Utopa.

Over many years, the animals evolved in isolated cities, with artificial genetic crossbreeding, they acquire language, bipedality, and build a life and civilization for themselves free from want. No one thinks of the world below any longer. One day, a single plant seed wanders into one of the sky cities, and the children Kui, Hiruma, and Root discover it. Unable to resist the temptation to look into the world outside, they decide to undertake an adventure to the outside world, unknowing that once they set foot on the ground, they will never be able to return.