Oh, there is a fan-theory out on the net that I find rather interesting.
It states that the reason that MF and the Businessmen ( and by extention Marceline), are what could best be discribed as proto-goblins, or in other terms humans who are a generation or so removed from normal humans to the point that they are a different species,one that did not last long evidently. Which could mean one of two things, realy.
One is that humans where already mutating before the war, which would have interesting implications with the mere existance of Finn.
The other is that magic is the cause of the existance of Marceline and such, and that the GMW was fought with magical weapons that left the very few survivers of the varying bombs where changed to the point they where not human anymore. Some became very powerful ( in the case of MD) or went utterly insane ( the lich). The mutents and zombies and such are the decendents of those who where effected by the fallout and, with the help of the power of magic, adapted to the newfound crazyness of the world that would be later called Ooo.
As for the fish people that look far more like humans, my personal guess is that they where probably the decendents of people who went underground before the bombs struck, and eventualy mutated into what they are now due to both radiation leaking in and magic screwing things up.
On the other hand, the varying blueish humanoids that exist, such as the Ice king and Billy, are very much related to each other and are the closest decendents of the varying survivers of the bombs, which would explain why they are all at least adults with at least a few years on the average inhabitant of Ooo( and in the case of the ice king about 600 years, and who knows how old Billy is!)