Anywho my theory can go multiple ways depending on how you view it so bear with me.
We all know that the majority of humans were destroyed during the mushroom wars and the remaining portion were left as goo monsters in a type of apocalyptic world. The radiation that was left mutated candy and gave it living properties that resulted in people like princess bubblegum or the candy people she created. If you're familiar with Adventure time you'll know this, if not search it up. Now my theory regards an episode where Finn went into one of his past lives and ended up as Shoko during the beginning of the construction of the Candy Kingdom. This construction of the candy kingdom occured nearly 400 years ago as it was already revealed that princess bubblegum is actually 827 years old. The construction of the Candy Kingdom occurred long after humans had gone extinct. Nearly 400 years later. So Finn was reborn later on from Shoko but we know that during Shoko's time there were no humans even though she was humanoid.
The only humans that we do know are Ice King and Finn. Ice King is undebatable because we saw flashbacks of his past life as Simon the archaeologist and protecting Marceline after the Mushroom war. We even saw his transformation into the Ice King. However as I said this happened nearly 1000 years ago.
Finn however is a bit more debatable. Because how is it that a supposed human exists even though humans went extinct nearly 1000 years ago. Now here are the possibilities. It's that Finn isn't actually the only human and he had a mother and father and some group of humans still survived even after the mushroom wars and the rise of the LICH King for 1000 years and extended that lineage into now(which I find highly unlikely) or something else I had in mind. We all know that Princess bubblegum has alot of secrets. Her experiments, her age etc. So what if it's possible that Finn is one of her experiments?
My end conclusion is a suggestion. You could tell where you think Finn might have come from.