I think it's obvious now that the mountain itself is alive, and people who get too close to it for too long will either get eaten or converted into its "children". I forgot which chapter it was, but one of the more intelligent cave dwellers referred to that strange hole with drawings above it as some sort of "birthplace" (alluding to a womb?). And then we have the recent cave dweller who was being friendly to Hajime refer to the mountain as "mama", right as Hajime is approaching that strange hole.
We also get Riku's wife thinking she finally managed to get pregnant after she becomes a 'shroom creature... There really are a lot of layers to this story.