What was it like for the first joined Trill?
I'm not evolutionary biologist, but symbiosis doesn't necessarily happen all at once.
This is going to be long and RHW may not want to read conjecture for writery legal reasons:
I think it involves addressing three questions: How are Trill hosts and Trill symbionts joined? Are symbionts sentient? What is the advantage to both for the first host to have a symbiont?
How are Trill hosts and Trill symbionts joined?
Modern Surgery - We've seen, I think, three transfers of hosts: Curzon->Jadzia, Jadzia->Verad, & Odan->Will Riker (Maybe Discovery had more?). In each case, the transfer was surgical, which points to a relatively modern origin.
PreHistoric Surgery - But then, trepanation has been practiced since prehistory, so a rough version of a surgical procedure could have been practiced amongst the Trill since their own prehistory. Unique factors of their physiology and planet may have made that type of procedure relatively safe before the development of more reliable medical technology (better immune systems or fewer invasive microbes or a better natural defense against those microbes).
Early Modern Surgery - Or hey, the Trill developed subspace communications before warp, so maybe--given the fact that their civilization has surgery as a major component--they developed sanitation and germ theory before sailing, or gunpowder, or masonry, or currency, or the wheel. Maybe.
Natural Late Symbiosis - Maybe there's a 'natural' way of joining. Maybe the Trill symbiote burrows into flesh while emitting an anaesthetic slime. Then the flesh grows over it with a little scar. Maybe the host Trill have a pocket used for young, an otter pocket, or a second orifice to slowly digest some foods over time, a natural way to gain slow-release nutrition from an external plant to make up a natural deficiency. Whatever it is, which Trill symbiotes can burrow into Trill hosts, but surgical transfer is quicker and safer when the option is available. This is kind of like StarGate's Goa'uld, which were parasites instead of symbiotes. There's just a happier ending (for the record, the Trill were introduced in 1991 and Stargate came out in 1994).
I don't want to impart exotic qualities to the less human member of this pair: it's entirely possible that the host Trill have an ability to 'pocket assimilate' some organisms and develop a biological relationship with them.
Natural Early Symbiosis - Maybe the two Trill species were symbiotic before that. Maybe even before both species were sapient. Maybe the pre-sapient symbiotes provided memory assistance to the hosts that encouraged symbiotes to develop or pre-sapient hosts needed to develop motor and communications faculties that made them more appealing to sapient symbiotes.
It's kind of like if humans kept breeding dogs until they were sapient enough to communicate the things they could smell/hear and understand complex commands.
Are Symbionts Sapient?
Yes - In the first appearance of the Trill, TNG's "The Host," it's understood that the symbiont Odan was the main life form and the host was just a shell that added some flavor. That was key to the episode's LBGT allegory.
We do hear in "Equilibrium" that the symbiotes can get cranky and make demands of their Trill hosts.
This means that the joining could have been an intentional choice by the symbionts who knew a joining was possible and thought it was a good idea. Or the hosts.
No - But in DS9, we get to meet every one of Dax's past hosts, but the asset that Dax brings to Jadzia's life is the experience and wisdom of its past hosts. We break down Jadzia, Ezri, and Curzon, but don't really talk about the Dax symbiont itself.
The symbiont has been treated as a non-entity: at most, a gestalt of the previous hosts' personalities.
That dog thing I said above might be a bit insulting and I don't want to dehumanize something because it's a lumpy worm, but it's entirely possible that the symbiotes aren't that sapient.
This means that it was most likely the Trill hosts who initiated the first joining. If you can host other organisms; why host this one. If there's a parasite out there: why put it into you?
What Is the Advantage to Both for the First Host to Have a Symbiont?
Symbionts - For the Trill symbiont, it's easy to be a little bio-centric and assume just having a bipedal form with thumbs is advantage enough, when it's a mammal that has to suck air, is a big target for predators, and--assuming Trill hosts are a mammal analog--have to eat a lot to fuel their stupid warm blood.
It could be that symbionts joined because the hosts were warm or because "yeah, the apes eat all the time but damn can they cook!"
