A theory about One Piece comes to my mind recently: the series contain a lot of impossible, non-existent and "fairy tale"-like elements, such as various giant animals, bizarre cross-species, giant or simply impossibly tall humans, sapient but animal-shaped species, and such.
Despite all these facts, the series tends to be much more realistic than actual fairy tales, making it surprisingly consistent and believeable. Moreover, a lot of similarities to actual human history and culteres also appear, implying that civilizations in this world developed near identically to some Earth countries, which is, I think, extremely unlikely.
Thus, I started wondering if One Piece takes place on Earth, but millions or even billions of years later.
In this question, I'd like to be interested only in the biological, to be precise: the genetic aspects. 65 million years was enough to evolve from rats to humans and enormous amount of other mammals, so evolution is pretty strong at making exotic species into existence, but what about such extreme results? Also, what about humans?
Is it possible that during either a natural or a supervised evolution, giant humans, giant animals, other intelligent species and other similar "fairy tale"-like beings form and live? Or is there a boundary for genetic recombination?