Star Trek has existed in one form or another for over half a century, so in that time it’s only natural to have developed some inconsistencies, many of which have been cleared up. However, the longstanding issue of why some Romulans have ridges on their foreheads and some don’t has only now been addressed by Star Trek: Picard showrunner Michael Chabon.
The Romulans were first introduced in the influential Original Series episode “Balance of Terror” (and probably its best alongside “City On the Edge of Forever”) where the race is encountered visually for the first time and appear physically identical to Vulcans, which itself becomes a plot point around Spock’s trustworthiness regarding them. It wasn’t until “The Neutral Zone,” the season 1 finale of The Next Generation, that their redesigned look was first seen, which has gone unexplained until now.
In a Q&A session on Chabon’s Instagram page, one fan asked him to clarify the matter of the differing appearances, and it was revealed to be caused by the point of origin on their home planet.
“If you are referring to the forehead ridges, they are a facial feature common to the inhabitants of the northern hemisphere of Romulus. Denizens of the southern hemisphere have smooth foreheads. There is an entire, quite scabrous vocabulary of mutual insult in the Romulan language based around this anatomical divergence.”
The explanation was alluded to in Picard’s third episode “The End Is the Beginning,” during an attempt to interrogate a Zhat Vash assassin after a group of them invaded Picard’s home. Laris, one of Picard’s Romulan companions, makes a comment to the other, Zhaban, declaring the zealot’s reticence to be on account of him being a “a stubborn northerner, like you,” flicking the agent’s forehead ridges to make the point.
Exactly why a mere difference in geographical location would lead to such distinctive facial features was not elaborated upon, although with Picard making pains to examine established Star Trek lore in a more detailed manner than has often been done in the past, it’s possible this might be revisited later in the season as a source of animosity between various Romulan factions.