In the re-imagined series, in the aftermath of a terrorist attack by the Army of the One, billionaire scientist Daniel Graystone designed and built the Cylons. The Cylons are robots that can carry out the jobs people don't want to do or can't. A pet project of Daniels is to build a human replica of his dead daughter who was killed in a terrorist attack.
Battlestar Galactica 2003 series established that the skin jobs as they became known were only created in the second war, they had not been unknown of before. Caprica only managed one series before cancelled. If it had gone on, it would probably have explained why skin jobs weren't known before. I would hazard a guess that Daniel Graystone's daughter Cylon would have been kept a secret.
Daniel Graystone became a pariah after his wife acknowledged that their daughter may have been the person who set off the bomb. The Cylons were hated, it was only when the Cylons prevented a terrorist attack that the Cylons began being liked. Cylons began integrating their way into society. Sister Clarice Willow began manipulating the Cylons almost immediately, planting the seeds of hate of humans in them.
In the original seventies series, the Cylons were people in metallic suits, controlled by Count Baltar and Lucifer, the re-imagined series had the Cylons controlled by superior model Cylon, the so-called Skin jobs. The Cylons in the re-imagined series computer generated Cylons which looked more robotic than people in suits.
The Cylons in both series are dedicated to wiping out humans and chasing the remnants across the galaxy.
They were going to make the Cylons a lizard like alien form but the executives that commissioned the series were worried that there might be a backlash that seeing non-human alien life being killed. A few years later, V was made where the aliens were lizards walking around wearing prostethic skin.
In the re-imagined series, the Cylons had evolved and were now looking and acting like humans. It was not possible to tell the difference between a human and a Cylon. Dr. Gaius Baltar was tricked by a female Cylon known as Number Six to hand over the defence codes to the Colonies defence matrix. A number of Cylons didn't know they were Cylons and one of them, Col. Saul Tigh was the second in command of the Battlestar Galactica.
Cpl. Tyrol has a relationship with Lt. Sharon Valerii ( Boomer ) without knowing either of them are Cylons. He later marries and has a child with Cally ( Battlestar ), still not knowing he's a Cylon. When Tory Foster finds out she's a Cylon, she switches sides and joins the Cylons. The only people who knew they were Cylons were one another.
In the original series, the Cylons were clearly people in shining suits. In the re-imagined series, they were replaced with C.G.I., plus also the characters who played the humanoid versions.
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