I'm currently reading the tie-in novel The Last Best Hope, and in that they do say that Romulans with ridges are the Northerners.
From what I've heard, their original plan for Trials and Tribble-ations was to just have Worf's forehead magically turn smooth when they traveled back in time and for his ridges to magically come back when they got back to the 24th century and for no one to acknowledge it. I kinda wish they'd gone that route.
Was that the original plan, or was that what some fans thought they should do? I know it's been talked about on here, but I can't remember if that was confirmed as the actual original plan or not.
Are the Northerners from the North of Romulus - or from the North of Vulcan? We never got even a semicanonical map of Vulcan or Romulus. Yet we could see that the latter at least has continents separated by seas. Perhaps the former had areas separated by nigh-impassable deserts, though allowing the Northerners to develop the different foreheads over the dozens of millennia - and then to foster ideas that resulted in them getting kicked out of Vulcan en masse (along with a precious few ridgeless Southerner symphatizers)? Timo Saloniemi
Now here's the real quandary... What is it about the North that would cause a biological genetic-hiccup to cause people there to develop heavy brow ridges to begin with?
More likely a recessive trait (i.e. like the Mintankans (sp?), called "proto-Vulcans") that became more dominant due to different breeding. Recessive traits are weird.
Come to think of it, I'm not sure. I recall reading about it on these forums and I recall them saying it was their original plan and I guess I just took their word for it. I can't even be sure they made that claim to begin with.
Rather they didn't explain it. It seems silly to them constantly being referred to Northerners. I just prefered the idea from the novels that both existed. It wasn't regional.
When a population splits off, they'll have a bit more or less.of certain genes than the main group, statistically, and over time can be a bit different even if the difference doesn't serve a survival or reproductive advantage. I believe the lack of epicanthal fold in many East Asian people's eyes is thought to be due to this, not to any advantage. Gene drift or gene flow, I can't remember. But not Gene Roddenberry. [trying hard not to make casting couch joke... so. difficult.]
Memory Alpha doesn't mention it. I think I've only ever seen the idea here. Annoying birds of prey that like to land on them and peck hard at their foreheads. Kor
Many Romulans wore helmets in TOS. Theoretically, they might have all had ridges, just hidden. For that matter, we also saw Vulcans wearing similar helmets in Amok Time. They, too, might have had head ridges.
Homo sapiens was able to breed with Homo Neanderthalis. There is no reason to believe that, had prehistory panned out a little differently, we'd be living alongside Neanderthals, Australopithecus and Homo Erectus right now. There's a good Stephen Baxter book postulating just such a scenario - Origin. No reason to believe that this isn't what happens on other worlds. I'm with others on here - the diversity is a refreshing change from Berman "wallpaper" Trek.
I find the Romulans in Picard odd to look at but I've realized it is because the Westmore Romulans didn't just have the forehead ridges, but a variety of facial, hues? Jarok's skin has an orange hue while Vreenak's skin has a greenish tint, hinting at Romulan green blood?