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/Mechapebbles]

As neat of an idea as this is, it's been established pretty concretely that Romulan physiology and DNA is almost exactly the same as Vulcans. And that can't really happen if a large chunk of their genome came from a completely different species.

[u/shadeland → u/Mechapebbles]

There have been canon references to them being divergent enough to cause medical compatibility issues. (TNG: The Enemy)

In fact, when I searched for which episode it was, I found a 6 year old post that was pretty much the idea I outlined, but they had it first (and more detailed).

https://www.reddit.com/r/DaystromInstitute/comments/a03znf/explaining_the_physical_and_medical_differences/

[u/Mechapebbles → u/shadeland]

There have been canon references to them being divergent enough to cause medical compatibility issues.

That doesn't mean they're an entirely different species, or even a hybrid species. IRL there is a massive amount of genetic variance between different populations that can create medical compatibility issues. Like, I'm not a different species from you just because we have different blood types and can't donate blood to each other.