The visitors don't really represent anything specific.
They're just based on random memories. The planet/alien doesn't understand humans any better than they understand it.
All life on the planet is a single being in the form of an ocean of goo. How could one creature have evolved to become intelligent? Obviously, it didn't start with one creature. The "thinking ocean" is the result of an extreme Assimilation Plot
The Ocean on Solaris is a technological artifact
The planet Solaris cannot exist naturally where it's located so one might conclude that it and the Ocean were placed there by some other intelligence. The Ocean is pretty weird so one has to speculate on the strangeness of its possible creators. Wonder if they're the same lot who descended on Harmont in "Roadside Picnic"?
All the characters are constructs of Solaris
Perhaps humans used to, or still do, visit Solaris, but none of the characters in the novel are real. They're creations of Solaris based off of people who died in the ocean. Solaris takes these templates, makes a few modifications, creates them and then sees what they do, in the way we might smash atoms together in a particle accelerator to see what happens. Or worse, maybe humans don't exist at all, and Solaris invented them to try and understand itself.
Solaris IS a baby of its galaxy spanning species
The 'species' of Solaris did evolve, but Solaris is just one individual of it. Solaris can control gravitational fields somehow, and so it makes sense that it, or other members of its species, could move through interstellar space. Perhaps they lay 'eggs' on suitable planets. The planet Solaris was chosen BECAUSE its eccentric orbit forces the baby ocean to react and learn to control its developing abilities.