Folks, this is getting dangerously close to story-idea territory. Probably best to curb speculation about where the Vulcanoid Rigellians came from.
As it happens -- and I should've clarified this already -- I've done a fair amount of research and thinking on the inhabitants of the Rigel system as background for Rise of the Federation
. As it happens, Nasat
's thinking on the overall system comes pretty close to what I'd arrived at. I count three main native races, one each from Rigel III (Chelons), IV (Vulcanoids), and V (beaded Rigelians from ENT season 4). Rigel II has colonies from both IV and V, going by Catalyst of Sorrows
. As for the Kaylar... well, I may get around to explaining that in a book.
Going by this transcript
of "Journey to Babel", Spock's line is "Rigelian physiology is very similar to Vulcan". It's not just a matter of blood chemistry, but the entire Rigelian organism is similar to the Vulcan. Especially since there were sufficient differences between recently-separated Romulans and Vulcans to complicate Crusher's treatment of a Romulan downed on Galorndon Core a century later, it's not unreasonable to speculate that Rigelians are as roughly closely related to Vulcans as Romulans.
Well, I'll grant that they said "physiology," but I don't agree with the second point. Two populations can have very similar physiology in one respect yet very different in another. For instance, two people with the same kind of colorblindness may have incompatible blood types, or two people with the same immunity to a given disease may have different degrees of lactose tolerance. It depends on what physiological factors are medically relevant in a given context.
When did this divergence take place? It may be that the Rigelians descend from an Arretian-era population.
I already pretty much established, or at least strongly suggested, in Watching the Clock
that the Arretians were actually two species coexisting as a single civilization -- one (most likely Sargon's race) that was the ancestor of many humanoid aliens (such as, perhaps, Bajorans, Betazoids, Deltans, Argelians, etc.), and one (most likely Henoch's race) that was the ancestor of Vulcanoid species (Vulcans, Romulans, Rigellians, perhaps Halanans, maybe even Ocampa).
Looking at Memory Beta's page, it seems that the consensus has been to write the Rigelians as a Vulcanoid population that emigrated from 40 Eridani at roughly the same time as the Rigelians.
That "consensus" comes mainly from two things, an RPG module and the novel Mind Meld
. RPG versions of Trek history are rarely followed by the novel continuity, and Mind Meld
's portrayal of Rigel V isn't consistent with what ENT and Catalyst of Sorrows
established. So I'm not going with that interpretation.