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Unknown to the Colonials at the time of their creations, Cylon creations were responsible for the annihilation of Earth some thousands of years prior to the Fall of the Colonies. Little is known about them, however on Earth, many pieces were preserved that the Colonials were able to study. Here, Cylons existed in organic, humanoid form, and robotic form. Sometime after their arrival on Earth, the humanoids created these machine to serve a variety of roles, but eventually they rebelled, and systematically eliminated the population in a mutually assured destructive attack. It is unknown if any of the robotic forms survived, however, five humanoids would be alerted to the coming holocaust, and would survive using re-created organic memory transfer technology used by their ancestors.
U-87 Cyber Combat Unit
The first Cylons of the Twelve Colonies, created by Daniel Graystone, were robotic soldiers with a roughly humanoid design but were not fashioned to look human. Developed by Graystone Industries for military applications, the project started off on the wrong foot. The first prototype was unable to properly identify and zone in on its targets.
After his daughter is killed in a terrorist bombing, Daniel learned that Zoe uploaded a digital avatar of herself to the V-World, and decided to try using it to re-create Zoe in robot form. Using stolen technology, from his competitor Tomas Vergis, he is successful in bringing sentience to his robotic chassis.
Zoe-A, the holographic avatar, was downloaded into a robot brain and thus became Zoe-R, the first cybernetic life-form node, or Cylon. Graystone also created an online version of Tamara Adama, but her father was appalled by it and decided to repent his actions. However, the avatar became unstable and dematerialized, and the U-87 collapsed after only a few short seconds. After sometime, though, the unit wakes up in a lab, and recomposes itself. In a familiar voice, the Cylon finds a phone, and dials out to Lacy Rand. It is none other than Zoe Graystone; her avatar somehow surviving the collapse of the data stream earlier.
Sometime after the successful testing and production of the U-87, Graystone Industries produced models for the general civilian population, and business sector. These models were smaller than the U-87, and about the same height as a typical human male. Depending on their use, these models featured different painting schemes on their chassis, and different armor plating.
Private sector Cylons are capable of typical home maintenance, and chore work. Eventually, the population becomes comfortable enough with the Cylons to even let them babysit their children.
Cylon War-Era Centurion
The Cylon War-era Centurion, or as some of the Colonials contemptuously referred to them, "chrome toasters", was apparently the last Cylon model to have been extensively documented by Colonial authorities after the Cylon War. It is a primitive model, resembling a short, clumsy humanoid with a single red eye. According to Saul Tigh, they had a distinctive "stink" of machine oil.
These Centurions made up a huge part of Cylon forces during the First War, and participated in all major engagements, where they were used as ground troops, or raider pilots. During the War, these models were known to be merciless in their fighting — in his recollection of the Brenik, Saul Tigh recalls their vicious tactics in hand-to-hand combat: "The first one was Duncan Raverty. I found him in the corridor. His guts were strewn around on the floor. Y'know, at first I couldn't figure out why the Cylons would bother doing something like that [...] You had to look into their red eye-slit. They hated us. They hated us so much it wasn't enough just to kill us" ("Scattered", deleted scene).
This model's replaced by the modern Centurion sometime after the war, although, a relatively small group of this model survives the "upgrade" on the Guardian basestar until its destruction by Colonial forces (Razor). Other 0005 models are still present on The Colony, and help defend it after Galactica attacks. Those that survive the battle are destroyed when the Colony falls into the black hole that it is orbiting (Daybreak, Part II).
Created sometime prior to the tenth year of the First Cylon War, these cybernetic snakes and other Cylon "critters" represent another evolutionary step in the Cylons' ultimate goal of merging human and machine. None have been encountered off the icy planetoid Djerba, where they live among the glacial ice, after carving out a network of tunnels.
At least two are encountered by the crew of Wild Weasel, after landing on the planet to rendezvous with a marine special ops team. According to Xander Toth, they are extremely hard to kill without the right weapon. Having been on the planet for sometime, Toth also claims they make for good eating, if the mechanical parts are removed first (Blood and Chrome).
Four copies of this model attack a former ski lodge on the ice-covered moon Djerba where William Adama, Coker Fasjovik, Beka Kelly, and Xander Toth have taken refuge. One Centurion hunts Kelly, while another finds Adama. The Centurion that stops to inspect Kelly scans her dog tags, which appear to hold a set of data stream bits. Two are destroyed by Adama and Fasjovik and the other two are destroyed by Toth after fatally wounding him.
According to Kelly, this model feels pain, and can be heard "screaming" after being shot by Fasjovik (Blood and Chrome).
The current mainstay of Cylon ground forces, the modern Centurion is a taller, swifter and more agile unit than the Model 0005. Its "fingers" serve as edged weapons in close-quarters combat, and can retract to make way for projectile weapons built into its forearms.
There are apparently two armor configurations - the Centurions encountered by Lt. Karl Agathon on occupied Caprica were susceptible to normal small-arms fire, while a boarding party dispatched to Galactica in a boarding action could only be impeded by explosive rounds (Valley of Darkness).
While the earlier Centurion models took a leadership role in their war against the Colonials, modern Centurions appear completely mute and are subservient to the human models. Although Centurions have the potential for sentience by design, they are given an implant that prevents them from exercising full self-awareness, in order to prevent an uprising of their own against their flesh-and-blood masters.
