system
Given the following post, respond as if you're commenting on Reddit:
human
[star trek] how did the romulans and vulcans separate before warp drive?
gpt
The Vulcan people have had a VERY violent past. Even more than our own bloody history, it's very possible the reason Vulcan is such a harsh desert is because of their constant warring. (Yes, they have physical adaptations to bright light and desert conditions that could have only come from millions of years, but it's doubtful the ENTIRE planet was like that.) 

Most human leaps in technology have come from warfare, and Vulcans were just the same. They had nuclear weapons in 400 A.D. and were fighting a war that very nearly destroyed the planet. Humans in 2014 basically have the capability to render Earth uninhabitable AND make some rudimentary leaps into space, so it's entirely possible the Vulcans were at a similar, but slightly higher, level of technology at the time. 

Those that left Vulcan to eventually found the Romulan Star Empire probably left on cryosleep-capable ships just like Khan used in the late 1990s. Or it's possible they left on generational ships, since there are "proto-vulcanoid" civilizations out there, which might have been founded by such vessels landing or crashing. And since Vulcan technology was probably better than Earth's in the 90s, they had engines capable of taking them much faster than simple chemical rockets. (I would even say they had very primitive warp drives, something on the level of Cochrane's ship. Vulcans had been major galactic players for a VERY long time by 2151, so why not coincide their near-downfall with their very first warp experiments?)