The Doctor (The Brain of Morbius)

Eight incarnations of the Doctor from before the First Doctor were seen during the Fourth Doctor's mindbending contest with Morbius. (TV: The Brain of Morbius; PROSE: Doctor Who and the Brain of Morbius, Cold Fusion; AUDIO: Cold Fusion)

Biography
When describing his "renewal" process to Ben and Polly, the newly-regenerated Second Doctor implied that he might have previously renewed himself in the past, and he opened a trunk that contained relics from his previous incarnations: Saladin's ornamental dagger; a large earring he used to wear; a solid gold-like thick bracelet with odd pictures; and Cameca's jade brooch. (PROSE: The Power of the Daleks)

When Polly, Ben, and Jamie found a shaving mirror on the control console of the second control room in the Doctor's TARDIS, Polly and Ben speculated that the room may have been used by a bearded incarnation of the Doctor from a time before either of the Doctors they were familiar with. (PROSE: Something at the Door)

During the mindbending battle, after Morbius said, "Back! Back to your beginning!" and the eight faces began to flash on the screen, the Fourth Doctor thought to himself, "You can't... not that far... I won't let you... Not even I." (PROSE: Cold Fusion) The mechanism broke down before any more faces could be shown. (TV: The Brain of Morbius)

The Fifth Doctor only had hazy memories of his life from before what he remembered to be his second regeneration; further back, great chunks were missing. (PROSE: Cold Fusion) The Sixth Doctor also mentioned that his memory was vague prior to his regeneration into the Third Doctor. (COMIC: The World Shapers.)

While in the Matrix on Gallifrey with the Master, the Thirteenth Doctor was shown images of her past incarnations, including these eight, the memory of which had been redacted from the Doctor's mind. (TV: The Timeless Children)

Behind the scenes

 * The faces included (in order of "earliest" to "latest" incarnation): Christopher Barry (director), Robert Banks Stewart (writer), Christopher Baker (production assistant), Philip Hinchcliffe (producer), Douglas Camfield (director), Graeme Harper (production assistant), Robert Holmes (script editor), and George Gallaccio (production unit manager). (DWM 329)
 * Many other stories would suggest that William Hartnell's Doctor was, in fact, the earliest. (TV: The Three Doctors, Mawdryn Undead, The Five Doctors, The Time of the Doctor, et al.) The novel Lungbarrow proposed a possible solution for this contradiction: following persecution from Rassilon, a Time Lord founder threw himself into a Loom to be resurrected, and the Doctor had many of his memories; this would suggest that the faces in the mindbending contest belonged to this founder rather than the Doctor himself, though this was only hinted and not explicitly stated in the novel as its foreword is Morbius' quote from the mindbending contest.
 * The original rehearsal script by David Whitaker for TV: The Power of the Daleks (then known as The Destiny of Doctor Who) stated that the Doctor renewed before, and that last time, he was wearing a metal bracelet stored in a drawer of the console, also containing a large earring he used to wear at some point. (DWMSE 4) The earring and bracelet were described in John Peel's novelisation of the serial.
 * Philip Hinchcliffe, producer of TV: The Brain of Morbius and himself one of the faces, said on the faces: "We tried to get famous actors for the faces of the Doctor. But because no one would volunteer, we had to use backroom boys. And it is true to say that I attempted to imply that William Hartnell was not the first Doctor." They all wore stock costumes. (REF: In-Vision #12: The Brain of Morbius, A History of the Universe) In Classic Who: The Hinchcliffe Years as well as A Day with Philip Hinchcliffe, Hinchcliffe reconfirmed this original intention and, while noting that many fans tried to say that the faces were Morbius' rather than the Doctor's (a claim repeated in Doctor Who The Handbook: The Fourth Doctor), he was certain that he had played the Doctor.
 * Script-editor and uncredited co-writer Robert Holmes, also one of the faces, confirmed Hincliffe's account: "We don't know which one Hartnell was, whether he was the first or not. In the phantasmagoric scene where they are mind-wrestling, we see the Doctor forced back through a number of regenerations." (REF: In-Vision #12: The Brain of Morbius) He would later introduce the idea of a twelve regeneration limit in TV: The Deadly Assassin, a story which also established that the Doctor's contemporary the Master had reached his last life as well, although, of course, it is possible for the Master to have burned through regenerations at an accelerated rate, the interpretation that mainstream continuity eventually settled on.
 * Issue 12 of In-Vision also included photographs of seven of the eight "Young Doctors".
 * Lance Parkin, author of the novel Cold Fusion, confirmed that Patience's newly-regenerated husband was intended to specifically be the Douglas Camfield Doctor (REF: AHistory); in an earlier draft of the novel The Infinity Doctors, Patience's husband would have been the Robert Banks Stewart Doctor.