Battlestar Galactica (2003)/Cylons
Cylons were created by man as a robotic worker class, used to do the work that nobody else wanted to do. Eventually, the Cylons revolted against their human masters, resulting in a long, bloody war between them and the Colonies. Finally, approximately 40 years before the destruction of the Colonies, the Cimtar Peace Accords were signed and the Cylons disappeared. Slowly, the Colonials forgot the lessons they learned during the Cylon War and began to design more and more advanced computer networking, leaving their defense mainframe and warships open to sabotage.
Cylons eventually created (or "evolved") a new model of Cylon, indistinguishable from humans except with specialized testing or under the most detailed autopsy. However, there are a few significant differences, including a spine that glows during sexual intercourse (the purpose of which is questionable), the ability to interface with human fiberoptic communications systems through something in the hand (at least with Boomer units), and the traditional Cylon silica pathway brain construction. These silica pathways may be what makes it possible for human-type Cylons and Cylon Raiders to upload their memories into other units of the same type at their time of death: Six claims that there are twelve models of Cylon (of which she is the sixth, hence her designation) and that when Cylon agents' bodies are destroyed, their consciousness is transmitted to a new copy of themself.
Apparently, at least some female Cylon models are capable of interbreeding with Human males, as it appears that a Boomer unit has become pregnant with the child of Lieutenant Karl "Helo" Agathon.
The human-type Cylons also appear to be used to command armies of less-autonomous, less-intelligent Cylons such as Raiders and Centurions.
With the development of advanced self-awareness, the Cylons apparently also developed their own version of the Colonial religion. Cylons worship a single "true God", as opposed to the Colonial polytheistic religion. Cylons view the Colonial religion as blasphemy, although their religion appears to have evolved from the same basic texts - Cylons seem extremely well versed in human religious texts and have been known to quote prophecy from the same scrolls as the President.
The Cylon Attack
Using the oversexed Dr. Gaius Baltar as an unwitting pawn, the Cylons sent Cylon agent Six to seduce him, pretending to be a fellow scientist. Baltar was working on the new Command Navigation Program, which would revolutionize navigation by using the advanced networked computer systems on modern Colonial ships, both civilian and military. However, Six rigged the CNP to act as a back door for a Cylon virus, allowing an external transmission from a Cylon ship to remotely deactivate Colonial vessels outfitted with integrated defense computers and the CNP in tandem. Additionally, Six used Baltar to get the codes for the Colonial planetary defense mainframes and shut them down as well, effectively rendering the Twelve Colonies totally vulnerable to a vicious nuclear strike. Upon the destruction of the Colonies and the presumed death of that particular Six, she began appearing to Dr. Baltar in hallucinatory visions. Six initially claimed that this was the result of a chip implanted in Dr. Baltar's head, but later changed her story to claim that Baltar was simply going mad, and then again claiming that she was an angel sent by the Cylon God to protect Baltar.