Doctor Who: Who are the Morbius Doctors, and how might they connect to Jo Martin's Time Lord?

For some fans, this is a sacrilegious suggestion, but this wouldn't be the first time that Doctor Who has played with the idea of there being versions of the Doctor pre-Hartnell.

In January 1976, Doctor Who aired the four-part story The Brain of Morbius – credited to Robin Bland, it was actually co-scripted by veteran Who writer Terrance Dicks and script editor Robert Holmes.

Like the recent The Haunting of Villa Diodati, The Brain of Morbius was a riff on Mary Shelley's classic Frankenstein, with the mad surgeon Mehendri Solon (Philip Madoc) having salvaged the (still-active) brain of the Time Lord war criminal Morbius and working to to create a new body for the villain using parts of other creatures.

Morbius is resurrected in a monstrous new form and is challenged by the Fourth Doctor (Tom Baker) to a 'mind-bending contest', the pair using the advanced apparatus in Solon's laboratory to engage in a literal battle of wits. And here's where things get both interesting and very complicated...

As he struggles against the powerful Morbius, the machine displays the Doctor's face, then that of his previous incarnation (as played by Jon Pertwee), followed by glimpses of the Second (Patrick Troughton) and First (William Hartnell) Doctors. But it doesn't stop there: the machine then displays eight more faces before the case containing Morbius's brain shorts out, forcing him to retreat.

So who were these eight faces? Though some fans have argued that they might actually depict past incarnations of Morbius, the intention of the Doctor Who production team at the time was absolutely that these were pre-Hartnell incarnations of the Doctor. "It is true to say that I attempted to imply that William Hartnell was not the first Doctor," producer Philip Hinchcliffe told Lance Parkin, author of 1996 reference book Doctor Who: A History of the Universe. "We tried to get famous actors for the faces of the Doctor, but because no one would volunteer, we had to use backroom boys."

The eight faces actually belonged to script editor (and Brain of Morbius co-writer) Robert Holmes, Doctor Who's production unit manager George Gallaccio, production assistant Graeme Harper, director Douglas Camfield, production assistant Christopher Baker, writer Robert Banks Stewart, director Christopher Barry, and Hinchcliffe himself.

Later that same year, another story The Deadly Assassin established that Time Lords were limited to 13 incarnations. At the time, this reveal didn't actually rule out the 'Morbius Doctors' being pre-Hartnell versions - perhaps the Tom Baker Doctor was actually the 12th, not the fourth? But retroactively, the regeneration limit does appear to contradict what was implied in The Brain of Morbius – how did the Doctor survive beyond the Peter Davison version if that was actually our hero's 13th and final form?

What's more, in The Three Doctors, a story that predates The Brain of Morbius by four years, the Time Lords refer to William Hartnell's incarnation as "the earliest Doctor". Then there's the alleged First Doctor referring to himself as "the original" in both 1983's The Five Doctors (where he's played by Richard Hurndall, stepping in for the late Hartnell) and 2017's The Doctor Falls (played by David Bradley).

Still, none of this means that Doctor Who in 2020 won't throw caution - and canon - to the wind and reveal that Jo Martin's Doctor comes before Hartnell's, with showrunner and series 12 finale writer Chris Chibnall diving headfirst into territory that Philip Hinchcliffe only dared dip his toe into.

Two series veterans, Chibnall's predecessor Steven Moffat and writer/actor Mark Gatiss, certainly think that he should, with Gatiss recently telling RadioTimes.com that he doesn't "give a flying monkey's" about Doctor Who rewriting its own history.

"For me it's all part of the joy of it, and it's sort of deadly to restrict it," Gatiss said. "I remember watching the Brain of Morbius and just going, 'Uhhh... what?!'."

"I hated the fact that they said the Doctor could only regenerate 12 times," Moffat agreed. "I had subtracted from me all the joy of imagining those other Doctors, by this bloody rule that came in, that for some reason we all decided was true! Despite the fact that there are many contradictions."

It's true – Doctor Who has always changed its own backstory and 'rules', and sometimes even changed them back again, whether it's in regards to the Doctor having two hearts, or being half-human, or the original name of the Dalek race (is it Dals or Kaleds?) or the show's wildly varied portrayal of how time travel and its impact on history actually works.

And if you really want to refer back to the past, then there's plenty in Doctor Who's long history that could explain away the 'problems' caused by Jo Martin's Doctor being confirmed as the real First Doctor – the Doctor was granted a new regeneration cycle in 2013's The Time of the Doctor to get around the limit imposed by The Deadly Assassin, so there's an established way around that, and as for Hartnell's version being "the earliest"... well, the Time Lords have lied about things before (including, apparently, the Timeless Child) and may have done so again.

"It is impossible," Steven Moffat said, back in 2008. "For a show about a dimension-hopping time traveller to have a canon." And he's right – whichever way you look at it, Doctor Who is both primed to and accomplished in changing the past.

Doctor Who continues on BBC One at 7:10pm on Sundays