It seems widely accepted that the Romulans are decended from opponents of Surak and his teachings and that they left vulcan during the Time of Awakening but I can't remember this actually being out right stated in any episode. Is this just some form of fanon or have I missed an important reference some where. Since the Vulcans were building temples like P'Jem on other worlds as early as 3,000 years before the time of Enterprise is it possible that Romulus was just an Vulcan colony that lost contact with the homeworld during the awakening?
I don't think it is ever stated outright. I believe it is just in books and inferred that they are descended from Vulcan's who left.
Though it isn't spelled out exactly, Surak kind of told Archer this when he carried his katra in the Vulcan arc in Enterprise (probably in the episode "Awakening"). They look out on a landscape devastated by nuclear war and Surak tells him that those who rejected his teachings violently were "marching under raptor's wings" which is a reference to the symbol the Romulans still use as their national symbol. Also, in TOS (in "Balance of Terror") Spock theorises that the Romulans are an off-shoot of the Vulcans that left because they rejected logic. This had been just a myth to him until he saw the face of the Romulan commander.
The Preservers from "Paradise Syndrome" are to blame. They had moved a colony of primitive Vulcans to a new planet when someone finally realized they had gotten the home address wrong. "Not these violent ruffians! That other planet over there!"
It makes me wonder exactly how long Vulcans have been a spacefareing species, if an offshoot of them on another planet had time to develop into another species. They should be all over the galaxy by now.
According to Enterprise, the Vulcans who opposed Surak were known as "those who marched beneath the Raptor's wings." The implication being they do become Romulans. Also, Voyager's Death Wish has Q accusing the renegade Q known as Quinn of starting the 100 year war that resulted in the split between Vulcans and Romulans in one of his failed suicide attempts. The idea of the Romulans being the descendants of Surak's opponents is by no means fanon at all. Maybe it's missing that One Direct Reference, but there's no real reason to assume it isn't so either.
Throughout the early years that is the stated philosopy among Vulcans, though the season 4 arc consisting of The Forge, Awakening, and Kir'shara kind of debunks this. Particularly, in The Forge, Syrran tells Archer the Vulcan High Command originally started off as Vulcan's space exploration service before taking over the government. Archer comments that he heard Vulcans weren't into space exploration and Syrran says "I'm sure you've been told many things which aren't true about Vulcans."
Romulans aren't a separate species. Their differences with the Vulcans are mainly cultural. Surak lived in the 4th Century and the Vulcans seem to have been a space faring, advanced culture at that time. Though the wars of that period probably caused a "Dark Age" where much advancement was lost and many things had to be rediscovered afterwards. My assumption is that contact with most of Vulcan's colonies was lost at this time and those worlds evolved into different cultures.
It's never stated outright how long ago Vulcans diverged genetically from Romulans. It's interesting to say they are the same species. Technically most of the races in the galaxy are the same species, because the scientific definition of a species is a group of organisms who can produce fertile offspring. So by the scientific definition, humans and klingons are the same species, as are most of the humanoid races in the galaxy. Vulcans and Romulans probably diverged more recently than some races of humans on earth, say a few thousand years ago, so they probably are the same species even though they're always referred to as different species. And I suppose if Simon Tarses could fool all the blood tests, that confirms that they are biologically indistinguishable.
Wasn't there a species in TNG that were specifically referred to as being proto-Vulcan? Perhaps Romulans are just another Vulcanoid species.
So are Humans Klingonoids or an offshoot of Humanity - Humanoids? or just adnoids, androids and simply annoyed?
This! It is possible that Vulcans were spacefaring prior to the ancestors of the Romulans leaving, then when Vulcan had their "world war" that devastated Vulcan, they entered a "dark age" of sorts, and maybe either stopped deep space exploration or even regressed to a pre-warp society for a while, as Vulcan rebuilt itself and under Surak's example. Well it is hard to say, as Vulcanoids could possibly evolve a lot faster than humans do, so it could only take a couple thousand years for enough differences appear to make Vulcans and Romulans technically two distinct races. Also, as the Vulcan Ancestors who became Romulans traveled from Vulcan, but before arriving on Romulus, it is possible that some, while along the way, grew tired of traveling and set down roots on other planets. The Debrune could be an example of this. Maybe the Mintakans are another example? Another thought is what if in order to survive, the ancestors of the Romulans intermingled with other humanoid or vulcanoid races along the way to Romulus or even Romulus itself, and are technically a hybrid species, with mostly Vulcan DNA?
According to TOS's "All Our Yesterdays," Spock's ancestors a mere 5,000 years ago were savages. That would be some awfully fast history, with starflight, a possible dark age and everything else thrown in. (Remember that Vulcan logic is a philosophy, not a behavioral trait. Yet Spock's trip through the atavichron is enough to strip that away, as though it were genetic.) Is is possible that Vulcan is the colony and Romulus the mother planet? Or perhaps both are colonies of a third planet?
Saying this though, the Romulans certainly had different physiology to Vulcans. In TNG Episode 'the enemy', Crusher tried to use methods used to treat Vulcans on a Romulan with no effect. And in TOS they were able to distinguish Spock's life signs from the Romulans.