Mondas

Mondas was the original homeworld of the Cybermen of this universe and also the twin planet of Earth, which, in ancient times, moved from its old position out into space.

Geography
Mondas was identical to Earth even down to the size and shapes of its continents. (DW: The Tenth Planet)

''Given what we know about the geology of Earth and the formation of its landmasses this is exceptionally unlikely and strongly suggests an unknown, vastly powerful agency somehow influenced the development of either or both planets.

Native life
Originally, the Mondasians resembled the humans of Earth in every respect. (DW: The Tenth Planet) One account shows that in the same way that parallels to humans existed on Mondas, Mondasian Silurians and Sea Devils shared the planet with dinosaurs and Cybermen. (DWM: The Cybermen: The Dead Heart) A reptilian god figure, Golgoth, also lived on Mondas at this time. (DWM: The Cybermen: The Dark Flame)

Conflicting accounts
One account shows the Voord of the planet Marinus using stolen time technology to "quick-evolve" themselves into the earliest Cybermen, which would make Mondas, in fact, Marinus, under another name. (DWM: The World Shapers)

Another account describes events which took place on Terra Nova, an artificially created duplicate of Earth and its cultures circa the 1st century BCE. Possibly this indicates that Mondas began as Terra Nova. (MA: State of Change)

Separation from Earth orbit
Originally, Earth and Mondas occupied the same orbit in the Sol System. It was stated that "Mondas" is another name for Earth in one of the ancient languages. For unknown reasons, Mondas left that orbit and drifted into space. (DW: The Tenth Planet)


 * This implies either contact between the two twin planets or else a degree of parallel evolution, with similar languages evolving on both worlds.

Origins
According to the Cybermen themselves, the Mondasians were once exactly like human beings, but to remove various weaknesses such as a decreasing lifespan, their scientists designed cybernetic spare parts to replace the organs of the human body. They also considered emotions a weakness, and removed those from their brains as well, becoming extremely powerful and long-lived while at the cost of becoming cold, logical and machine-like in their ruthlessness. (DW: The Tenth Planet)

Early history
During a time when parts of Mondas had a tropical climate, Cybermen met and warred with Silurians and Sea Devils. (DWM: The Cybermen: The Dead Heart) The godlike reptilian humanoid Golgoth appeared and destroyed all but one Cyberman and finally destroyed the lone survivor. (DWM: The Cybermen)

Some centuries after, human-appearing Mondasians of roughly 20th century technological development had appeared and Mondas had begun to move out of its orbit. Archeologists had discovered remnants of the Cybermen. (DWM: The Cybermen: The Ugly Underneath)


 * This last story implied that the discovery of the extinct Cybermen would lead to the Mondasians deciding to use Cybertechnology, beginning the cycle of creation anew and creating a new race of Cybermen.

The Doctor and Nyssa arrived on Mondas prior to the rise of the Cybermen, while it was still drifting through space. There, they discovered the Mondasians living underground in a culture virtually identical to that of England in the 1950s, although with more advanced technology. They witnessed the slow slide of the Mondasians towards cyber-conversion, conceived by the cybernetic gestalt intelligence controlling Mondas as a solution to the planet's increasingly desperate situation. (BFA: Spare Parts)


 * The gestalt intelligence had similarities to the Cyber-Planner. This event may have happened after the other account or it may have even happened before

Return of Mondas to the Sol System
By 1986, the Cybermen had developed a means of propulsion for Mondas itself. This allowed them to pilot the planet through space, returning it to its original orbit. The Cybermen invaded the South Pole while Mondas drew closer, draining Earth's energy to replace and supplement its own. However, the energy absorbed was too much, and Mondas disintegrated, killing the Cybermen on Earth. (DW: The Tenth Planet)

The "New Mondas"
In 1988, the Cyber-Fleet traveled to Earth, planning to take the Nemesis statue, a Time Lord weapon that would allow them to conquer Earth. The Cyber-Leader planned for there to be a new and final era where imagination, thought, freedom, and pleasure would end. The Human-race would be enslaved and Earth would become the Cybermen's base planet: the "New Mondas".

The Doctor, however, launched the Nemesis in space to meet with the Cyber-Fleet. When it did, it exploded and destroyed the entire fleet. Earth remained unchanged.(DW: Silver Nemesis)