The Doctor (The Brain of Morbius)

According to many accounts of the Doctor's early life, they had at least eight incarnations before being the First Doctor, glimpsed during the Fourth Doctor's mindbending duel with Morbius. (TV: The Brain of Morbius, The Timeless Children; PROSE: Doctor Who and the Brain of Morbius, Cold Fusion; AUDIO: Cold Fusion)

Some accounts had these individuals already being known as "the Doctor", and being remembered to some extent by post-First Doctor incarnations, (PROSE: Cold Fusion, PROSE: The Power of the Daleks) while others suggested that the Doctor's memories from before the First Doctor, of which the duel with Morbius had been a glimpse, were from the Other. (PROSE: Lungbarrow)

Early life
According to one account, the Doctor was raised on Gallifrey from a family of explorers, his father being part of the Supreme Council. (PROSE: Cold Fusion) In an alternate universe, after Patience was widowed by Omega, she became the Doctor's tutor, just as she had been his grandfather's tutor and his father's tutor. (PROSE: The Infinity Doctors) He and Patience ultimately fell in love. (PROSE: Cold Fusion, The Infinity Doctors)

Marriage
He embarked on an expedition to the Time Vortex, being one of the first to explore it, with the Machine, a prototype timeship. On the day of his comeback, returning with "charts and trophies from every corner of the universe" and bringing back with him "travellers' tales of monsters and lost civilizations", he married Patience. They lived at Patience's ancestral home, the House of Blyledge, and had thirteen children. (PROSE: Cold Fusion)

On the Supreme Council
After regenerating, considering himself old, he became part of the Supreme Council as his father was before him. At this time, his firstborn son was a Cardinal and a Time Lord of the first rank, whose wife was expecting to give birth to their first grandchild. (PROSE: Cold Fusion)

As only the Loom-Born were allowed to inherit the Legacy of Rassilon, the then-Lord President sent Chancellery Guards to terminate the pregnancy and seize the family. They culled Patience's twelve children and her husband was accused of consorting with aliens. It is implied that the First Doctor transgressed the Laws of Time to travel into the ancient past and rescued Patience and her grandchild, placing Patience in the Machine for safety and assureing her that her granddaughter would be taken away from Gallifrey. (PROSE: Cold Fusion)

Legacy
The Fifth Doctor recognizes the name of Patience's husband as a pioneer amongst his people - which is odd, because his people have forgotten the name. (PROSE: Cold Fusion)

When describing his "renewal" process to Ben and Polly, the newly-regenerated Second Doctor implied that he had 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 thick gold 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 after the eighth face appeared. (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)

The Eighth Doctor saw a series of men in pseudo-Edwardian clothes in the Tomorrow Window. (PROSE: The Tomorrow Windows) After being trapped in the Matrix by, the Thirteenth Doctor escaped by overloading the system with memories of her past incarnations and of all she had seen; some of these eight faces, as they had appeared to the Fourth Doctor during the mindbending contest, were among them. (TV: The Timeless Children)

Appearance
The screen of the mindbending device counted down from the Fourth Doctor to the First Doctor before showing the Morbius faces in reverse chronological order. (TV: The Brain of Morbius) When the Thirteenth Doctor blasted the Matrix with her memories of the mindbending contest, she remembered the faces in a different order. (TV: The Timeless Children)

First incarnation
He was fair-skinned and had a short dark full beard which went lighter at his temples. In the screen of the mindbending device, he wore a dark narrow brimmed hat with a dark feather. (TV: The Brain of Morbius)

Second incarnation
He was fair-skinned and beardless but had lengthy dark straight hair. In the mindbending screen, he wore a dark wide-brimmed hat. (TV: The Brain of Morbius)

Third incarnation
He was fair-skinned, had a goatee and moustache and straight dark lengthy hair. In the mindbending screen, he wore a dark wide-brimmed hat and a top with light coloured collar. (TV: The Brain of Morbius)

Fourth incarnation
The incarnation that married Patience (PROSE: Cold Fusion) was fair-skinned and beardless but had long curly dark hair. In the mindbending screen, he wore a dark cavalier hat with a light-coloured feather. (TV: The Brain of Morbius)

Fifth incarnation
The incarnation that sat on the Supreme Council (PROSE: Cold Fusion) was fair-skinned with blond curly hair and a dark full beard. (TV: The Brain of Morbius) Shortly after he regenerated, Patience thought he was much taller and hairier than his previous, beardless body, and his board was much coarser. (PROSE: Cold Fusion) In the mindbending screen, he wore a light coloured necktie with a top with dark high collar. (TV: The Brain of Morbius)

Sixth incarnation
He was fair-skinned with light coloured curly hair and a full beard. In the mindbending screen, he wore a dark top with no collar at the front but a very high stand behind the head. (TV: The Brain of Morbius) This face and outfit were identical to those of Martin Jurgens, an Adjudicator who was working for the Earth Empire circa 2472. (TV: Colony in Space)

Seventh incarnation
He was fair-skinned with no facial hair but curly light-coloured hair of mid length. In the mindbending screen, his hair was held at the back with a bow tie, and he wore a tricorn hat and a light-coloured necktie over his a dark collar. (TV: The Brain of Morbius)

Eighth incarnation
The incarnation which immediately preceded the First Doctor was fair-skinned with a dark moustache and dark straight hair of mid length. In the mindbending screen, he wore a top hat. (TV: The Brain of Morbius)

Behind the scenes

 * The faces were played by various crew members for the episode: in order from "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, 541) Although they are sometimes wrongly attributed.
 * The montage of The Timeless Children uses a different order: six of the seven Timeless Child incarnations of the episode from earliest to latest, skipping the sixth one; then Philip Hinchcliffe, Christopher Baker, Robert Banks Stewart, George Gallaccio, Robert Holmes, Graeme Harper and Douglas Camfield's incarnations; the elderly Brendan, as played by an uncredited actor; and finally Jo Martin's incarnation.
 * The picture of Graeme Harper is reused from the earlier story Colony in Space, in which it was the ID of Martin Jurgens, the real adjudicator impersonated by.
 * 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. Similarly, The Timeless Children, taken in its entirety, implies (though it does not state) that these eight Doctors were some of the lives the Doctor experienced as regenerations following their Timeless Child self, and prior to being regressed into a child and having their memories erased, both of these events being established in the same story.
 * 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 who regenerated into a Geoffrey Bayldon incarnation at the end.
 * A screenshot of these Doctors' appearance in the mindbending contest appears in Strax Saves the Day when Strax notes that The Day of the Doctor featured "all the original Doctors, except for most of them".