Sef was the very first Trill symbiont to become joined to a humanoid Trill host.
Twenty-five thousand years ago, Sef was one of many "swimmers" who broke the surface of the water. Sef used its telepathic abilities to entice four-legged creatures to the water's edge where it would attach itself to them and experience life on land. It would then return to the water and share its experiences with its fellow swimmers.
One day, a "walker" answered Sef's telepathic call and, after an initial fear response, allowed the symbiont into its pouch, becoming joined. This was the first symbiosis.
In the year 2376, Ezri Dax experienced the memory of this event during her visit through the Annuated's stored memories. (DS9 - Worlds of DS9 novel: Trill: Unjoined)