Voord

The Voord were a race of beings who had discovered how to resist the Conscience of Marinus. They sought to take control of the Conscience, thus taking control of the planet, but were defeated by the Doctor and his companions. (DW: The Keys of Marinus)

The Voords began experimenting with a World Shaper, these accelerated time and the Voords began evolving much faster. Over time they replaced there body part with Cybernetic parts. The Doctor Frobisher,Peri Brown and Jamie McCrimmon attempted to stop the Voord. Jamie scrificed himself to destroy the World Shaper, this caused a blast that excellerated time so much that Marius became known as Mondas and the Voords evolved into entirely Cybernetc creatures called Cybermen. (DWM: The World Shapers). This revelation has largely been ignored by fans, though it is possible that The Voord's experimentations with the World Shaper simply set off a Cyberman trap and "test fired" a succesful terraforming weapon with which to recreate Mondas.