Adventure Times back story is actually sad. The great mushroom war they mention in the show was a major nuclear war, nearly destroying the human race. On one of the episodes, there is a hologram with earth, but a huge chunk gone. That huge chunk is Russia, and most of Europe. The Utopia underground is a nuclear bunker. Those people underground were human, however evolution took place, and turned them all to half human, half fish, because of all the water in there. In the episode where Fin faces his fear of the ocean, and they are in the underwater city, there is one building with the roof off, and there is a dead family on a couch. The city was flooded. And with the candy people, those are also human like lifeforms, but they were formed from the nuclear radiation. You can look at the wiki for Adventure Time about the great mushroom war, and there are great theories about this.