context: Doctor Who has appeared on stage numerous times. In the early 1970s, Trevor Martin played the role in Doctor Who and the Daleks in the Seven Keys to Doomsday. In the late 1980s, Jon Pertwee and Colin Baker both played the Doctor at different times during the run of a play titled Doctor Who – The Ultimate Adventure. For two performances, while Pertwee was ill, David Banks (better known for playing Cybermen) played the Doctor. Other original plays have been staged as amateur productions, with other actors playing the Doctor, while Terry Nation wrote The Curse of the Daleks, a stage play mounted in the late 1960s, but without the Doctor.
question: What is the earliest period mentioned in this paragraph?