A planet's geophysical conditions are a greater component of biological evolution on that planet than its environmental conditions. The combined synergistic effect of a planet's particular magnetic field and particular gravity field has a submolecular-level influence on organisms that induces specific mutations and genetic variations. Because of that, specific geophysical conditions unvaryingly direct evolution and speciation along and towards a specific pattern of physical development when added with a specific natural selection path caused by specific environmental influences such as a particular atmospheric composition and climate system.
As a consequence, physically identical life forms can evolve on different planets because those planets happen to have both very similar environmental conditions and the same precise gravitational and geomagnetic conditions. It is under those circumstances that on Kobol, a planet environmentally similar and geophysically identical to Earth, evolved a sentient mammalian bipedal species anatomically indistinguishable from modern Earth humans right down to the exact same morphological traits and range of phenotypical variations. Kobol-evolved 'humans' and Earth humans are even genetically compatible with each other to the point of being able to interbreed.
The humans of Kobol achieved dominion of their world while the humans of Earth remained in the Stone Age. Eventually the people of Kobol developed an advanced civilization with a high level technology, such as space travel. However, they became a wasteful people, and didn't care for their planet. Waste piled up, the air and seas became polluted, and many species of animals died out. Simultaneously, the star that Kobol orbits began showing the early stages of imminent destruction. The end result was that all life on the planet would be doomed by either humanity or nature's hands. However, the people of Kobol united to act before it was too late for them. They built great space arks to transport the majority of their population to other worlds, and they set out across the galaxy in search of a new home. They eventually found Earth (and gave it that name). However, some time later, explorers found a system which had twelve habitable planets circling three stars, and it was here that twelve tribes of Kobol settled, and these worlds were to become the Twelve Colonies of Man. The worlds were: Aquarus, Aeries, Cancera, Caprica, Gemoni, Leonis, Libris, Picon, Sagitaria, Scorpia, Taura, and Virgon.
Kobol was ruled by thirteen tribes, but the thirteenth didn't follow the other twelve. Instead, it left Kobol before the other tribes and settled on Earth (although some subgroups settled worlds such as Terra and Paradeen). The members of the Thirteenth Tribe were the forefathers of the ancient Egyptians, the Toltecs, and the Mayans. They were also the architects of the great pyramids, as well as the lost civilizations of Lemuria and Atlantis. As for the other twelve tribes; once they landed on the twelve colonies, they turned on the technology that had saved and corrupted them - destroyed it along with the great arks - and returned to a very simple life without it. Most records of the time before this were also destroyed or lost, and soon Kobol was just a legend and a myth as most elements of its civilization were lost.
However, the Kobol-evolved humans possess a capacity for genetic memory. This genetic memory does not take the form of passing on personal memories, but 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 several thousand years, all the technological and cultural aspects from Kobol completely reconstituted within the Twelve Colonies 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.
Space flight was rediscovered, and with it, the door was opened to establishing contact between the Twelve Colonies once more. Once cultural differences were sorted out, trade and help was forthcoming amongst the various worlds. Unfortunately, one of the 'cultural differences' would actually be the "Unification War." Up until this point, all of the Twelve Colonies had developed with their own governments, cultures and way of doing things. As such, many misunderstandings would occur before the Colonies reunited, and as a result of this, there would be many small skirmishes between the various planets. None of this could be called a war as such, and indeed most of the action would take place within the political theaters, but sometimes it spilled out, and a lot of lives would be lost before the Quorum of Twelve could be established, ending the strife and bringing on a new era of peace and prosperity.
Eventually, the Colonies started to launch probes and expeditions to explore the galaxy that they lived in, and it was then that the colonies established new settlements on other planets and asteroids, and made peaceful contact with intelligent life from other worlds. Some of the races that were contacted by the Colonials included the Hasaris, a peaceful amphibious race, and the Orions an insectoid species of traders and merchants, making and selling goods. They used their own money but became very important to the Colonials, providing technology and luxury goods. It was during this time that a central Colonial Government was established and that with the advent of extra outposts and trading links, piracy soon got itself wired up, which lead to the formation of the Colonial Fleet to counter this threat. The Colonials' technology became greater, and artificial intelligence was soon common place, the goal being that of robots doing all the boring and dangerous jobs that society didn't want to do. However this backfired when the computers and robots revolted against their human masters.
A short 'war' against the machines was instigated, and when the revolt was finally put down, the Colonial Code was passed, limiting the sort of intelligence that could be programmed into a computer, and effectively meaning the end of AI for the Colonies. While each world was free to define exactly what that meant, Caprica was the harshest in it's ruling, and had to be restrained from banning computers all together. Scorpia on the other hand, was the least firm in implementing the Code, and continued to use robots and artificial intelligence to almost the same levels as before.
