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The title says it all. How did the people of Trill decide that it was a good thing to put symbionts inside themselves?

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The symbionts seem to be quite wise, but unjoined symbionts lack any obvious means of communicating with other life forms, and therefore don't have an obvious way of saying,

"Hey, humanoid! Yeah, you! Put me in your abdomen!"

Only a fraction of the Trill population get joined, and in the DS9 episode "Equilibrium" we see a pool of healthy, unjoined symbionts being tended to, and so joining is not essential to the survival of either species.

How was the practice of joining Trill humanoids and symbionts initiated?

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    I'm assuming trills are much like humans and have experimented with putting every possible thing into every possible orifice to see what works.
    – Valorum
    Commented Apr 23, 2016 at 21:42
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    Dave got really drunk one night... Commented Apr 23, 2016 at 23:25
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    How did humans get the idea to try eating lobsters? If lobsters came out of spaceships instead of the sea we’d flee in fear and do everything we could to destroy the terrifying aquatic insectoid alien menace. Commented Oct 18, 2017 at 10:06

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Main Canon

No major explanation is given within the shows for how Trill society developed or how the first bonding occurred, although it's mentioned that Trills have been doing so for a very considerable amount of time. In TNG: The Host, the Trill symbiont is referred to (by Crusher) as having all of the characteristics of a parasite, suggesting that the relationship may not have begun as a purely symbiotic one, but one that developed that way over time:

CRUSHER: It's as though there's a parasite at work. Odan, Odan. I need to do exploratory surgery. You may have a parasitic infection.

...

ODAN: This body is just a host. I am that parasite. That is what must survive. It has always been this way. The Trill are a joined species. A host and a symbiont, and in this fashion we have survived for millennia.

Its also noteworthy that some unjoined Trill are powerfully psychic, at least to the level shown by the Vulcans. It's likely (and indeed almost certain) that they possess the means to communicate with the trill Symbionts prior to insertion and it's also possible that this skill developed before the first implanation. It can easily be envisioned that primitive Trill telepaths attempted to 'scan' many plants and animals before determining that symbionts were a sentient species and worthy of further study. Along with that, the use of a Trill symbiont as a potential repository of knowledge or as a hind-brain (or even just a means to transfer knowledge between hosts) can't be overlooked.


Out of universe this was addressed in the DS9 "screenwriter's bible" (written by showrunner Rick Berman) which gives some additional backstory to our characters.

Dax is a Trill... a "joined" species first encountered in the ST:TNG episode "THE HOST". A Trill is comprised of two separate but interdependent entities: a host and a symbiont. The host provides a humanoid body. The symbiont is an invertebrate, androgynous lifeform that lives within the host. It looks like a short, fat snake. Many centuries ago, the symbionts lived underground while the humanoids were on the surface... and due to an environmental disaster, they were forced to 'join' to survive. As time went on, this mutual support evolved into a biological interdependency and thus two individuals became one -- they speak with one voice (you can't ask to speak with the symbiont or the host, only the combined lifeform).

You may also wish to note that Trek Producer and Writer Ira Behr described the Trill as an astoundingly "medical" society, suggesting that their decision to bond with the Symbionts may have come about as an offshoot of early medical experimentation:

"It's a fascinating subject. So fascinating it's deep waters to get into. I just love the scene with the guardian and the Trills – weird kind of stuff we don't quite understand. I also like a society that's basically medical. Its a bunch of people really involved in their bodies."

And heck, it's not like nobody's ever had the idea of stuffing a parasite into themselves to see what happens...

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Richard has mentioned that the symbionts may originally have been parasites. This is actually the way in which symbiosis in nature generally happens. You start with a parasite that takes resources from its host. But that damages the host and makes it less likely to survive and reproduce. And of course when the host dies so does the parasite (in most cases: exceptions are microscopic parasites that actually want to get eaten by the host's predator).

So there is evolutionary pressure on the parasite to be as inoffensive as possible, and the logical extreme on inoffensive is to actually help, for instance by providing biochemical support. It is generally believed that this is how our mitochondria happened: as parasite cells inside microbes. The parasites became symbionts that provided ATP, and those twinned cells eventually evolved into us.

In the case of the Trill, it is possible that the parasites started out as a straightforward invasive worm, perhaps something similar to a leech that can actually burrow into the host and stay there. In order to evolve the symbiotic neurological link these parasites would have to be able to survive the death of the host, but still be endangered or disadvantaged by it. Presumably once outside the host they were likely be eaten by predators, and of course if the host gets eaten then so does the parasite. So these proto-symbionts would have evolutionary pressure to help their hosts, but still be able to survive in a series of hosts. And as it happened the hosts were (like humans) evolving to survival and reproduction based on intelligence rather than muscle, so neurological assistance was selected for.

This would suggest that the symbionts are primarily there to provide memory, with the host' personality remaining intact except for changes due to episodic memories. But you'd probably have to ask a Trill for details.

Mitigating against this theory, however, is the acute shortage of symbionts. If this theory was true one would expect symbionts to be universal.

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  • Thanks for your answer. But how would the symbiont parasite originally invade the host? The symbionts have no apparent locomotion, burrowing ability, etc. Jonah, in the comments below the question, suggested that the symbionts may have had such abilities in the distant past, and then lost them through evolution after the Trill humanoids began handling the joining themselves. You might want to add something like that into your speculations.
    – Praxis
    Commented Nov 7, 2015 at 20:24
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    Someone went swimming in the pool and a symbiont just latched on to them? Also this whole discussion begs the question of if Trill primate equivalents have the pouch and if any symbionts "hooked up" with life forms further down the evolutionary ladder. Commented Oct 27, 2017 at 0:40
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According to the novels and Memory Beta (the noncanon "Star Trek" wiki, that takes novels, video games, etc. into account) the Trill are marsupials, and the symbionts could simply be inserted into their pouches (both male and female Trill have puches). The Trill just began joining that way, and brought in doctors and medical procedures later on.

Even onscreen, we clearly see that Dax has a kangaroo-like pouch that they take the symbiont in and out of. I assume Beverly only had to cut Riker opened in "the Host" because as a primate, he has no pouch.

As for why the TNG Trill are almost nothing like the DS9 Trill, there are a few plausible theories; but the most likely, in my opinion, is that TNG Trill are too boring for any of us to care.

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    – Politank-Z
    Commented Oct 18, 2017 at 6:23
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    “we clearly see that Dax has a kangaroo-like pouch that they take the symbiont in and out of” — uh, when do we see that? Commented Oct 18, 2017 at 7:32
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Phil Farrand, in The Nitpicker's Guide to DS9, wondered how the trill practices of surgically implanting slugs starting. He speculated that some cruel tyrant of the host species, Vlad the Impaler, implanted various small creaturies into his victims and it was accidentally found that the slugs could bond with the hosts.

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