I started with these known or agreed facts:
There are 8 galaxies or rather galaxy sectors.
Galaxy 1 is just outside the region containing Sol.
At some point in the future all other species are driven out of Galaxy 1. [This assumes that we actually want to be consistent with Frontier and FFE.]
There are 8 main species types: Rodents, Frogs, Lizards, Lobsters, Birds, Humanoids, Felines and Insects. Additionally there are Humans and Thargons.
Each world is has a variant of one of those 8 species or is Human populated. The Thargons originate elsewhere.
I can not go with the idea that all the non-human colonial worlds all evolved such a limited range of species types so close to each other independently.There are no unpopulated worlds in the 8 "galaxies".
With the above facts and thoughts I considered the following scenario:
Further thoughts:Humans discovered interstellar space flight and expanded into the region around Sol discovering a handful of non-space-faring inhabited worlds.
Scientists invented the galaxy hyperdrive most likely somewhere in the space that is Galaxy1 (if invented by Humans). This allowed jumping to other regions of the galaxy that form a loop that leads back to Galaxy1.
Each galaxy (2 to 8) are situated in the Milky Way near regions of space which have other space-faring species are expanding into. Each "galaxy" is near one species except one "galaxy" (which is to be decided) that has two species nearby.
GalCo-op is set up to keep the peace between the species and by treaty is limited to 8 equally sized cuboids of space that is recognized by us players as the 8 "galaxies". Inside each "galaxy" is a mixture of worlds populated by any of the 9 species and maybe a handful of native species which just happen to look similar to some of the others. Outside of each of the GalCo-op "galaxies" there are empires that are mostly one or, in one case, two of the species.
Time passes and GalCo-op's "galaxies" are fully populated and the regions around them are overrun by worlds of the appropriate species.
The Thargon wars keep the various species more engaged in dealing with an external threat than at each other's throats/thoraxes/etc. With the defeat of the Thargons other political machinations lead to the collapse of GalCo-op and each species takes control of their neighbouring GalCo-op "galaxy" (such as human factions take control of Galaxy1). This leads to major migrations of species back to their home "galaxies".
At some point the galactic hyperspace jump system fails (or its jump points start to migrate) breaking the cycle and isolating each "galaxy". Humans populate all worlds in Galaxy1. There are some members of other species on these worlds but they become a minority except for the native species worlds. Similar occurs in the other "galaxies".
Human Colonists worlds occur more often due to either:
a) Humans are more expansionistic.
b) GalCo-op colonial policy is biased
c) Both a) and b)
Thargons threat comes from the space in the centre of the loop of "galaxies" and therefore explains why the war is spread through all 8 "galaxies". Also some Thargon technology homed in the centre of their space is causing the galactic hyperspace drive to loop around in a circle. [I have an idea to what that is but that's for another day] When the the Thargons are eradicated the galactic hyperdrive start to fail.
Galaxy4 would have the Feline empire near it (thinking of the Lovecats OXP) [not used that OXP so that idea could be incompatible]
The above are my thoughts on "Origin of Species in the Ooniverse" maybe others can expand on this idea or point out the holes.