Peter Cushing's wig in this was also used by Christopher Lee and various other star names in the first season of this show (Joan Collins, Margaret Leighton, and Leo McKern).
The confrontation between Carter and Mathias (when Helena was about to "pull the plug" on Koenig's life-support) didn't go as planned. Anton Phillips insisted on doing his own stunt and didn't perform it properly, resulting in Nick Tate accidentally knocking him unconscious with a haymaker (which appears on screen; a shocked look on Tate's face can be seen briefly before it cuts to the footage filmed after Phillips recovered). Phillips contradicts this, claiming that Tate really did hit him, but he remained on his feet.
The title refers to a popular term for the theoretical link between ancient simians and prehistoric man for which there lacks actual fossil evidence.
Originally filmed April 22nd to May 7th, 1974 (7th episode in production order) but reshoots took place in July immediately after Alpha Child (1975), due to the producers deciding it would be more logical to have the Alphans investigating one of the nearby planets instead of lunar mineral deposits. First appearance of Kano's rotating desk (the platform of which Martin Landau was standing on while it was filmed rotating for the hallucinatory segue sending Koenig from the duplicate Main Mission into the interior of Raan's home; the rotation being achieved by an unseen crew member laying on the floor).
Peter Cushing was cast in this at Christopher Lee's behest. Lee had been in an earlier episode--Earthbound (1975)--but would only agree to do it if they would cast his good friend Cushing in an episode as well. The producers willingly agreed.