A little under a thousand years before the 20th century on Earth, the Colonials received a distress call from their alien allies - the peace loving Hasaris - who were under attack from a race known as the Cylons. It is not known why the Cylons attacked the Hasaris, but a generally held belief is that the Cylons perceived them as a threat of some kind, either as to Cylon expansion, or as a race that would attempt to curb Cylon intentions. Either way, the Cylons decided that they couldn't be allowed to exist, and thus in typical fashion decided to remove them - permanently. This started out by the Cylons simply attacking civilian ships and lone warships in a pirate style series of raids, but then when the Hasaris started defending their convoys and ships, the Cylons sent in Baseships with the edict of extermination. The Hasaris tried to defend themselves as best they could, but they were simply no match for the size of the Baseships and the sheer numbers of Raiders that the Cylons could throw at them.
The Colonials sent aid, in the form of a large battle fleet, to help defend the Hasaris from the robotic invaders, and were horrified to learn first hand of the brutality of the Cylon fleet and their ships capabilities. Whilst it was clear from the start, that even flying outmoded equipment, Colonial pilots were superior to their Cylon counterparts, the Cylons could employ far greater numbers than the Colonials could even dream of. And skill alone couldn't fend off the Cylon fleet.
In the span of two years, much of the Colonial Fleet, and the entire Hasaris race, were destroyed. Humans could outfly Cylon Raiders in any battle, but they would never be able to match sheer numbers, nor the destructive power of the Cylon Baseships. Because of this intervention, the Cylons learned about humanity, and felt that it too, constituted a major threat to their well being and their order of the universe, and thus had to be removed. There could be no chance for peace or negotiation. Only the complete extermination of the human race would satisfy the Cylons. Exactly what had happened to the Hasaris. The Cylons moved on the Colonials' worlds, and the war began in earnest.
The war went badly for the Colonials. Not having learnt as much of the Cylons as the Cylons had of the Colonials, they were unprepared for the all out nature of the Cylon assaults. As the Cylons wanted the Colonials exterminated, prisoners were not taken, unless the Cylons needed information, and then the prisoners were executed afterwards. Outposts were destroyed, civilians butchered without mercy. The Colonial Fleet wasn't up to the task of stopping the Cylon might completely. While Colonial pilots fought bravely - and always outnumbered - there were simply too many Cylons, and the early ships of the line weren't up to the task of taking on the Cylon Baseships. However one thing that the Cylons hadn't counted on, was the human tenacity and refusal to bow out and allow themselves to be killed. This meant that Cylon advances were never as great as they should have been, and the Colonials hung on, something the Cylons could never understand. However the Cylons were stopped. In their arrogance, they couldn't believe that the Colonials could stop them - maybe slow them, but not stop, and they launched an all out invasion of the Colonial space. The Colonials had not only anticipated this attack, they stopped it dead and had the Cylons running.
The middle stages saw the most action and the least conquest. The Colonials had some initial breathing space as the Cylons decided how to react to the humans recklessness and backed off to mere probing attacks instead of all out assaults. Another reason for the Cylons pulling back, was that the Colonial destruction was not the sole plan of the Cylons. They had an empire to forge out, a galaxy to explore, other races to contact, and be ordered into the Cylon Empire or enslave or destroy. The Cylons did venture out far into the galaxy, further out then the Colonials ever imagined or did themselves. They did ally themselves to some races, and it has never been made clear what constitutes a threat to Cylon Order. It is due to this that the entire Cylon fleet was never actually employed against the Colonials, which was a good thing, as the Colonials would never have stood a chance.
The Colonials however, put this breathing space to good use. They overhauled their fleet, firstly building the new light cruisers to escort the aging Heavy Carriers and other ships of the line, before retiring them when the Battlestars entered service. Other ship types soon followed, as did the Viper fighter. While these designs were updated and modified over the course of their long service history, the Colonials preferred to overhaul old ships rather than design new ones, and hence the amount of ship designs over the 1,045 years of the war was pitifully small. With these new ship designs, and a better understanding of how the Cylons waged war, the Colonials conducted their own attacks, and finally brought the war back to the Cylons.
While the Cylons could never actually be driven back, neither could they advance, and the middle stages of the war was one of probing assaults, strikes and counter strikes into each other’s territories, but very little actual conquest. The current permanent combat footing and way of life for the Colonials was developed here. During this period, there were many lulls in the fighting, and these lulls sometimes lasted tens of years. To civilians growing up in this era, these lulls were often misinterpreted as separate wars of their own.
While this was happening, various societal ideas, concepts, and cultural aspects of Kobol's civilization were appearing on Earth as a result of genetic memory passed on from the Thirteenth Tribe. Although they appeared only in rudimentary, modified and even fragmented forms, eventually those fragments coalesced into complete reconstitution of Kobolian aspects. For instance, the Kobolian 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 dominant language of Kobol (and hence also of the Twelve Colonies).
