Fans of the classic series of DW will know that the original Cybermen were from Mondas, a planet that was apparently Earth's twin in the distant past before it drifted away. Even Mondas' continental landmasses were the exact same as Earth's, albeit upside-down. In his book Cybermen, David Banks theorises that it was the arrival of the Moon that pushed Mondas out of its orbit (This would put the date of Mondas' departure at the same time as the end of the age of the Silurians on Earth; I discount his given date of c. 10,000 BCE and assume this happened a few million years ago). But does anyone have any theories as to how a planet like Mondas came to exist in the first place, being a near-exact duplicate of Earth, and how its indigenous human population was more advanced than their Earth cousins?