Genesis of the Cybermen

The creation of the Cybermen in the Doctor's universe occurred on the planet Mondas sometime prior to the 20th century, probably in the distant past.

Overview
One account describes the planet Marinus, as in fact, an earlier name for Mondas. According to this account, the Voord acquired Worldshaper technology, which could accelerate time, and applied to themselves to turn themselves into full cyborgs. The Cybermen would later remember Mondas/Marinus as Planet 14. (DWM: The World Shapers])

Another account did not actually explain the earliest origins of the Cybermen, but showed how Cybermen existed on Mondas alongside Mondasian versions of Silurians and Sea Devils. These Cybermen died out, until centuries later, Mondasian archaeologists discovered the remnants of ancient Cybertechnology. (DWM: The Cybermen)

Yet another account shows that as Mondas travelled beyond the outermost planets of the solar system, conditions on the planet continued to deteriorate. The Doctor and Nyssa arrived on Mondas prior to the rise of the Cybermen. There, they discovered the Mondasians living underground in a culture similar to that of England in the 1950s, although with more advanced technology. They witnessed those Mondasians who had remained on their planet slowly slide towards cyber-conversion, as the CyberFaction had. The Mondasian cyber-conversion program was conceived by the cybernetic gestalt intelligence controlling Mondas as a solution to the planet's increasingly desperate situation. (BFA: Spare Parts)

The Cybermen would continue to evolve and change on Mondas. The CyberFaction, a group dedicated to more radical cyber-conversion, moved to Planet 14. The CyberMondasians stayed on their home planet. (NA: Iceberg)


 * This obviously cotradicts the explanation for "Planet 14" given previously.

Behind the Scenes
Taken at face value, the origins of the Cybermen given in the Doctor Who Magazine comics feature The Cybermen conflicts with that given in Spare Parts and both do not accord very well with The World Shapers, which had earlier appeared in Doctor Who Magazine. Lance Parkin has speculated that perhaps Cybermen have originated in different planets and times independently, just as they had in different universes, as illustrated by the parallel universe Cybermen having no connection with the Cybermen of the main universe.