Millions of years ago, the first evolution of humanity know as the Alterans (from Latin alteri, "the others", though in canon, it descends from the Ancient's own language), lived in a distant unknown galaxy while proceeding along the evolutionary path to ascension. At the time, those who were later to be known as the Ori and the Ancients were not distinguished from each other, and lived in one society. However, a philosophical division emerged. Over time, the Ori grew more and more fervent in their religious belief, while the Ancients adopted a more scientific outlook. Ultimately, their viewpoints diverged so much that the two groups split apart and began to oppose each other, culminating in the Ori attempting to destroy the Ancients. Instead of going to war, the Ancients decided to leave their galaxy, and began searching for a new home. The Ori later used their departure to their advantage, including their departure in the Book of Origin, claiming that the mountain the Ancients leveled while leaving contained evil and that it was buried as the mountain collapsed.
After much time, believed by Daniel Jackson to be thousands of years, the Ancients discovered a "great belt of stars", which they named "Avalon" (Milky Way Galaxy) where they eventually built an impressive empire. The Ancients built Astria Porta (Stargates), which were spread out throughout the Milky Way. Furthermore, the Ancients sent out "seed ships" to spread Stargates to other galaxies, along with Destiny, which was sent out after the "seed ships" to further investigate a pattern discovered in the Cosmic Background Microwave Radiation (Stargate Universe). Among the planets they settled was Earth, which they called Terra. The status of Earth in the Ancients' empire is currently unknown. However, as the planet's original Stargate, eventually discovered by SG-1 in Antarctica, was the oldest gate ever encountered, and being the eventual birthplace of the second evolution of humanity, Earth was probably one of their oldest, if not the first world, they settled upon arriving in this galaxy. Also the nine-chevron address that dials the ship Destiny must use Earth as the point of origin, no matter what planet the Stargate is dialing from, so Earth must have been very important, possibly the capitol to their empire. It was also once the location of one of the Ancients City Ships, Atlantis, before it moved to the Pegasus Galaxy, and it was where their descendants returned to after their defeat by the Wraith. Other important planets they have colonized were Praclarush Teonas, Dakara and Valos Cor.
Although exiled to another galaxy, the Ancients still remained in contact with their distant cousins, the Ori, evidenced by the discovery of long-range communication devices in both the Milky Way and their home galaxy. Still, the two factions remained bitter enemies. Eventually, the Ancients were afflicted with a terrible plague that wiped out most of the civilization they had built in the Milky Way. It would later be discovered that what was known of this plague is very similar to the disease used by Ori Priors against non-believers, which had led Daniel Jackson to speculate that the pre-Ascended Ori might have been responsible for this plague.
Some Ancients apparently attempted to travel back through time to escape the plague, without much success. Many others left the Milky Way to travel to the Pegasus galaxy. It has been stated that the Ancients who remained in the Milky Way galaxy either ascended or died out. However, according to Anubis, the Ancients had attempted to recreate life after the plague in the Milky Way using the Dakara Superweapon. The evolution of humanity on Earth could be the result of this attempt.
Five to ten million years ago, several Ancients departed Earth in their city-sized spaceship, 'Atlantis' which at that time was situated on Earth, on the continent Antarctica. The exact reason for their departure is unknown, although Daniel Jackson speculated that they might have left to escape the plague that was devastating their civilization at the time. One known Ancient (later named Ayiana by the researchers who discovered her perfectly preserved in Antarctica) was left behind. The fact that Ayiana was infected with the Ancients plague when she was revived would seem to confirm Jackson's conjectures.
