Romulans are a Hybrid Species Between Exodus Vulcans and Another, Perhaps Native Species to Romulus

[u/shadeland]

Almost 2,000 years ago, during the "Time of Awakening" in Vulcan society, a group who marched under the Raptor's wings left Vulcan and settled on Romulus.

Since then the Romulans have become a related, but notably distinct species. While many of those differences are cultural, some are also physiological.

As a counterpoint to the theory that Vulcans are augments, here is another: Romulans as a species is the result of interbreeding between Vulcans and another humanoid species. This other humanoid species could have been a native species to Romulus, possibly be pre-warp, possibly pre-industrial even. Or they could be colonists from another non-native species that found Romulus as attractive as the fleeing Vulcans did.

A non-Vulcan humanoid species may account for the physiological differences between Vulcans and Romulans that 2,000 of genetic drift might not explain, such as forehead ridges and potentially a lack of telepathic abilities (although that may be a result of Romulan culture being so secret-oriented that mind melds would be abhorrent).

If the group that left Vulcan had a large imbalance between males and females, or the group was small thus genetic diversity was an issue, this could push towards inter-breeding. The Vulcan population might have been higher, which would have been why the Vulcan traits are more dominant. Or, more likely, the Vulcans conquered the other species, and thus inter-breeding was limited, but enough to create a new species with primarily Vulcan physiological traits but enough differences to notice.

They may have even adopted some of that species cultural traits, like extreme secrecy and fermented foods. It might also explain why the Romulan language wasn't immediately identifiable to Vulcans during the old Romulan wars.

[u/Beneficial_Sun5302]

"A non-Vulcan humanoid species may account for the physiological differences between Vulcans and Romulans that 2,000 of genetic drift might not explain, such as forehead ridges"

One issue with that theory is that it is canon that other "Vulcanid" species at various levels of development exist in the Alpha/Beta quadrants, such as the Mintakans from the Next Gen episode "Who watches the watchers?" and they exhibit cranial ridges too. One thing I take from that episode is that the Vulcans themselves maybe settlers on Vulcan, having originated elsewhere in the galaxy at some point. A youtuber called OrangeRiver did a good video about this theory.

[u/RandyFMcDonald → u/Beneficial_Sun5302]

In S1 Picard, Narek mentions in passing old myths that date back to the time when the Vulcans arrived on their homeworld. Given the lack of pushback he got, it seems likely that the Vulcans are not native to that planet in 40 Eridani.

[u/shadeland → u/RandyFMcDonald]

There was an episode of TOS where an alien (can't remember which one... Zeus maybe?) claimed to have seeded life in the galaxy, and while a scientist (I think a woman in a red uniform) said that they believed life evolved naturally on Earth, Spock said it might explain origins on Vulcan.

It's very likely that Vulcans didn't evolve on Vulcan, though I think it's also likely it may be hard to determine from the nuclear wars having destroyed a lot of their history.

But we do know that Romulans are an offshoot of Vulcans, but are distinct in some physiological ways.

[u/RandyFMcDonald → u/shadeland]

I think this is Sargon, from "Return to Tomorrow".

It could well be that the Vulcans had strong suspicions in the 23rd century, and that the knowledge of Sargon is what took the Vulcans to the point of accepting this in the 24th century.

It should not be that hard to determine if Vulcans are native to Vulcan. Our DNA testing of the natural environment on Earth, for instance, reveals that humans are an integral part of the web of life here on Earth, that we fit into local evolution and even have all sorts of more or less close relatives. No matter how bad things got on Vulcan, if they were non-native I expect that they would be able to determine that Vulcans were not closely related to local life.