The Cybermen are cybernetically enhanced humans which developed on the parallel Earth visited by The Doctor, Rose and Mickey in Rise of the Cybermen (2006). They lack any emotion or individuality, the latter they view as a weakness. Similarly, another faction of Cybermen rose on Mondas, Earth's twin planet.
Origins: The Rise of the Cybermen/The Age of SteelEdit
John Lumic's Cybus Industries experimented with synthetic replacement of living issue in an alternate version of London in 2006. Lumic believed that he could heal the pain that humanity suffered through emotion, in what he called the 'Ultimate Upgrade'. This entailed removing the human brain and encasing it in a cybernetic body which would live forever.
Despite the success of his experiments, Lumic was unable to commerically provide the upgrade to the public. However Lumic triggered an override system which could control everyone in London through ear pieces, brainwashing a significant portion of the population into receiving the upgrade.
Lumic disabled the government the same night, many of whom attended Jackie Tyler's 40th Birthday.