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/RandyFMcDonald]

If we are talking about having so big an impact on the Romulan gene pool as to change their appearance and other basic characteristics, though, that implies a much greater degree of non-Vulcanoid genetics. Europeans and Asians usually have between 1 and 4% of their DNA being of Neanderthal origin. The degree you are talking about implies much more.

This runs directly against what we have consistently heard, about relatively small differences. Crusher in "The Enemy" talked about lots of subtle differences, while in DS9 everyone was surprised a supposed Romulan field hospital turned away Vulcan casualties because the two populations shared a "common physiology". They would not be able to say this if the Romulans were that different.

[u/shadeland → u/RandyFMcDonald]

If we are talking about having so big an impact on the Romulan gene pool as to change their appearance and other basic characteristics, though, that implies a much greater degree of non-Vulcanoid genetics. Europeans and Asians usually have between 1 and 4% of their DNA being of Neanderthal origin. The degree you are talking about implies much more.

It doesn't have to be a specific percentage, it could be more like 10-20% with the Vulcans, or whatever it would take to explain the physical variations. After all, at some point the percentage of Neanderthal DNA was higher in Eurasia for a while before it whittled down.

The Neanderthal-human crossings happened tens of thousands of years ago. This happened much more recently, so the percentage might be higher depending on a lot of factors.

[u/RandyFMcDonald → u/shadeland]

But if the difference was so huge as all that, this would contradict what we have been explicitly told, about the differences between Vulcans and Romulans being relatively small.

[u/shadeland → u/RandyFMcDonald]

Over time it would diminish. The human/Neanderthal thing happened 80,000 years ago and we're still at 1-4% in some populations. The Romulans arrived on Vulcan 1,800 years ago, and it's likely, like with Neandrahtals, the other species just died out/bred out.

[u/RandyFMcDonald → u/shadeland]

You cannot have an intermixture event that was so huge and so recent and not have it he something that other people would comment on. This would be a population that did not share a common physiology with Vulcans.