After arriving in the Pegasus Galaxy, the Ancients settled on a planet they named 'Lantea', calling themselves 'Lanteans' from that time forward (also, they have been referred to as 'Atlanteans' by members of the Atlantis Expedition - a name often used by the peoples of Pegasus to describe the expedition members). Finding Pegasus devoid of sentient life, the Ancients dispersed humans and Stargates across their new home galaxy, protecting the newly populated planets and occasionally inhabiting them (usually concealing their true identities as Ancients). The humans seeded by the Ancients came to know them as 'the Ancestors'. At some point they encountered a leech-like organism called the 'Iratus bug', which possessed impressive healing and immunological capabilities. Through feeding on humans (and processes of evolution), this organism absorbed genetic information from humans and eventually evolved into the vampire-like Wraith. Although it remains unknown precisely how this evolution occurred, the Ancients have confirmed that they were responsible for the species' emergence.
The Wraith, over a vast period, developed into an advanced but violent society (with vestiges of territorialism that were later harshly revived when their prey became scarce) - all the while avoiding the attention of the Ancients. When the sentient Wraith were discovered by the Ancients, it appears their ferocity, belligerence and large population were underestimated. The Ancients sent a delegation protected by their most powerful warships to negotiate with the Wraith, but the ships were ambushed and destroyed. The Ancients and Wraith went to war, and despite the Ancients' technological superiority, the Wraith overwhelmed the Ancients with their sheer numbers. During their war which lasted for centuries, the Wraith eventually conquered every planet in the Pegasus Galaxy until only Atlantis, the capital of their empire, remained protected by its powerful city shield.
The Ancients were able to hold the Wraith back with their superior weapons for a hundred years, until they were forced to submerge the city and to leave the Galaxy, for no matter how long they stalled the destruction of Atlantis, the Lanteans were unable to overcome the endless number of Wraith. They started to make more desperate experiments, such as Project Arcturus, a failed attempt to draw Zero Point Energy from their own universe, instead of from within an artificially created region of subspace as in Zero Point Modules. However, containing zero point energy in our own space-time caused serious containment issues, killing the Ancients and the people of Doranda, the planet it was based on. They also developed nanites designed to fight the Wraith on a cellular level, giving them aggression codes surpassing that of the Wraith. The nanites took human form, becoming a race known as the Asurans. They begged to have the aggressive nature removed from their programming, but the Ancients refused. Once it became clear the Asurans would never be the weapon the Ancients hoped for, they were subsequently exterminated by the Ancients. Not wanting anyone to find out their mistakes, the Ancients deleted all references to the Asurans in the Atlantis database, save for the gate address. A few of the nanites survived and brought up a second evolution, re-creating Asuran society. It is believed that at one point in time the Asurans even created several nano-viruses specifically designed to kill humans, so as to deprive the Wraith of a food supply. One such virus, which causes hallucinations and death caused by a brain aneurysm, was encountered in a lab on Atlantis. Janus, an Ancient scientist, also built a device called the Attero device, which emitted a specific frequency of subspace distortion, causing Wraith ships to explode when attempting to enter hyper-space. It was later shut off and ordered to be destroyed (an order Janus, as usual of him, disregarded), because the same frequency which damaged the Wraith ships also caused an explosive overload in active Stargates.
The Ancients also pursued technological means of accelerating the process of Ascension through a variety of mediums. Before the war reached its final stages, they selected a hospitable planet and enclosed a portion of its surface inside a time-dilation field. Within the field, time passed much more quickly than outside, allowing the humans within to spend many generations undisturbed in seclusion and meditation in order to reach ascension, whilst remaining undisturbed by the Wraith. On Atlantis, they built a DNA Resequencer designed to modify DNA, fulfilling the physiological component of ascension. This had varying results.
Eventually, the Ancients realized that the war was lost, and those who had not already ascended, decided to return to Earth, the planet that had been home to them for millions of years. As they were making the final preparations to leave, Dr. Elizabeth Weir appeared from the future in a modified Jumper. The High Council of Atlantis, fearing further damage to the timeline, ordered the scientist who built the machine, Janus, to dismantle it. They offered Weir the chance to return to Earth with them, forbidding her to return to her time. Janus, in order to help preserve the city in the future, reconfigured Atlantis's three ZPMs to run in sequence instead of in parallel, and left Dr. Weir behind in stasis to rotate them out. The Ancients evacuated the city via the Stargate, hoping it would eventually be rediscovered by their descendants. A few isolated groups of Ancients were left stranded in the Pegasus Galaxy and either lived out the rest of their lives with the humans, or found a way to ascend. At least one warship, the Tria, attempted to reach Earth using modified sublight engines that ran at 99.9% of Light Speed after the ship's Hyperdrive was destroyed.
