The fictional backstory is established over the course of the series. In season 4's "The First Ones", it is stated that the Goa'uld evolved on the planet P3X-888, where there are still populations of primitive Goa'uld. Their original hosts were the Unas (meaning "First Ones"), a race of large and primitive humanoids also native to the planet and whom SG-1 had first encountered in season 1's "Thor's Hammer". The Goa'uld then found and ruled over Earth for thousands of years, masquerading as gods from ancient mythologies and transplanting humans throughout the galaxy to serve as slaves and hosts. A faction of Goa'uld symbiotes named the Tok'ra (meaning "against Ra"), whom SG-1 first encounter in season 2's "The Tok'ra" and become close allies with, formed in opposition to the Goa'uld culturally and militarily millennia ago to live in true symbiosis with their hosts, both beings sharing the body equally and benefiting from each other.