The people who were to become the "Colonials" originated on the planet originally known as "Kobol." Human life evolved on Kobol. Over time, numerous societies collectively developed a civilization with all the amenities of an advanced culture (e.g. automobiles, television, advanced medicine, etc.). They even developed a space-folding technology that enables space ships to make instantaneous "jumps" across vast distances in space.
Eventually, concepts of artificial life emerged. Scientists theorized about the notion of a "cybernetic living organism node", also known as a "Cylon." This type of artificial life-form would be grown from constructed synthetic DNA and have a consciousness with some of the characteristics of a computer software program - meaning, its mind could be transmitted, stored, programmed and even interfaced with computer systems. However, the idea of Cylons remained a theory; they were never created on Kobol.
Despite the high level of technological advancements, these advanced humans still did not have total reign over the planet. Instead, they lived within enclaves spread throughout Kobol, while most of the planet remained untamed with primitive tribal humans and other hominids roaming the land.
At some point, the inhospitable natural climatic conditions led 'civilized' humanity to decide to abandon Kobol. A very distant trinary star system with a total of twelve habitable worlds had been discovered. The worlds would be named Aerelon, Aquaria, Canceron, Caprica, Gemenon, Leonis, Libris, Picon, Sagittaron, Scorpia, Tauron and Virgon. Most went to the those worlds, becoming the "Twelve Colonies of Kobol". However, one group (later known as the "Thirteenth Tribe") went a different way and instead colonized an even more distant planet they named "Earth" (the first planet to bear that name). Upon leaving Kobol, the people completely stripped their cities of all non-biodegradable materials to the point that within a few years there would be no archaeological evidence whatsoever that an advanced civilization ever existed on Kobol. This was done so the primitives left on Kobol didn't find any technology that would one day lead to their destruction.
Several centuries after the Thirteenth Tribe colonized Earth, they managed to create Cylons. Outwardly indistinguishable from humans, these fully organic yet purely artificial beings would serve as a slave caste for centuries. At some point, they rose up and waged war against their masters. The war eventually ended in nuclear conflagration with the destruction of both the Cylons and the Thirteenth Tribe. The sole survivor was a single Cylon. All alone he then set out to find the Twelve Colonies in order to seek revenge against the last of humanity.
Because of religious reasons, the humans on the Twelve Colonies initially lost most elements of civilization from Kobol after they settled on the twelve worlds. However, humanity possesses a capacity for genetic memory. This genetic memory does not take the form of passing on personal memories, but the passing on of an unconscious knowledge of certain invented concepts and ideas such as technology, language, engineering and even fashion and music along with various other aesthetic cultural aspects. Over a period of a couple thousand years, all the technological and cultural aspects from Kobol completely reconstituted because the genetic memory of these concepts and ideas 'leaked' into the subconscious minds of various people who gradually and collectively reinvented these aspects of Kobol society. Eventually, even the genetic memory of the concept of cybernetic living organism nodes was reconstituted in the conscious minds of several scientists.
Some time later, Cylons were created within the Twelve Colonies. However, these Cylons were different than those created on Earth by the Thirteenth Tribe; they were composed of strands of Cylon brain matter encased in mechanical bodies. Eventually these "Centurion" Cylon robots were used as slave labor within the Twelve Colonies. As had happened on Earth, the Cylons eventually rose up and waged war against their masters.
After ten years of fighting, an armistice was declared and the Centurion Cylons left for another world to call their own. Soon after, the surviving humanoid Cylon from Earth made contact with them. He became their "Imperious Leader." With the Centurions' help, the Imperious Leader created twelve new models of fully organic humanoid Cylons.
A significant aspect that was endowed into the new humanoid Cylons was that upon "death" their consciousness was externally transmitted and downloaded into another Cylon brain of the same model line. Over time, the humanoid Cylons became the dominant force within the Cylon hierarchy. They became the quintessential Cylon form. During this time, the Cylons never stopped hating humanity. For them, the war never ended. It had simply evolved as they had evolved.
