Dalek in-universe history has seen many retroactive changes, which have caused continuity problems. When the Daleks first appeared in The Daleks, they were presented as the descendants of the Dals, mutated after a brief nuclear war between the Dal and Thal races 500 years ago. This race of Daleks is destroyed when their power supply is wrecked. However they reappear in The Dalek Invasion of Earth, where they have conquered Earth in the 22nd century. Later stories saw them develop time travel and a space empire. In 1975, Terry Nation revised the Daleks' origins in Genesis of the Daleks, where the Dals were now called Kaleds (of which "Daleks" is an anagram), and the Dalek design was attributed to one man, the crippled Kaled chief scientist and evil genius, Davros. Instead of a short nuclear exchange, the Kaled-Thal war was portrayed as a thousand-year-long war of attrition, fought with nuclear, biological and chemical weapons causing widespread mutations among the Kaled race. Davros experimented on living Kaled cells to find the ultimate mutated form of the Kaled species and placed the subjects in tank-like "travel machines" whose design was based on his own life-support chair.
Genesis of the Daleks marked a new era for the depiction of the species, with most of their previous history either forgotten or barely referred to again. Future stories in the original Doctor Who series, which followed a rough story arc, would also focus more on Davros, much to the dissatisfaction of some fans who felt that the Daleks should take centre stage rather than merely becoming minions of their creator. Davros made his last televised appearance for 20 years in Remembrance of the Daleks, which depicted a civil war between two factions of Daleks. One, the "Imperial Daleks", were loyal to Davros, who had become their Emperor, whilst the other, the "Renegade Daleks", followed a black Supreme Dalek. By the end of the story both factions have been wiped out and the Doctor has tricked them into destroying Skaro, though Davros escapes.