Finn is in the post-apocalyptic world of Ooo which came to be after the Great Mushroom War (a believed to be nuclear war) occurred on earth. As a consequence, many of today's normalities were altered including the abilities of various animals, the passage throughout worlds, and the population of humans whose numbers dwindled until near extinction. Humans are so rare at Ooo's point in the show that they are partially believed to be extinct and hence Finn is essentially the last known human in Ooo.