From Starsider Wiki
The Republic, as it was simply called by it's citizens, spanned many millennia. The Republic was initially established over twenty-five thousand years before the Battle of Yavin after the re-discovery of Hyperspace by various Core worlds. Many restructuring events took place over the history of the Republic, the last one being the Ruusan Reformation. Ultimately the Republic was restructured into the Galactic Empire.
The Republic was formed as a direct result of the Unification Wars. This conflict was borne out of the re-discovery of Hyperspace by Human and Duros scientists who reverse-engineered long dormant technology of the Rakata.
Coruscant emerged victorious from the Unification Wars and dictated the new Coruscant Accords, which outlined the criteria for a planet's membership in the new Galactic Republic. The Galactic Constitution was signed at the war's conclusion by many planets which came to be known as the Core Founders. Nearly all of them were human, with the Duros being a notable exception.