Millennia ago, in a distant galaxy, the Alterans were a human society on an evolutionary path to ascension, but a philosophical division grew. The Alterans had believed in science and free will, but a faction of their society grew more and more fervent in their religious belief and attempted to wipe out those who did not share their faith. Rather than go to war, the Alterans built a ship and left their homeworld, eventually coming to the Milky Way Galaxy. A book found among the treasures buried in the caves deep below Glastonbury described the Alterans as ancient travelers: "Once upon a time, there was a race of people that went on a great journey through space, across the universe. They were called the Altera. After much time (probably thousands of years), they found a great belt of stars. The Alterans named their new home Avalon, and they built many astria porta (or stargates)."
The Alterans settled on Earth, where they came to be known as the Ancients, the most powerful race ever to inhabit the galaxy, and the builders of the stargates. Those they had left behind came to be known as the Ori. Both the Ancients and the Ori eventually evolved physically and mentally to the point that they were able to shed their physical bodies and live as energy on another, higher plane of existence. This ascension, or enlightenment, brought with it a much greater understanding of the universe and all its knowledge. As ascended beings, the Ancients fervently believed in free will and strict noninterference in the lower planes, however the Ori passed down a religion called Origin to the next evolution of humans they created, and they believed that humans must worship them or face annihilation. The philosophical rift has continued through the ages. The Ori believe that the Ancients are evil because they do not share the secrets of the universe, whereas the Ancients have been shielding the existence of the Milky Way Galaxy from domination by the Ori.
Cross Reference: Ancients, Ascension, Merlin's Treasure, Ori
Episode Reference: Avalon, Origin, The Fourth Horseman