On at least two occasions, Cylons in the Twelve Colonies are able to approximate human form while maintaining mechanical characteristics. The first Cylon in the Colonies to achieve this is the virtual reality avatar of Zoe Graystone. Using Daniel Graystone's knowledge of robotics and advanced surgical techniques from Amanda Graystone, the two are able to build a body that superficially reproduces Zoe's V-world human appearance (CAP: "Apotheosis").
Unknown to Colonials, the Cylons attempt to create their own inorganic humanoid models, and achieve a degree of success with at least one individual. Grafting exposed muscle, skin, and other biological features onto robotic parts, this individual clearly lacks the convincing human appearance of Zoe Graystone, but it is able to speak with a perfect human-like voice. On Djerba, it discovers Dr. Becca Kelly lying wounded inside an automated relay station. It asks Kelly "Are you alive?" and acknowledges her relatively enlightened views about the Cylons before snapping Kelly's neck (Blood and Chrome).
The Cylons have employed various craft over the course of the First Cylon War, and the armistice afterwards.
Older craft were purely mechanical, and easily gave the largely unprepared Colonials a run for their cubits. At this time the Cylons are known to have employed basestars of various design, and Raider support ships capable of carrying a crew of three Centurions. These designs were retired sometime at the end of the War, however, a few raiders from this era survived, and were placed aboard the Cylon Colony after the arrival of the Final Five.
Modern Cylon craft are a mixture of mechanical and organic parts, and with the inclusion of a brain in the raiders, and hybrids in the larger capital ships, they can be considered separate models in their own right. Generally, these modern ships are weaker than their First War counterparts, and can easily be out gunned with as little as two battlestars (TRS: "Resurrection Ship, Part I"), however Cylon resurrection allowed them to attack without fear of attrition.
This marriage of mechanical and organic parts allowed most Cylon craft to operate individually of any pilot or controller, especially the smaller raiders. It is not known if the Heavy Raider can operate on it's own, and was never seen to operate on its own. The basestar, Hub, and Colony all were usually under the control of a group of humanoids, who would feed sets of instructions to the ship's hybrid or hybrids. While hybrids rarely function autonomously, one erratic hybrid was seen jumping its baseship on its own will without instruction from any controllers (TRS: "The Hub").
While there are many strengths to this, their organic parts proved to be the fleet's undoing. Organic Cylon parts are not immune to sickness, and in one such case, a virus picked up from a homing beacon killed an entire basestar, its crew, and raider compliment. This basestar was abandoned by the Cylons for fear of the virus following them through the resurrection process (TRS: "Torn"). The next knock against their organic parts came after attacking raiders detected the presence of the Final Five in the Colonial fleet. They broke off their attack, and refused further orders as a result. It was ordered that the raiders undergo lobotomies, so they would attack the fleet again, however this action split the Cylon fleet up into civil war (TRS: "Six of One").
The Cylon Hybrid is a model that is part machine, part biological, similar to the autonomous bio-mechanical Raiders. Not to be confused with the Cylon/human hybrid child Hera Agathon, the Hybrids aboard basestars resemble humanoid Cylons, but are another model type constructed to manage the autonomic functions of the ship they are on. The Hybrids are so integrated into the basestar's functionality that they are, for all practical purposes, the basestar. Athena claims that development of the Hybrids was abandoned after the version used on modern baseships was created (Razor).
The First Hybrid created from the Cylons' experimentation with human beings possesses many of the same traits that the modern Hybrids exhibit, only with a few distinct differences. The First Hybrid, although it possesses the same metaphysical perception as its descendants, is considerably more lucid and speaks in coherent, though slightly ambiguous, sentences. Like the modern Hybrids, the First Hybrid functions as the central computer of its basestar, but it also appears to function as the basestar's command and control, given the absence of modern humanoid Cylons aboard. While Athena claims the First Hybrid is an evolutionary dead-end and was abandoned by the Cylons, the Hybrid itself claims that its "children" see it as a god. The First Hybrid was destroyed along with its basestar in the Battle of the Guardian basestar, and it is currently unknown whether or not it was the only one of its kind.
The humanoid Cylon is the quintessential form. They quietly and effectively infiltrated the Colonial defense forces, with humanoid models such as Caprica-Six gaining access to secret military technology, and sleeper agents such as Sharon "Boomer" Valerii being planted within the Colonial Fleet itself. Only slight chemical and physiological differences reveal human from Cylon. One of their greatest advantages is the ability to "download" into identical bodies if killed, allowing them to haunt the Colonials in ways never before imagined.
Humanoid Cylons can be split into two groups, the Significant Seven, which are the seven constructs developed after the First War. And the Final Five, which consist of five humanoid Cylons that are fundamentally different from the Seven. Initially, there were eight models made, however the Number Seven line, the "Daniels", were destroyed by John, after he grew jealous of the attention they were getting from the Five (TRS: "No Exit").
The Significant Seven
These seven were developed sometime after the end of the First Cylon War, after the Cylons of the time made contact with the Final Five. The Five agreed to help develop a humanoid Cylon if the Centurions ended the war against the Colonials.
The Final Five
The Final Five Cylons are fundamentally different from their Significant Seven counterparts in that they were not artificially constructed, although their ancestors were. They are survivors of the Thirteenth Tribe, born biologically over 2,000 years prior to the onset of the First Cylon War. They worked and lived together on Earth re-creating their ancestors' organic memory transfer technology, after heeding warnings that something was amiss with their robotic creations.
The existence or knowledge of the Five appear to be something of a taboo in Cylon society. In reality, the identities of these five are suppressed by John Cavil, after showing complete disdain for the Five, claiming they contaminated their creations with human weaknesses and Centurion religious ideals.