I prefer to think a simple spirit of exploration might have inspired two otherwise compatible species--parasite Trill symbiont or assimilation pocket Trill hosts--to work together and the extra vistas of experience offered by that is what kept them together. This makes Jadzia Dax's actions in "Meridian" extremely true to her nature as a Trill, but let's not do anything as dramatic as trying to justify "Meridian."
It's an easy answer, and a very Star Trek one too.
But let's assume a slightly more complex/cynical possibilities. If you're still reading I have to assume you're down with that.
Maybe it was a migratory advantage to ease overpopulation; merge with a life form and then when it hits another compatible pool which can host Trill symbiote life then you bail and get a new, roomier habitat.
Trill symbiotes could actually use hosts to compete for limited habitats, possibly due to sapients Being Like That. It's possible that the limited Trill symbiotes we see now are carrying the cultural baggage of their own self-inflicted apocalypse, now living in leftover habitats deep beneath the planet's surface.
If we assume a shrinking habitat, then host Trill become another habitat for the symbiotes. It's possible that Trill hosts are a place where lowest-status symbiotes are sent while higher-status symbiotes can stay in the pools where they're safe and comfortable. Making the pool worms the 1% of Trill society.
Or imagine if there are very smart, lake-dwelling fish whose environment is vanishing. You can give them scraps from hunting and they bring you...shiny lake rocks or seaweed spices or maybe they herd fish that you kill and split with them.
Well their environment is vanishing and Jobiah says that they if they live in water and mud and the four humors of the Trill body include water and mud, so maybe they could live in us because there's an allusion in a Trill religion to living within good people and they look at a few volunteers uteruses (uteri?) and go, "let's try a thing." Maybe there's some animal testing?
Hosts - What does the first host gain? Obviously, once a symbiont lives with a Trill host and then outlives that host and goes into a second host, the advantages are fucking huge.
But the first host doesn't see that deferred advantage. How could they?
Sure, each member gains some memories of the others and as a land-dwelling ape I'm underselling the experiental value of being a swimming worm with EM emissions as a sensory organ. I don't think that's enough. Maybe you do.
I've suggested improved memory, because I feel like the advantages would be soft and slightly deferred. Trill hosts don't gain the kind of resilience that Gou'ald hosts do. And they don't gain any of the symbiont's environmental abilities like breathing underwater or communicating/sensing via electrical impulses (maybe they do but we've never seen it because no story has ever needed two Trills to touch stomachs to quietly communicate some information).
Would some additional universal perspective be that helpful? Would observation of aquatic life forms inspire enough people to do the same? Would a host provide a tangible, immediate benefit to the first host ever that would inspire others to do that, but wouldn't have ever come up in Star Trek before?
Maybe they do. Maybe Trill host physiology does benefit from an altered body chemistry that makes them more resistant to poisons, chemicals, or other stimuli. Small advantages not too relevant for a space-dwelling Trill, but one that's very noticeable for one that's farming, hunting, or gathering Trill.
It might be as simple as getting the second-hand enjoyment from the symbiont enjoying food, sleep, and sex as a host does. A dopamine hit from someone enjoying those things for the first time.
There's also the ever-present social issue. Maybe there's just one weirdo in a cave that merges with a symbiote who is also a weirdo in a cave and they have a little cult. Depending on the difficulty of the joining in the social milieu, it's possible that Trill history has several cults of joining (and fake cults of joining).
Widespread Adoption
There are also questions about how Trill society adopts it on a widespread basis. The earlier the first joining happens--which might be something that the Trill could capitalize and even have First Joining myths--the earlier it would get widespread adoption. If someone on 21st Century Earth learned you could stick a lobster in your chest and gain the ability to overcome a nut allergy, and then you could give that lobster to someone else after you died and they could have your memories...there would be a lot of resistance to that.
Eventually, on Trill, the practice is legitimized by science or a joined Trill rising to a position of power and making joining either required or a fashion amongst the nobility.
Or maybe it's something common Trills do that nobles don't think is important but eventually--when combined with the symbiotes-as-exiles theory above--results in a unique type of class-consciousness among the Trill.
This assumes some very Terran assumptions about historical societies, yeah, but it also assumes a citizen of the 24th century could meet the Trill version of Karl Marx.
