Short answer: Genetics is hard, but not that hard.Hearly wrote:How would the Humans know and not the Faey, I don't think Fel has every wrote anything in the stories implying that they even think it... The only reason we know it is because Fel told us so.
Long answer: All life forms on Earth are part of a common family tree. There are about six things you can base this tree on (Anatomy, embryonic development, geographical distribution, cellular chemestry, etc) and for every basis, the tree comes out identical. This is part of why we know evolution is real. Genetics is one of these bases. If you take two organisisms, you'll find they have huge chunks of DNA in common. You can in fact use the comparative differences and similaraties to very accurately tell when the ancestors of the two species diverged, and how long ago that happened. In other words, slotting new species, and new groups of species, into the tree of life is now super easy. The same is true of RNA. This is grad student level gruge work - and that was with pre-invasion human technology. With Faey technology it is going to be one of the easiest biology experiments in the world. And now that exobiologists have aliens that confound all their theories, it will almost certainly be literally the first thing they do. It is absolutely impossible for humans not to know this. It is also (I suspect) absolutely impossible for at least some Faey to not know this.
To answer your first question though, I would suspect it's similar to the situation in North America, where everyone above the 49th parallel and everyone below (on average) the 30th parallel is moderately reasonable and sane - but half the people between those lines are too fucking stupid to understand that evolution is real, and the world is more than 6000 years old.