Perhaps there's a life-supporting, Earth-like planet orbiting close to a red dwarf star many light years away. Perhaps this planet orbits so close to the star that only one half of it ever faces the light. Maybe some sentient species evolved during that infinite day, on some massive supercontinent. And maybe one day, the sun set on part of that continent, stranding half of the species in the dark, where they evolved under a changing atmosphere. The light and dark species, unable to survive in the other's environs, separated forever.