After the Ancients returned to Earth, they found only a harsh environment and primitive inhabitants. It was believed that there was no hope of rebuilding their former civilization. So they spread out to distant lands and helped to populate them. Others would depart the planet through the Stargate located in Antarctica, possibly seeking to live out their days on distant worlds within the Milky Way. The remainder chose isolation in which to meditate and seek enlightenment. This would eventually lead to a spiritual ascension for some.
Those Ancients who stayed on Earth and interbred with humans passed on the Ancient Technology Activation gene marking the humans of Earth as their "Second Evolution" as well as giving civilization to the people. They would also lay cultural foundations by way of the Indo-European languages. Through this cultural and linguistic inheritance many Roman myths were also influenced by the Ancients, such as the myth that the "Ancient ones ascended to the Heavens". According to mythology, the Romans learned how to build roads from gods known as "The Ancient Ones" in mimicry of the "astria porta" constructed by the ancients. Indeed, Daniel Jackson later determined that the Ancient language vaguely resembles Old Latin. This was first discovered when Colonel O'Neill had the entire database of an Ancient repository of knowledge downloaded into his brain, after which he became incapable of speaking in anything other than the Ancients' language. Using this vital insight, Dr. Jackson compared previously indecipherable Ancient writings with Old Latin, factoring in millennia of linguistic drift, and was eventually able to produce functional translations of much of it. Two years later, Earth and a pocket of surrounding space became trapped in a repeating time loop caused by a malfunctioning Ancient time machine. Only O'Neill and Teal'c were not affected because they were standing near the device when the loop began. With Dr. Jackson trapped in the loop and unable to study the time machine's writings thoroughly before his memory was reset, the only choice was for O'Neill and Teal'c to learn the Old Latin/Ancient language from Dr. Jackson in incremental steps, until they learned enough of it to discover the nature of the device.
The Arthurian Legends were also shown to be influenced by the Ancients, in particular the Ancient Myrddin/Merlin, (Known as Moros, in Atlantis where he headed the Council prior to his Ascension), who appeared to have ascended after returning from Atlantis, but later de-ascended himself to create a weapon that can destroy ascended beings, which he hoped would be used against the Ori. Merlin became known as Arthur's wizard counselor in legends, and the weapon he developed became known as the Sangraal or Holy Grail. He also entrusted his secrets to a small group of nobles which consisted of King Arthur and the Knights of the Round Table.
Some of the Ancients, presumably those who left Earth, set up the Alliance of Four Great Races including themselves, the Nox, the Furlings and the Asgard. Together, they created a "true" universal language. According to writings discovered on Heliopolis, this alliance lasted for centuries.
After the Ancients' ascension they continued to oppose their former nemesis, the Ori, who had also reached Enlightenment. Although the two factions were deadlocked in strength, the Ori were still bent on destroying the Ancients once and for all. The Ancients probably outnumbered the Ori as the ranks of the ascended Ori were not swelled by new ascendants, rather they preferred to hoard the energy they derived from their own mortal worshipers, sapping power from this second evolution of humanity created in their own galaxy to gather enough strength to, ultimately, attack the Ancients. In response, the Ancients chose to hide their own second evolution created in the Milky Way from the Ori with the sole purpose of protecting the Milky Way humans from being coerced into forced religious servitude to the Ori, and thus used as a resource for the Ori to empower themselves. To this end they also probably influenced the association of fire, the central icon of the Ori religion, with demonic imagery. Still, they refused to take direct action because of their strict self-imposed edict of self-determination. It is also likely that the Ancients knew the slippery slope of addiction to empowerment by way of the worship of `Lowers` and understood that there was a danger in this reliance, giving the Ori and later Adria a weakness despite `God` like powers. There are several examples of "lowers" praying and giving thanks to the Ancients/Ancestors in the Pegasus Galaxy. Such as "The Tower", even an exiled Ancient on a planet where the people worship her as "Athar" in the Stargate Atlantis episode, "Sanctuary."
