I came up with a pretty convoluted theory when I was in high school. The ridges do come from interbreeding, perhaps from Klingons - or other species - but it goes way back to the Sundering times. Because the time between TOS and TNG is too short to account for the necessary evolutionary genetic drift. So when the would-be Romulans left Vulcan and started their nomadic exodus across the Beta Quad, they began interbreeding with other races - with Klinks being the most obvious. Of course, given Rommies obsession with purity, the offspring where shunned. This was later supported by Nemesis, with the Remans being the most impure banished to the moon.
The remainder basically became the lower class menial workers. But of course, there was still intermixing between the 'pure' and 'impure,' over the centuries (or even millennia) to the point where, by TOS time, the purists were just a small gene pool clinging to power. However, in the intervening years - during the time of described internal conflict - the 'impure' where just too many and took control of the Tal Shiar and usurped control of the Senate. But, by then, the sole ridge thing had become the dominant genetic trait.
Then with ST09 I just figured that the "pure" had been the subjugated race and the tattoos were like a brand representing their masters.
They may just have always had it. Some Vulcans too. It may be a trait no different to the epicanthic fold on earth.