP to have tentacles near the spine and all other limbs as well, to give it more information and anticipate things like the need for balance.
  5. Either the hosts carry them around and feed them prior to someone losing a limb, or the LP has its own normal body it controls. Once it needs to attach it could shed most of its body quickly like a salamander its tail. A more fantastical alternative is that the LP functions as a hive for a range of insects that live inside it, the insects find and kill food like other insects or honey to feed the LP and themselves.
  6. Either the insect idea remains in effect, or the LP needs to create a reverse umbillical cord effect with its host. Assuming a humanoid species then both male and female have had an umbillical cord stuck to them wich was part their own and part that of the mother. The facia that seperates the LP and host would contain a lot of host bloodvessles, which the LP would grow its mirror-bloodvessles against to trade nutrients, vitamins and part of the immune material with. Possibly a cavity in the bone connecting the LP and host could be used more readily for a full umbillical cord without risking a massive immune response by the host.
  7. A hand is way different than a foot. Since the LP will likely need a period of adaptation to properly figure out what signals mean what movement it could use that time to also create the rudimentary shapes of the limb it needs to replace, then start a long (months) process to properly convert itself to a fully functioning limb.
  8. The "mouth" of the creature would have lips with a snail-like structure in them. This snail-like structure would move itself across the skin to fully seal the wound, excreting slimes and fagocytes to deal with any bacteria and virusses that try to pass. It might be a bit gross, but it would likely be wise to slowly push the slime outwards, making the edge of the mouth dirty but reducing the risk of infections reaching the wound.
  9. If it can alter its shape this shouldn't be too much of a problem. Potentially it could shed access material through the umbillical cord to the host in order to speed up recovery and reduce the amount of food the host needs to eat. Otherwise it needs to consume food from the host to grow in size.
  10. A bit far-fetched, the LP could try to stimulate the old nerves. Unfortunately knowing the full extend of which nerve did what is very hard to determine. Which nerve did what sensation and for which part of the body is nigh impossible to find out when it is severed, and isn't mirrored exactly in the other limb(s). I doubt this is possible even if the LP can completely circumvent the host immune system and can directly connect to the nerves.
  11. Immensely difficult and likely not possible, at least not with ease. You would need to attach the muscles for example to something while they grow while the body is stimulated to think it is trying to heal a tiny gap in exactly the shape it needs to be.
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a kind of symbiosis in which two organisms evolve together one get transport, almost free nutrients and relative security and the other gets the possibility to suplement lost limbs by the body of an specific part of the body from the "living prosthetic".

Don't service dog do already something along the line of what you are asking?

Take the example of service dogs for blind people: they become the eye of their owner, and get food and shelter.

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    $\begingroup$ A service dog would qualify as an orthosis (support for human body, like a wheelchair or cane) rather than a prosthetic (replacement of human bodyparts). In Iron man he says the wrong thing, claiming his suit is an advanced prosthetic rather than orthosis. $\endgroup$
    – Demigan
    Aug 21 '21 at 9:34

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