The first ascended being ever encountered by the Tau'ri was Oma Desala, when SG-1 arrived on the planet Kheb in their search for Shifu, the young son of Apophis and Sha're. During their stay, the team discovered a Buddhist temple, where a single monk appeared to reside. The monk spoke in riddles, but spoke about a being called Oma Desala, a name Daniel Jackson translated as "Mother Nature". Eventually, when the team was preparing to defend the temple from several Jaffa warriors, Daniel discovered a non-corporeal being living on Kheb. He eventually formed a personal bond with Oma Desala, who even helped him to ascend on two occasions.
Three years later, an Ancient woman, Ayiana, was found frozen in Antarctica near the location of the Antarctica Stargate. She revived herself when thawed, but was still infected with the plague and died exhausted from healing SG-1 and others from the Plague. Apparently, she was left behind in the Ancient outpost in Antarctica, for carrying the plague, when Atlantis left for the Pegasus Galaxy.
When an Aurora class battleship was discovered by the Daedalus, between the Pegasus and Milky Way galaxy, the crew was still alive. As the ship was traversing the inter-galactic expanse between Pegasus and the Milky Way Galaxies, close to light speed, the crew had only experienced a handful of years, because of the time dilation effect of near speed of light travel, since the evacuation of Atlantis. After learning that Atlantis had survived the Wraith, they reclaimed the city. When the Asurans returned and captured the city, all the remaining Ancients were executed.
After Daniel Jackson and Vala Mal Doran inadvertently alerted the Ori to the existence of another galaxy filled with humans, starting the Ori invasion of the Milky Way, millions of humans were converted to the oppressive Ori religion, feeding them only more power. According to the Ori themselves, the entire conversion of the Milky Way would probably give them enough strength to destroy the Ancients once the crusade was over. Still, most of the Ancients refused to intervene, even though their own existence is threatened. Exceptions include Morgan le Fey and Merlin.
SG-1 eventually discovered Merlin in stasis inside a cave on a barren planet. He was revived by the team, and assisted them with constructing the Sangraal, the weapon he had designed to combat the Ori. Although he died before he was able to complete the device, he downloaded the necessary information into a Repository of Knowledge.
Eventually, when the Sangraal was complete, the Ancients didn't interfere for unknown reasons and allowed it to go to the Ori galaxy. The Sangraal did its job and destroyed, although Daniel Jackson says a better translation would be "cancels out or neutralized", the Ori. Adria, the de facto leader of the Ori army ascended, taking on the full mantle of the dead Ori. When the humans from the Milky Way traveled to the Ori galaxy and discovered the Ark of Truth, Adria's power was significantly reduced when the Ark was used to "show" the truth to the Doci, and subsequently all the Priors in their home galaxy, which enabled Morgan, who was exiled by the Others, to engage Adria in eternal battle.
Although the Ancients appear to be human in appearance their physiology is much more advanced. The main characteristic differentiating the Ancients from ordinary humans is what modern humans on Earth have come to call the Ancient Technology Activation gene, a protein that allows for a contact-based neural interface with late-era Milky Way and all Pegasus-era technology. This protein is unique to, and naturally occurring in, all Ancients. However, due to genetic inheritance from the Ancients who returned to Earth after they were forced to abandon Atlantis, a small percentage of modern humans on Earth possess the gene. Shortly after the beginning of Stargate Atlantis, Dr. Carson Beckett developed a gene therapy that integrated the genetic marker into those whom it was not already naturally occurring, allowing them to interact with all of the Ancient technology on Atlantis and elsewhere in the Pegasus galaxy.