Far away amongst the stars, the Colonials continued the fight against the Cylons for over a thousand years. In that fight, they never forgot their neighbors. When the Cylons attempted to attack allied planets, the Colonials always tried to help. Sometimes this worked and they freed their allies from the oppression of Cylon rule, and sometimes they lost. Inevitably, the Cylons had had enough. Destroying a race was never meant to be such a slow and tedious process, and so they put their collective heads together to destroy the humans. They had an empire to manage, and the Colonials were a thorn in their side to be removed for good. Thus they started learning from their enemies, and became sneaky and treacherous, employing any sort of ambush and bait to trap and destroy the Colonials. It is in this fashion that they got their reputation for being underhand. Any trick to bait Colonials - distress calls, hostage bait, Trojan horse traps - all were deemed acceptable if the end result was the death of more Colonials.
One thing that never changed though, was that their tactics always involved overwhelming odds and enough firepower to make sure that the Colonials were destroyed. Sometimes it worked, sometimes it didn't. The end result though, was always a drain on ever shrinking Colonial resources. All the decisive battles of the war were played out in this stage of the war, and while the Colonials managed to inflict major damage on the Cylons (nearly always destroying more than they lost, even in their defeats), they were generally on the defensive just trying to stem the relentless advances of the Cylons. Despite this extra offensive state of the Cylons, no one suspected that the end was nigh and that the endgame was finally being played out.
The Cylons managed to convince Lord Baltar of Picon, a Quorum of Twelve member, that they would spare his world if he helped them conquer and enslave the people of the other eleven worlds. They also told him that he would rule over the Colonies as the representative of the Cylons. Baltar spoke to the Quorum of Twelve that they Cylons wished to sue for peace. This peace would be sealed at the old moon of Cimtar, a system only a short journey away from the Colonies. The president of the Twelve Colonies, Adar, had had enough of war at this point, and he believed Baltar - as did most of the Quorum, a notable exception being the military Commander Adama who represented the planet Caprica in the Quorum - that the Cylons truly wanted peace. As such, he let himself be led by Baltar, and the heavy elements of the Colonial Fleet journeyed to the moon of Cimtar to seal the truce.
The Cylon plan was three fold. The first two parts operated together to destroy the final defenses of the Colonies, and then the third part would be the ultimate destruction of the human race. Part one was a huge trap for the Colonial Fleet. Surrounding the moon of Cimtar is a vast mist layer, and hidden within this layer were Cylon tankers. They fueled a vast armada of over a thousand raiders, which would allow them to operate without baseship support. These would destroy the Colonial Fleet. At the same time, part two of the trap consisted of three fully loaded baseships jumping into the Colonial system to attack the twelve worlds. Targets would include any Fleet elements and planetary defenses. Part of this plan was to use spies of Baltar who would attempt to disable defenses and provide information on Fleet movements. Then, when the raiders and baseships had done their jobs, they would rendezvous with the remaining Cylon fleet elements, and once refueled and rearmed, the entire fleet would then jump back to the Colonies and complete the destruction. The plan worked almost perfectly.
The Colonial Fleet moved to the moon of Cimtar, but a lone patrol from the Colonial battlestar Galactica discovered the massed raiders and the Cylons had to spring their trap early. Thus during the attack, the Colonials detected the presence of the baseships in their home systems, and the Galactica managed to break away from the ambush and jump back to the Colonial system. The rest of the Colonial Fleet, aside from a few vipers, was destroyed.
The Galactica arrived too late to save the Colonies. The first group of baseships had already arrived and left, and with the help of human saboteurs, had disabled the planetary defenses, allowing them and any Fleet elements present to be easily destroyed. Commander Adama realized that life on the Colonies had come to an end, and there was no hope of remaining there alive. So he sent word to all survivors who assembled in any ship they could find, and lead this fleet away from the remains of the Colonials and from the Cylons. Having no choice to stay, the battlestar Galactica assembled as many survivors as it could, and a space-dwelling caravan of human survivors left the Colonial system for good. This rag-tag fleet is on the search for the Thirteenth Tribe and legendary planet known as Earth, which they believe will be a new home for them.
Shortly afterwards, the Cylons arrived back in Colonial system, and this time they completed the destruction of the Twelve Colonies, and the extermination of all life there. All of the twelve worlds were rendered uninhabitable as a result of nuclear fallout. Despite their deal with Lord Baltar, the Cylons attacked Picon along with the other colonies. The Cylons have kept Baltar alive as a prisoner because their commander-in-chief, the Imperious Leader, believes that, to find the surviving Colonials, another human is needed that thinks as their prey does. And so Baltar aids the Cylons in their pursuit of the Battlestar Galactica and its fugitive human fleet, who have dedicated themselves to finding Earth.
Meanwhile, the people of Earth live in complete ignorance to the fact that, in part, life here (on Earth) began out there...far across the galaxy, and that there are brothers of man who now fight to survive beyond the heavens.