So, I'm designing A species of sapient extraterrestrials for a series of short stories, and I've decided it would add flavour if they relied on a species of symbiotic organism to enhance their intelligence to the point of sapience.

The mechanism by which this works is that the symbiote would release hormones and other chemicals which cause the brain to grow faster and for longer than it otherwise would. The symbiote has NO intelligence or cognitive processing on its own. It does NOT override the mind of the host, it only enhances host intelligence.

With that said, what kind of organism exactly should this symbiote be?

Some things that should be the case if at all possible, but which can be omitted if there is no other way to make the general idea work:

  • The symbiote should live inside the host.
  • The symbiote should be something which primitive societies with minimal knowledge of anatomy might realistically not realise the existence of.
  • The symbiote should be something which cannot pass through the placenta or through breast milk, requiring young to be deliberately infected, even if primitive societies do not realise what the ritual by which the young are infected actually does.
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    $\begingroup$ Want your children to do well in school? Come on down to Brainworms-R-Us and we'll get them infected for you today. $\endgroup$ Jan 26 at 12:20
  • $\begingroup$ To avoid confusion with brain control vs brain enhancement aspect, avoid the worm-like creature. It's been done and those engaging with your story (and assumedly would have also engaged in that other story universe) are bound to conflate the two. $\endgroup$ Jan 26 at 12:41
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    $\begingroup$ Personally I'm partial to a tiny crablike critter going in through the eye or ear! But that's an opinion not an answer :) $\endgroup$ Jan 26 at 12:41
  • $\begingroup$ Beowulf's Children had a similar scenario: the mainland "grendels" could be infected with a brain parasite. If this happened at the right stage in their life cycle, it caused an increase in brain size and intelligence. $\endgroup$ Jan 26 at 16:05
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    $\begingroup$ I’m voting to close this question because it is not seeking a solution to a specific problem, as required by the help center. Worse, it's basically asking for an aesthetic and appears little different to me than asking, "what color should my bicycle be?" Finally, the title question and body question do not appear to match. Is there a question here beyond "what should my creature look like?" $\endgroup$ Jan 26 at 21:40

4 Answers 4


Fungal symbiote.

I was looking for an image from "The Long Afternoon of Earth"; in that story the protagonist is infected by a fungal symbiote that augments his intelligence. The symbiote in that story is intelligent in and of itself and communicated with the host. But on searching images for that story, Google image gave me this!



Perfect! I am sure the artist did not intent for this to depict a brain symbiote but if that is what you are thinking of when you see the image it works very well. All of those globules and shimmer is the symbiote. An exobrain if you will. It grows and ramifies over the life of the host. It has gotten medium large on this individual but they can get much larger. Symbiotes are not all alike and the specific augmentations they confer are not the same individual to individual. A young person will be infected with a symbiote she chooses or perhaps her clan chooses for her.

It might be possible to switch symbiotes later in life. Symbiotes might compete for their host. And symbiotes have a reproductive cycle of their own which can be a interesting time for the host.


Fungal Ancilla.

enter image description here

This should be an extension to Willk's fungal symbiote answer.

The exact nature of the brain symbiote -- It does not work by rewiring your brain to make you smarter. This crops up often in fiction and I find it hard to believe.

The symbiote works in a less invasive manner, by attaching to the brain stem and monitoring the nerve signals in and out of the brain. The nerve impulse are sent to an exterior brain like organism for processing. The symbiote sees what you see and feels what you feel, in addition to its own senses which includes a better sense of smell.

For examples in fiction consider Iron Man's Jarvis AI or Cortana from Halo. They are AIs built into a super suit who have access to sensory data from the suit, and can be given tasks and take over if necessary. The difference here is the symbiote does not have a personality.

If necessary the symbiote can assume control of your body, and do things it learnt to do from monitoring your sensory data. When the symbiote takes over the host experiences a state of "flow" for the new task where they know exactly what to do and don't need to think consciously. For example the symbiote might know how to juggle even if you do not.

The symbiote could also assume partial control to combine both your skills. For example someone who does not play videogames might have a symbiote who knows how to work a controller. So the human decides the tactics and the symbiote decides to turn the control-stick left and press the A button.

It cannot however hear your thoughts and it cannot communicate with you directly. You do not hear the symbiote talking to you in your head. It bypasses the head directly and makes you dodge to the left at the correct moment.

