The Orcs are native to Tuchanka, a world known for its harsh environments, scarce resources, and overabundance of vicious predators. The Orcs managed to not only survive on their unforgiving homeworld, but actually thrived in the extreme conditions. Unfortunately, as Orc society became more technologically advanced, so did their weaponry. The end result is that they destroyed their homeworld in a nuclear war that reduced their race into primitive warring tribes.
With the help of the Gnomes, the Orcs were “uplifted” into galactic society, and lent their numbers and military prowess to bring an end to the Rachni Wars. Ironically, after the Rachni were eradicated, the rapidly-expanding Orcs became a threat to the galaxy in turn, starting the Orc Rebellions and forcing the Tieflings to unleash the genophage. This genetic “infection” dramatically reduced fertility in Orc females, causing a severe drop in births secondary to prenatal and postnatal death and, ultimately eliminating the Orcs’ numerical advantage.