It's essentially a slime mold that can interfere with its host organism's endocrine and neural systems. Its lifecycle encourages it to keep its host alive as long as possible, so it evolves a mutually beneficial relationship with its host organism. The primordial symbiote organism could adapt relatively rapidly, so soon after it evolves (evolutionarily speaking), most organisms on the planet have a corresponding symbiote organism.

One host species evolves specialized structures that allow its symbiote to grow complex neural structures, allowing it to evolve complicated group behaviors like large social groups, pack hunting, animal husbandry, agriculture, and eventually writing. This species pair quickly becomes the dominant species (pair) on the planet, similar to humans' position on earth.

The host species also develops the ability for abstract thought at a similar pace with its symbiote, as each increasingly complicated instruction needs to be understood by the host. However, the symbiote is generally has far more influence on its host, via endocrine and neural signaling, than the host has on the symbiote.

Recent medical advances have made it possible for host organisms to live without a symbiote, and while synthetic symbiote hosts are a promising area of research, no viable solution exists for a symbiote to live without a host.