I have this theory that humans have made a nuclear weapon and it caused many lives and it may mutated the animals causing them to envolved and having to stand up straight and having English speech and other animals like bugs became monsters we seen in episodes in Mao Mao while animals have mutated and evolving, Humans have been dying out and they have lost their language they speak and only speak like cavepeople or grunts like in the prehistoric era, They would've make a underground Home with each tunnel leading into their each home and If the humans go to the outside world they might get eaten by monsters or even captured to be sold of as food, livestock, Etc.

That's all about theory#1