The Romulans were a humanoid race from the planet Romulus in Star Trek. The Romulans were biological cousins of Vulcans, descended from those who rejected Surak's reforms during the Time of Awakening. By the 24th century, the Romulan Star Empire was one of the major powers in the galaxy. After a supernova destroyed the Romulan sun, the Romulan Free State became the official government. Eventually, the Romulans reunified with the Vulcans and settled on the planet Vulcan, which was renamed Ni'Var.
Romulans were aware of Humanity for some time before Earth knew of them. Infiltrating the highest levels of the Vulcan High Command, the Romulans were impressed and seemingly confused by Humans. Even after fighting the Earth-Romulan War, it wasn't until the 23rd century that Humans actually made visual contact with Romulans. After the Treaty of Algeron went into effect, the Romulans retreated into political and social isolation from the Federation. In late 2364, an unprovoked attack on a Romulan outpost near the Federation neutral zone occurred. The Romulans initially suspected the Federation had executed the attack but it was later learned that the Borg may have been responsible. This event marked the end of Romulan political isolationism with the Federation.
Romulans and many of the Milky Way's species all shared a common ancestry back to the Ancient Humanoids and the DNA code they produced and seeded across the galaxy. Though each evolved under separate conditions from each other, their shared DNA allowed these species to remain roughly compatible and viable with each other.