SG-1 learns in season 1's "The Torment of Tantalus" that although most known habitable planets in the "Stargate" universe are populated by humans, there was once an Alliance of four great races. In season 2's "The Fifth Race", the Asgard tell Jack O'Neill that this strategic alliance had consisted of the Ancients, the Asgard, the Furlings, and the Nox, and that the humans from Earth had taken the first steps towards becoming "the Fifth Race". (This comes full circle in the "Stargate SG-1" finale "Unending", when Thor declares the humans from Earth the Fifth Race.)