I have a hypothesis. What if this is just a rehash of "Planet of the Apes", and he is back on Earth, not just a couple hundred, or couple of thousand years later, but tens of thousands, hundreds of thousands, or even a million or 2 years later, enough time for every animal to anthropomorphize and build a society, as well as the continents to change, and humans to go extinct.
And yes, this sudden change out of no where seems to me as a grasp to keep interest in the manga going. It's just so out of place compared to the previous chapters.