The sci-fi series Stargate SG-1, a spinoff of the 1994 film Stargate, follows scientist Daniel Jackson, US Special Operations Colonel Jack O'Neill, Colonel Samantha Carter, and former First Prime to System Lord Apophis, Teal'c of Chulak, as they venture across the cosmos via the Stargate. The Stargate — first used in the 1994 film before being utilized throughout the Stargate SG-1 series — is a machine that was created thousands of years ago to act as a galactic wormhole that is used by the Stargate Command squad on Earth.
As is inevitable when traveling across the galaxy and galaxies separate from the one earth is most familiar with, the members of Stargate SG-1 encounter countless alien species. Of these many races, some were good and others, not so much. Some of those who were good, though, found it necessary to form an alliance against a common enemy: the Goa'uld.
After exploring throughout the Milky Way and Pegasus galaxies, four races found themselves terrorized by the Goa'uld and decided to band together against them. This team would come to be known as the Alliance of Four Great Races. It was made up by the Ancients (formerly the Alterans), the Furlings, the Nox, and the Asgard. The fifth race wasn't appointed until many, years later, after a certain amount of evolution of their species was achieved, and they proved themselves to be effective protectors of the galaxy.
Who Are the Four Great Races?
The Ancients
The first members of the alliance are the Ancients. Once known as the Alterans, they lived as a developed civilization millions of years ago in the Alteran Home Galaxy in the form of humanoids, both in appearance and anatomy. At one point, their people split into two — the Alterans and the Ori — who differed in their beliefs and practices. The Alterans were science-oriented, and the Ori adapted a more religious lifestyle. This caused a civil war, forcing the Alterans to flee the galaxy to keep their people from being wiped out.
Thousands of years later, the Alterans found themselves in the Milky Way Galaxy as they settled on the planets Dakara and Terra — the latter which would eventually be known as Earth. As they began colonizing planets in the Milky Way, their most prominent presence was on Earth, and they left behind their knowledge and architecture. Eventually they suffered a widespread plague which wiped out much of their species, and they departed from the Milky Way Galaxy to venture towards the Pegasus Galaxy.
Thousands of years later, they returned to Earth to find the humans more evolved, and they decided to breed with the Earthlings to keep their bloodlines alive. The Ancients were also the constructors of the Stargates that were placed all around the Milky Way and Pegasus galaxies.
The Furlings
The Furlings were also part of the Alliance of Four Great Races, though they are the least documented throughout the Stargate saga. Their people resemble the Earth-native species of koalas (and are often compared to Ewoks from Star Wars). A peaceful people, the Furlings joined the alliance to both gain protection against any conquering enemies and to provide their own pacifist resolutions to political and war-like issues.
The Furlings once resided in the Milky Way before being killed by the Goa'uld. Although it is unclear what planet they are from, evidence of their existence was found on P5X-777 in season 6, episode 15, titled, "Paradise Lost."
The Nox
A humanoid species, the Nox reside on the planet Gaia and share similar pacifist beliefs as the Furlings. Part of their lifestyle is represented through their deep anatomical connection with the nature of their native planet. Unlike their allies, they did not provide particularly advanced technology, as they believed it would taint their peaceful relationship with nature.
Instead, they had gained special abilities thanks to their eco-relationship. These included extraordinary memory and comprehension, heightened sensory skills, and even the ability to heal living creatures through the Ritual of Life. Their usefulness to the alliance was in their pacifism and stealth, as they were able to help refugees escape from the devastation of the Goa'uld.
The Asgard
Lastly the Asgard are a humanoid species who originated from the planet Hala in the Othala galaxy. Although they were once humanoid, by the time the members of SG-1 first encountered them, their appearance was extremely altered due to the many years of cloning themselves with the ultimate goal of achieving immortality. It backfired, causing sterility and unnatural changes in their evolution. This resulted in them appearing as the classic "grey alien," with enlarged craniums and bulging black eyes.
The Asgard once served as the sole protectors of the Milky Way after freeing the many captive humans of the galaxy from Goa'uld rule. Once liberated, they stood as an even bigger threat against the conquerors and came to terms with a treaty, known as the Protected Planets Treaty. For a long while, the treaty proved effective, until the Replicators invaded the Asgard's home galaxy. This forced them to leave the Milky Way vulnerable as they attempted to fight off the Replicators.
Eventually, the Asgard became extinct, but not before learning the knowledge of the Ancients and helping relay their resources to SG-1 and the people of Earth. They were also responsible with introducing the fifth race to the Alliance of Four Great Races, and informing them that they were in fact on their way to becoming the fifth race.
What is the Fifth Race?
The fifth race was first introduced as such in season 2, episode 15, appropriately titled "The Fifth Race." While Colonel Jack O'Neill and company are exploring planet P3R-272, they stumble upon a device. Jack approaches it and is quickly trapped in its grasp as it "downloads" contents of the Ancients into his brain. He then starts to retain Alteran knowledge, and eventually can only speak in their native tongue.
Eventually, after deciphering certain mysteries of the Stargate and opening gates across the galaxy that they had yet to explore, he enters the portal and is greeted by the Asgard. Once there, they turn him back to normal and inform him that his species, the Humans of Earth — known to the Asgard and the rest of the alliance as the Tau'ri — is taking their first steps into becoming the fifth race of the alliance. They also reveal that the device that implanted the Ancients' knowledge into him was a Repository of Knowledge, a device placed across the galaxies by the Ancients that holds everything they knew.
O'Neill returns to Earth, but retains no knowledge of the Ancients. However, he does return with the comfort that his native species will be okay, as the Asgard had informed him, and with the hope that the Tau'ri will one day serve as great as protectors of the galaxy as the Alliance of Four Great Races once did. Their power is thus explored throughout the remainder of the Stargate SG-1 series.