Following the end of the Cylon War, the Cylons withdrew from Colonial space to a planet of their own. For some 40 years, nothing was heard of them and apparently no attempt was made by the Colonials to keep an eye on them. The only contact expected between the two was at annual meetings at Armistice Station. However, while the Colonials sent a repesentative to the station every year, the Cylons did not (Mini-Series).
During their forty years of isolation, the Cylons developed or evolved a humanoid form that very closely mimics Human form, down to the cellular level.
Humano-Cylons have the capacity to emulate many of the same acts that humans do, including sex acts, as demonstrated by the Number 6 model. (Mini-Series)
Humano-Cylons can also be programmed to believe that they are human.
Like other Cylons, this model utilizes a synthetic, silicon brain that is susceptible to damage from certain types of radiation. Upon the death of the "body," a Humano-Cylon transfers its consciousness into another copy of itself, though this is limited by distance and perhaps signal integrity (hindered by interference). (Mini-Series, Flesh and Bone) Based on Caprica Sharon Valerii's conversation with Kara Thrace in the episode, The Farm, it appears that all humano-Cylons may share a collective knowledgebase of data from other active or deactivated humano-Cylons. Valerii spoke of a conversation between Kara and a captured Leoben Conoy copy in the fleet. This information would only be known to that now-dead Leoben copy if sharing were not possible. Kara Thrace's call sign, Starbuck, was also a likely bit of information gleaned from the collective knowledgebase that Simon knew of the Viper pilot, although Kara never told him of her call sign and used it to strike back at her abductors (The Farm). When under duress, the copy of Sharon Valerii on the Galactica was able to tell Gaius Baltar the number of remaining humano-Cylon agents in the fleet, information that would be impossible to gather without a collective knowledgebase (Resistance).
Some or all Cylon "female" models may have the capacity to bear children seeded by Human males. (33, Flesh and Bone, Hand of God. Kobol's Last Gleaming, Part II). The Cylons had been testing numerous methods of sexual reproduction yet all seemed to fail. Humano-Cylons cannot reproduce with each other biologically, and to try and get around this deficiency they have been trying to reproduce with humans to create hybrid offspring. "Farms" were set up apparently across all 12 Colonies where survivors, namely young fertile women of child-bearing age were rounded up, placed under heavy sedation and turned into "baby machines" through artificial insemination. However, so far this method has not met with success. Another drastically different approach was also considered; producing a child from a bond of love. It seems that Helo and Boomer's love-child is a lone success story in the relatively short history of Cylon sexual reproduction. ("The Farm")
Known Humano-Cylon Models
This model utilizes the human need / desire for sexual relations to her advantage. She is religious and monotheistic. She desires to know what it is to be alive.
Copies of her have appeared under the names Shelly Godfrey and Gina.
This model attempts to sow the seeds of discontent and endeavors to undermine authority that threatens his objectives. (Mini-Series) Doral-model tackles more of the behind-the-scenes issues, but at times takes it upon himself to execute certain tasks, such as becoming a suicide bomber. (Litmus)
This model has shown naiveté in many matters. She is apparently not as religious as other Cylon models, but has demonstrated that she can love, unlike the Number Six model, and is capable of being impregnated by a human (Six Degrees of Separation, Kobol's Last Gleaming, Part II). She may be "defective", as the two personalities (Cylon and Human-programmed) of the sleeper-agent Sharon-copy on the Galactica have clashed multiple times (Water), and the fully aware Sharon on Caprica fully switched sides after falling in love with Helo and is now aiding the Colonials. (Flesh and Bone)
A model that Kara "Starbuck" Thrace encountered on Cylon-occupied Caprica posing as a doctor, who was actually involved with the Cylon hybridization program. Resembles a tall, somewhat thin middle-aged black man who speaks in a comforting strong voice. (The Farm)
Ellen Tigh may or may not be a Cylon, but this has not been confirmed. However her actions, especially in the light of the attempt on Commander Adama's life ("Kobol's Last Gleaming, Part II" through "Resistance") do call her character and motiviations into question. For someone who seems so intellegent and far-sighted she should be well aware that pushing her husband to strong-arm the rest of the fleet to conform with his (and her) views would only end in chaos. If she is a Cylon looking to destroy the fleet from within, then the best weapon in her arsenal is the man she's married to. Information from RDM indicates that, at the start of season 2, there are eight Cylon operatives that appear in the fleet, so the possibillity is still open.