This is good for the host, as they get to retain a sense of self. It is not the organism taking control. It is me making use of the organism.


Hazing a child with leeches

Q: "If possible, it should be something that can neither pass through the placenta nor the breast milk, requiring young to be deliberately "infected"."

Aha, so you'll get it later, as a child.. Good, the creature I have in mind depends on oxigen, so it cannot live in the womb.

The leech

I propose to choose a creature that won't need the brain itself for food. Easiest is to have it fed with blood. Your creature "induces mutated neural growth" so I guess there would be a certain toxic substance emanating from the animal, which penetrates the blood-brain barrier, influencing the brain DNA and causing local cell division. It could inject its "roots" into the brain itself. These threads are organs attracted by electricity: the toxic would expand the brain in places where it is often activated. The ability to learn is enhanced.

enter image description here Leech

The culture

This brain leech is not only culture or tradition. Your aliens have adjusted to them in evolution, they developed special artery to host them. So they've lived with these leeches for a very long time. Applying the leech was done in all cultures in the past. A precious tradition, especially for parents and family. Not so nice for the subject.

It's scary, but it is for your own good

At some age, a child will be ready for it. Nearly all children get it. It could be postponed by some parents, they hope the child will be able to decide for itself. In most of these cases however, that would not work: the child will actually not really be able to decide, without the symbiont. In some religious circles, there is a controversy about that.

You get it behind your left ear and then.. you'll get a party and many presents :)

Anyway, the procedure is done by professionals, you'll get your bloodsucker ! You'd have some special arteries ready - through evolution - that feed this creature. When the leech-like creature is applied, in the presence of your happy family, it will immediately attach at a certain spot behind your ear and penetrate the skin, to find the artery, which is quite painful. In the culture of your planet, that pain is seen as hazing to prepare for adult life. After 2 hours or so, you'll get your presents.. and cheer up

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    $\begingroup$ It's not exactly what I'm going for, but I really like the idea, so, whilst I can't accept this as the definitive answer, I am going to give this an upvote for style $\endgroup$ Jan 28 at 6:15
  • $\begingroup$ @Chloroflorocarbon thx for the upvote. The above describes how it would develop in humans. That was my primary frame of reference and I think the WB ways of doing things cannot do without the human frame of reference. We find plausible ways to escape pitfalls or inconsistencies in world building, these plausible ways actually refer to an Earthly situation. Your readers are humans. $\endgroup$
    – Goodies
    Jan 28 at 11:20
  • $\begingroup$ I now see you changed your question text completely! I'll review and see if I can vote to reopen. $\endgroup$
    – Goodies
    Jan 28 at 11:28
  • $\begingroup$ @Choroflorocarbon I've looked at the changes.. it improved, but it invalidates above answer, because you require the symbiont to be inside the body. That's unfortunate, I'll have to change the first part.. I vtr, but you need 4 more votes now: you could improve quality further by focusing the question more, ask about a specific aspect. If you ask "what type of..." there are many answers for that, opinion based is often the closure reason when that happens. Are there specific design problems ? Try focus on some aspect of this, that would make it easy for the reader to upvote ONE answer. $\endgroup$
    – Goodies
    Jan 28 at 11:48

Some sort of cellular endoparasite. Think "mitochondria". This probably means that it's an obligate (can't survive long outside of a cell). Reproductively it can't act like a virus where it replicates inside of the cell until the cell bursts, but that at various times in its cycle it somehow causes the cell to eject copies of itself.

This behavior likely favors only specific cell types. So it could be in brains only, if your story wants to go that direction. But really, the mechanism doesn't require this, since the endoparasite is either producing a chemical/hormone its host wouldn't produce on its own, or alternatively consuming such a substance to the point that it no longer interferes with the host.

Done right, this could lead to very bizarre evolution... the organism didn't shape the host species over many generations, but that the first generation to be infected early in development looks (and behaves, with amplified intelligence) vastly different than the parent generation.

You can also have many specimens who are somewhere in between being unintelligent and highly intelligent, the organism infected them far too late (past their developmental stage), or the infection was uncharacteristically too mild.

This can be exclusively limited to a single species. For instance, if we were talking about humans, it might infect them easily, but chimps and gorillas are simply different enough that the microbe does not infect them. Perhaps it uses a single receptor site on the cell exterior.


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