It has been established that the Ancients are actually the "first evolution" of humanity, originating in a distant galaxy from the Milky Way. At some point in their races history, an ideological schism occurred between two factions who later became known as the Ancients and "Ori", the former advocating the values of science and reason, while the latter championed religious devotion, respectively. This schism resulted from the threat the Ancients felt from the Ori's religious zeal, prompting a mass exodus by the Ancients from their home world via a large ship. After a presumably long voyage the Ancients arrived in the Milky Way galaxy where they colonized a number of planets, notably Earth. After tens of millions of years they emigrated to the Pegasus galaxy to survive, having seeded life in the Milky Way before they left. Upon returning to the Milky Way approximately ten million years later, the Ancients encountered an Earth populated by a new evolution of humanity, thus it is implied that the current human species evolved as such due to the actions of the Ancients, since two species progressing along identical evolutionary paths is extremely improbable.
Toward the end of their evolutionary development, but still before they ascended, the Ancients developed advanced abilities, such as telekinesis, extraordinary sensory abilities, and the ability to heal others.
Eventually, the Ancients were able to ascend, thus shedding their physical form and existing as pure energy on a "higher plane" of existence. Although this new state of being grants them god-like powers, they adhere to a sort of "Prime Directive" which forbids them from influencing the unascended.
The ability to ascend is not limited to Ancients, as both humans and Goa'uld have been able to ascend.
The Ancients are well known for their fierce belief in free will. As such, they do not interfere on lower planes of existence at all, not even to save their own kind from being exterminated by the Ori. As punishment for breaking this law, the Ancients have been known to wipe out entire civilizations and abandon the one responsible to correct the grave mistakes caused by one of their people. They have also forced (or tried to force) transgressors to descend back to corporeal form. Ancients are, by every means, capable of drastically altering courses of events at will.
What exactly constitutes law-breaking is open to debate. Interfering "a little bit", as Vala Mal Doran described it, will be tolerated. Examples of this include the killing of several Jaffa troops and giving the coordinates of Sahal and Castiana to Daniel Jackson. Also, some Ancients, like Oma Desala, repeatedly assists "those beneath" in ascending. Those who follow her are forced to walk a very fine line. Other Ancients who have been reprimanded for interfering in the mortal planes include Orlin and Chaya Sar.
The Ancients' viewpoints completely oppose those of their distant cousins, the Ori. The Ori religion states that failure to share the secrets of the universe to those on the lower planes of existence is an evil act and that anyone not following it must be eliminated. They also have no rules against taking direct control of living beings or completely changing them to behave as they desire.
This difference in beliefs is not without its reasons. According to a de-ascended Ancient, Orlin, ascended beings can be empowered by massive numbers of humans worshiping them. The Ori have fabricated an entire religion based on the false promise of ascension to drain power from their followers. The Ancients firmly resent using their powers this way, and therefore refrain from interfering in the lower planes of existence as manipulating and aligning lower life forms in some order could result in exactly this type of abusive corruption. As Morgan le Fay said, if they were to interfere, they would be no better than the Ori.
As a result, the Ancients have shielded the second evolution of humans (i.e. current human culture) in the Milky Way from the Ori (and although it has never been stated, they most likely did the same for the seeded humans in the Pegasus Galaxy and Ida galaxy [the Asgard home galaxy]) and still prevent the Ori from taking direct action in the Milky Way. However, as the Ancients will not interfere in the lower planes of existence, the Ori are allowed to send their human followers to the Milky Way to convert it, and anyone who wishes to worship the Ori will be allowed to do so.
Even though the Ancients do not believe in interfering in mortal affairs, it is speculated that they are responsible for generating the associations of fire with Demonic imagery in the Milky Way. This would be another method of shielding humans from the Ori, as the central symbols of Origin involve fire. As Daniel Jackson pointed out, there is no immediate illogicality with fire as a symbol of all that is good: it provides warmth, light, etc.
