The latest episode of Doctor Who, "Arachnids in the U.K.", dropped a tantalizing hint that there have been previous female incarnations of the Doctor. If true, this revelation would completely rewrite the Doctor's history, and potentially even suggest that the Doctor had another (never-before-seen) regeneration cycle.
Doctor Who has always enjoyed hinting that viewers have only seen a fraction of the Doctor's adventures. Pretty much every Doctor has delighted in name-dropping encounters with everyone from Amelia Earheart to Julius Caesar, yet just a small number of those historical adventures have been shown. But it goes bigger than that. When Russell T. Davies relaunched the series in 2005, he revealed that the universe had been caught up in a Time War while the show was off the air, one that had only recently come to an end. The series eventually revealed that the War Doctor was an entirely new regeneration of the rogue Time Lord.
But a single line of dialogue in "Arachnids in the U.K." hints that there have been other, previously unseen incarnations of the Doctor - including a female one. In one scene, Jodie Whittaker's 13th Doctor is desperately attempting to make small talk, something she's clearly not very good at. She drifts from talking about her own siblings to remembering a time when she was "a sister in an aqua hospital" that turned out to be a training camp for space assassins. A "sister," of course, is a female title for a nurse; this may well imply that there's been a previous female Doctor.
This follows on from a line of dialogue in last season. The episode "World Enough and Time" included a scene where Peter Capaldi's 12th Doctor remembered back to his days at the Academy, when he'd been friends with the Master. "I think she was a man back then," he observed. "I'm fairly sure that I was, too. It was a long time ago, though." Bill responded by asking if Time Lords are a "bit flexible on the whole man-woman thing," a question that prompted the Doctor to complain about humanity's petty obsession with gender. The suggestion is that there was a female Doctor in the past.
However, this risks causing a continuity problem. Time Lords can only regenerate 12 times; although they can be given a fresh regeneration cycle, this is rare. The Doctor apparently reached the end of his natural regeneration cycle in "The Time of the Doctor," with Matt Smith believing he was finally about to die. Capaldi was the beginning of a whole new cycle, courtesy of intervention from the Time Lords. So there doesn't appear to be any room in the Doctor's history for further previous incarnations, let alone female ones.
Unless, of course, the Time Lords had extended the Doctor's regeneration cycle once before. If that's the case, then William Hartnell wasn't the first Doctor after all. Curiously enough, there is one hint in the classic series of Doctor Who that supports this. In the story "The Brain of Morbius," the Doctor became locked in a psychic duel with another Time Lord. Memories of previous Doctors were displayed, with Morbius declaring, "How far, Doctor? How long have you lived? Back, back to your beginning…" This was during the Tom Baker era, and three familiar faces were shown on a screen; Jon Pertwee, Patrick Troughton, and then William Hartnell. But the images continued, displaying eight more faces, apparently in period dress, before the machine exploded. Producer Philip Hinchcliffe told the Radio Times that he definitely intended to suggest there had been other versions of the Doctor before Hartnell. "I just reasoned that it was entirely possible that William Hartnell may not have been the first Doctor Who," he explained. "So yes, as far as [writer] Bob [Holmes] and I were concerned, the other faces were meant to be past Doctors… it is true to say that I attempted to imply that William Hartnell was not the first Doctor."
If there were indeed eight previous Doctors - then there must have been 13, an entire regeneration cycle that has never been shown. And there's no reason some of those incarnations couldn't have been female. This, of course, comes from an older era of Doctor Who, so may not be viewed as strict canon. Although, as season 11 is delving into a Timeless Child mystery, anything is possible.
Doctor Who continues on BBC America on Sunday.