I believe that all the Prominent Species of the World weren’t from the same planet to begin with (not including ones that just might be mutations or result of isolationism). I believe each race originated from different Planets.
Also might be that people of One Piece believe their planet be the centre of the World, and hence the model. The races would be:
- Humans – Earth
- Merfolk and Fishmen
- Sky people – the OG moon
- Long Leg tribe
- Long Arm tribe
- Devil Race (King & Magellan’s race) – confirmed to be a separate race from Sky people
I believe that most of these planets were either drained of resources, or simply abandoned in favor of Earth, which had optimal conditions to live. They might even be too harsh to survive. Also, it has been shown that space travel was possible as the sky people moved to earth atop balloons, as shown in Enel’s cover story.
Now, when they reach Earth and started settling on the planet, it was normal, unlike the earth with 4 seas divided by Red Line and Grand Line that we see today. Most kingdoms were hostile to them, but few tribes/people/kingdoms welcomed them with Open Arms. Over time, this led to the formation of the ANCIENT KINGDOM, which was the result of an aAlliance between the Human factions, consisting of Kozuki Clan and other human factions of Earth, with the different Species from other planets. This kingdom was all-powerful and through sharing of knowledge, led to scientific and technological progress.
The World Government was both scared and jealous, as the Ancient Kingdom were using up resources at an alarming rate, and at the same time were growing incredibly powerful. They feared that they would become an inferior race, or that Earth would be drained of its resources and started the War that ended the Ancient Kingdom.
I will not however, speculate about how the Ancient Kingdom was taken down. But I’m sure the World Government, to keep the tribes/species divided, split the world into 4 seas divided by the Red Line and Grand Line, through the use of the technology of the Ancient Kingdom, and buried these tools/weapons forever in history.
Do let me know what y’all think.
*Theory by RokudaimeSama46