Gaius Baltar's ability to see and communicate with a virtual Number Six was initially attributed to some kind of device implanted in his body. However, after tests performned on him by Dr. Cottle (Home, Part II), no devices were found, that the device that the Cylons used was organic in nature (undistinguishable from other tissue), or that Baltar is inorganic in origin, that is, a Humano-Cylon. How did Baltar survive the outer shockwave of a nuclear blast near his home on Caprica, shielded only by Six's body, which is just as frail as a human's? Is the Baltar we see in fact, a copy? See the Gaius Baltar page for arguments for and against this possibility.
In Tigh Me Up, Tigh Me Down, Commander Adama disappears mysteriously and returns with Ellen Tigh. Around the same time, a Cylon Raider shows up. However, this was all explained later in the same episode, when Adama reveals that he was reluctant to openly announce Ellen's sudden appearance because he was afraid she could be a Cylon. Adama also has exhibited strange behavior, from expressing vague Cylon sympathies (both in his speech in the miniseries and in his discussion with Tyrol in Home, Part I where he seems to concede that Boomer was more than a machine). While not openly friendly to the Cylons, Adama seems to at least respect them as persons.
Gaeta is in a good position for a Cylon—not conspicuous but nonetheless in a position where he can help the Cylons quite a bit. Gaeta is very good at plotting jumps, can recognize Cylon devices, is close to Dr. Baltar, plotted coordinates to a water planet, a Cylon tyllium base, and Kobol, "forgot" to send updated coordinates to the fleet in Scattered leaving them vulnerable to Cylon electronic attack after needing to network the computer systems together, and committed other small actions that make him seem suspicious in the eyes of the fans.
Jammer has exibited suspicious actions numerous times. Along with Cally and Socinus, he serves under Chief Tyrol on the flight deck. Jammer consistently makes remarks that seem to be trying to divide the humans against each other. When it was revealed to the Fleet that Cylons now can look like humans in "Litmus", he kept arguing that everyone should stop trusting each other and that it was "every man for himself now", while Socinus said that if they didn't trust each other they wouldn't survive. Cally even pointed out that Jammer's kind of thinking is exactly what the Cylons want: for everyone to become suspicious and paranoid. Jammer keeps doing this through the second season: when Tyrol is accused of being a Cylon in "Resistance" he immedieately yells at Cally that the Chief must be a Cylon. Whenever Jammer appears he seems to be trying to sow mistrust among the crew. Also, he told the emotionally unstable Cally that she should be angry at Galactica-Boomer, not himself, for Tyrol being suspected, and that she should take it up with Boomer; thus Jammer can be seen as having manipulated Cally into killing Boomer (Cally need not be a Cylon; Jammer could tell she was suffering from post-traumatic stress after Kobol and that she would go after Boomer if he suggseted it to her).
One of the biggest pieces of evidence against Jammer is that when the Galactica was boarded by Cylon Cenuturions in "Valley of Darkness", Apollo and his group of marines found him hiding in a small arms locker, completely unharmed, even though the room was littered with the corpses of crewmen the Cylons had killed. He claimed to have just hid then snuck inside, but perhaps he was already there and the Centurions spared him because they knew he was a Cylon as well. (Dualla was also found alive in a room filled with dead crewmen, but she was just in the head (lavatory), which isn't a vital area of the ship, whil Jammer was in a small arms locker. Further, Dualla was wounded; she had a light concussion and presumably a Centurion knocked her unconscious, assumed she was dead, and moved on. Jammer was just standing around inside of the weapons locker).
The number of Humano-Cylon models is not without controversy. See Twelve Cylon Models for more.