Even if they are monsters... =.="
Yeah, had the same thought about human origin for a while, for them as a whole, didn't want to sound it to spoil the fun. But since we have a bunch of revelations coming might as well. An interlopers origin is likely, but for plot twist it wont be Earth, though many things factor that it is. Then again it might not be just decades since "then" (when?) but much more, would make sense for any drastic changes.
If that's the case im shipping Musica x Ray. (=・ω・=)
I do think that Mama lied about the year they live in, like that the story can take many ways. But I doubt the mangaka would do that...
And if they are in a planet that is not Earth then wow, 'cause I didn't see that coming.
We can see in the last chap that they were praying, maybe they don't eat humans because of their religion (?) :/
Musica and Ray...that's a ship I didn't even think about, they didn't talk much yet with each other, so I'll wait a while before shipping Ray with her. Even if the idea is good.
Maybe humans who were transformed into "demons"? Or a disease? I want to believe that those "demons" aren't aliens or any kind of monster.... ╮(￣△￣)╭