The thing to remember is all those pesky p_worlds. Which Terra did the Earthmasters import Terran lifeforms from?
The p-a-Earthmasters imported the p-1-lifeforms from the p-1-Terra in p-A-Harn-year-xxxxx-BT. The p-b-Earthmasters imported the p-2-lifeforms from the p-2-Terra in p-B-Harn-year-yyyyy-BT.
The p-c-Earthmasters went to the p-2-Terra in p-B-Harn-year-yyyyy+n-BT and didn't find many lifeforms they actually cared about because they had already left with the p-b-Earthmasters. And The p-d-Earthmasters "hit the jackpot" and grabbed the humans from p-3&4&5-Terra and dropped them all in p-F-Harn. That greatly confused the p-f-Earthmasters because when they showed up in p-F-Harn with pre-humans they found in p-6-Terra they found Harn already occupied but not knowing what else to do with them they added them to the pile.
Unfortunately the intense interest in p-6-Harn by so many groups lead the p-b&j-airmasters right to them and almost all the p-d&e-Earthmasters were wiped out and the p-j-airmasters opened the plane as a hunting preserve for just about any human/earthmaster/airmaster/sentient animal race that found use in killing humans for fun or food.
Then the p-x-airmasters found out about the terrible and illegal way they were hunting other sentient races and went back in time to warn the humans on p-3&4&5&6-Terra. They trained and armed the humans so that when the Earthmasters showed up they were ready for them. They waited until they had visited each of the worlds and first shipments of humans were exported before the humans and airmasters ambushed the Earthmasters. They then followed the Earthmasters to p-6-Harn, killed them and freed the humans there. The humans and p-x-airmasters again employed time travel to get the jump on the Earthmasters and sent warning to most of the early p-airmaster planes and worlds which resulted in the Air-Earthmaster wars which not only spanned the universe but also into most p-planes and led to the destruction/disappearance of both races in most p-worlds (but at different points on the timeline as not all p-worlds are at the same point of time). It also led to the abandonment of the various human and sentient animal half-races under various conditions that existed at the time all were wiped out.
There are still various groups of each race who didn't figure out about the p-world issues and got lost from their own worlds & times and so missed the great war. Whether they eventually figure it out or not they end up on random worlds seeing the devastation and confusion left behind and took various actions in the aftermath to either help/hurt/leave/set-up kingdoms in the places discovered, etc.
Oh, I forgot about the planes where the Earthmasters never existed, didn't discover inter-world travel and/or other random races showed up instead...