Also, even before their ascension, the Ancients had strict rules on time travel, although this rule has been broken as well. The Ancient scientist Janus developed time travel technology, leading to Elizabeth Weir using it to travel back to Atlantis 10,000 years previously, and to a column containing a history of future events on the Milky Way planet ruled by Harry Maybourne.
Ancient technology is among the most advanced encountered in the Stargate universe rivaled only by the Ori, who are the same race as the Ancients, and the Asgard who revealed some of their most advanced technologies only shortly before their extinction. Among their many achievements, the Stargates and Atlantis are perhaps their most famous, but they have also developed technology as diverse as flying cities, advanced power sources, advanced healing devices and DNA Resequencers. Much of the Ancients' weapons technology was used or designed for use in the centuries-long war with the Wraith in the Pegasus Galaxy.
Much of Ancient technology requires the presence of the ATA gene to use, such as the Ancient weapon system in Antarctica, but numerous instances requiring only the gene to activate it, once active anyone can use it regardless, for instance much of the day to day technology of Atlantis.
The Ancients' language is a precursor of Latin, but it is still distinct. There are many similarities between the two languages, and someone with a decent knowledge of Latin may, given time and practice, be able to understand Ancient as well. The Ancient language and writing system is also used by the Ori and their followers, who share the same ancestry. Since the Wraiths were created by the Iratus bug absorbing Ancient DNA, the Wraiths' language seems to be a derivative of the Ancient language, and the two use the same writing system. The Wraiths' language is therefore relatively easy to translate by experienced Ancient speakers.
The Ancient language has a unique writing system that has no analogues with any known Earth alphabet. Ancient letter are squarish and dominated by right angles, and as a full letter, a letter contains 24 squarish dots. There are 25 distinct letters ('F' and 'U' are the same character) and 10 numerals (from 0 to 9) in the Ancient-English conversion. It has been mentioned though that the Ancients have an Octal counting system, meaning based on 8, not like the decimal system of the Tau'ri, based on 10, although the Stargate Universe symbols include symbols for 8 and 9. Exactly how the characters were used by the Ancients is unknown. It has been mentioned that some Ancient characters have different meanings when read upside down. The Ancient writing system has been shown to be learnable with moderate difficulty by humans. However, the pronunciation of Ancient words has fluctuated during the millennia the language was used.
Passages written in Ancient on the shows can be transliterated into English (there are no characters for spaces and punctuation marks). These transliterations show that the in-show writing system rules do not necessarily apply in reality, but they sometimes reveal in-jokes or offer further (non-canon) information instead. For example, parts of the writings on Atlantis main screens include the beginning of the nursery rhyme Where Oh Where Has My Little Dog Gone?, and the text shown on Daniel's laptop in the episode "Affinity" can be transliterated into "Actual words are hard to deciph(er) so this(')ll work". Playback supervisor Krista McLean said that when she originally put the graphics together for the Atlantian holo screens, she carelessly used things that were going through her head. When fans translated the language on the internet, she became more careful and tried to not use direct English translations anymore.
There are also many examples that are in keeping with what the Ancients and associated races intended – the writings on the stairs in the gate-room on Atlantis translate into a sort of greeting for travelers; Puddle Jumper consoles include instructions on how to use the ship; the Ancient wall in "Reckoning" describes the use of the Ancient object hidden behind, and the responsibilities that go with it. The sign in Harrid and Sallis' house in the Ori galaxy in "Avalon" says what Daniel translates it to be ("This is the nuptial home of Harrid and Sallis Cicera"). The engraved Sodan flagstones in "Babylon" give instructions on how to behave in Sodan society.
The Athosians in the Pegasus Galaxy say their prayers in Ancient.
Although most of the Ancients had reached ascension long before the dawn of modern civilization on Earth, the Tau'ri have come in contact with Ancients on several occasions, both in ascended and corporeal form. Most of the corporeal Ancients were encountered in the Pegasus Galaxy, where their empire had collapsed only some 10,000 years ago. Also, some Ancients helped humans on the lower planes of existence ascend, like Oma Desala helped Daniel Jackson.