Mondas was identical to Earth, even down to the size and shapes of its continents. When it approached the Earth in 1986, the planet was oriented such that its continents were upside down relative to Earth. However, it had no visible clouds in its atmosphere at that time. (The Tenth Planet)
Originally, the Mondasians resembled the humans of Earth in every respect. (The Tenth Planet) One account shows that in the same way that parallels to humans existed on Mondas, Lizard Kings and Sea Devils shared the planet with dinosaurs and Cybermen. (The Cybermen: The Dead Heart) Golgoth, a bipedal reptilian god in physical form, also lived on Mondas at this time. (The Cybermen)
The Museum of the Last Ones had specimens of extinct species from Mondas. (The Last Dodo)
Mondas was created by the Constructors of Destiny as an experiment of sorts to see how it would differ from Earth. (The Quantum Archangel) Mondas had, like Earth, at one time, an intelligent reptile species. (The Cybermen)
ORIGINS OF THE CYBERMEN
Originally, Earth and Mondas occupied the same orbit in the Sol System. It was stated that “Mondas” was another name for Earth in one of the ancient languages. (The Tenth Planet)
This implies a high degree of parallel evolution and/or contact between the two worlds.
During a time when parts of Mondas had a tropical climate, Cybermen met and warred with Silurians and Sea Devils. (The Cybermen: The Dead Heart) Golgoth appeared and destroyed all but one Cyberman and finally destroyed the lone survivor. (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. Archaeologists had discovered remnants of the Cybermen. (The Cybermen: The Ugly Underneath)
This last account 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.