The Cylons, realizing that the Colonial Fleet would likely be too strong to engage in a direct military action, devised an elaborate plan to infiltrate the Colonial ships' operating system software, leaving a backdoor that could be exploited to disable any Colonial ship with its own programming. The Cylons softened up the Twelve Colonies for attack by infiltrating their military and civilian society with numerous copies of the humanoid Cylons. One such copy seduced the brilliant Dr. Gaius Baltar, fooling him into contract work that gave her access to the Colonial Defense Mainframe. Her work compromised Baltar's Command Navigation Program, writing in the planned programmer backdoors.
The plan was successful; the Colonial Fleet was destroyed, the Colonies was subjected to nuclear bombardment, and Colonial civilization was all but wiped out in a matter of hours, except for a space-dwelling caravan of approximately 50,000 humans that managed to escape the Cylon onslaught. Led by the battlestar Galactica under the command of Commander William Adama and President Laura Roslin, this rag-tag fleet began a search for the Thirteenth Tribe and Earth, which they believed would be a new home for them.
Along the way, the Cylons continued to pursue the remnants of the Twelve Colonies of Kobol. However, President Roslin and the people of that surviving fugitive human fleet dedicated themselves to finding Earth. They had many death-defying exploits along the way, such as constantly battling Cylons, locating the thought destroyed battlestar Pegasus commanded by Admiral Helena Cain, only to lose it again when it went up again three Cylon baseships single handedly.
Eventually, the fleet found the location of Earth. Arriving on the planet, they discovered an irradiated wasteland and the ruins of the destroyed civilization of the Thirteenth Tribe. After less than a day of excavating, they were able to piece together the truth of the origin of the humanoid Cylons. Devastated at this discovery, the Galactica then led the fleet away from Earth in the yet another search for a new home. Several months later, the circumstances of a cataclysmic battle saw the final destruction of the Imperious Leader and the rest of the Cylons. The circumstances of that battle also enabled the crew of the Galactica to acquire the coordinates to Kobol. Ironically, it was Gaius Baltar who heroically sacrificed himself to allow both of these things to happen.
Upon arriving at Kobol, the people of the fleet renamed the planet "Earth" as that was the name of the dream for which they had searched for so long. They then left the ships of the fleet (including Galactica) and set them on autopilot to fly into the Sun. These last survivors of the Twelve Colonies decided they wanted a whole new fresh start. So they settled and abandoned their technology to join with the tribal hunter-gatherer population that had remained during the original exodus from the planet.
The Colonials spread throughout the world to the point where eventually every person on the renamed Earth would have some Colonial ancestry. Due to climatic circumstances, it took more than 20,000 years before civilization was redeveloped. But eventually it was. After the end of the Stone Age, the unconscious genetic memory of Colonial society caused certain Colonial ideas and concepts to reemerge. However, they reappeared only in rudimentary, fragmented and even modified forms. For example, the Colonial religion gradually reconstituted in the minds of the people of ancient Greece - but in a modified form (e.g. the "Lords of Kobol" became the "Gods of Olympus).
Over a period of several millennia, a multitude of Colonial ideas, concepts, and cultural aspects reemerged because of genetic memory. Although they would continue to reemerge in rudimentary, modified and fragmented forms, eventually these fragments coalesced into complete reconstitution of Colonial aspects. For instance, the Colonial language eventually reconstituted as the "English" language. The English language did not suddenly appear, but instead evolved through centuries of modifications along with amalgamations from numerous other languages. It was all the result of the genetic memory of the language 'leaking' into the subconscious minds of people who unconsciously directed the language toward a linguistic endpoint that is an exact replication of the Colonial language (apart from a certain profanity word).
As a result of genetic memory, by the early 21st century virtually every cultural aspect of Colonial society has reemerged: language, names, fashion, engineering, architecture, furniture, music, art, cuisine, household appliances, television, automobiles, federalized government, judicial systems, labor unions, military protocols, etc. Countries such as the United States, Canada, and the United Kingdom collectively form a near perfect replication of the society of the Twelve Colonies of Kobol more than 30,000 years ago – just without advanced technology such as jump drive and sentient machines.
The people of modern Earth live in complete ignorance to the fact that their world was once known as "Kobol", that tens of thousands of years ago most of humanity was much more technologically advanced than it is now, that their ancestors traveled amongst the stars and colonized other worlds, that human civilization has rose and fell several times...and that whether it will happen again remains to be seen.