Well, not exactly aliens since it was stated in various places that Elves, Faries and Humans did have a common ancestor. Though how those 3 major species have diverged that much is still unknown. I would also argue that elves and faries are probably closer related compared to humans, but maybe humans evolved out of elves? It would take a really long time of strict isolation for such a strong diversion. That includes the various elf races.
It's theoretically possible that a prior highly advanced civilization was responsible for spreading some "farming animals" across the various islands where they then evolved into the species we know now. That old civilization may have left the planet or simply went extinct. This may tie into the religious background (Gaius, the seeder, etc). But up to this point there isn't much known about the origin of the major species. So far we just know that they are related.