Throughout the history of the Doctor has regenerated fourteen times on screen, including one fake-out regeneration during season 4's "The Stolen Earth" and "Journey's End," with the questions surrounding the ability not being answered until Jodie Whittaker's run as the Thirteenth Doctor.
Over the years, regeneration has come to be seen as something of a religious miracle for many Time Lords, with nobody really knowing the true origins of the ancient species' ability. However, several theories have been posed including unusual experiments, natural evolution, and even a viral infection, though nothing had ever actually been confirmed until the Timeless Child reveal in 2020. By finally explaining regeneration, Doctor Who runs the risk of ruining the mystery behind the Time Lords' mystical ability, though many more questions have been asked that Sex Education star Ncuti Gatwa, will have to tackle. Altogether, eight theories had been suggested about the possible origins of the Time Lords' regenerations before "The Timeless Children."
Rassilon's Experiments
One origin story suggests that a young Rassilon, one of the most notable figures in Gallifreyan society, had been investigating regenerating decaying tissue with self-replicating molecules, which would enable the cells of a Gallifreyan, including the brain cells, to repair, restore, and re-organize themselves. This would result in a completely new physical form and different personality while retaining memories from their previous versions. Deeming true immortality a curse, Rassilon put a limit on the amount of times a Gallifreyan could regenerate, and intended this process to be reserved for the Gallifreyan elite, dubbing the lucky few the Time Lords. Sacha Dhawan's sinister spy Master reveals in "The Timeless Children" that the Founders of Gallifrey wanted to hide the truth of regeneration from the Time Lords, instead coming up with a "noble creation myth."
Thremix's Virus
Another myth comes from the Black Scrolls of Rassilon, ancient Time Lord relics that hold forbidden knowledge from the Dark Times, an era on Gallifrey before Rassilon had risen to power and established the Time Lords. The Scrolls detail the works of Thremix, an ancient Gallifreyan biologist, who was said to have created a virus that would infect the general population of Gallifrey, gifting the species an elongated lifespan and the ability to regenerate. Before Thremix could challenge Rassilon's power with his creation, the scientist was killed and the truth about the origins of Time Lord regeneration were buried in the Scrolls, which would end up being reviled among Gallifreyan society.
Triple-Helix DNA
A separate suggests further experiments by Rassilon on the Gallifreyan elite. The Fourth Doctor serial, "The Deadly Assassin," confirmed Rassilon started his career as an engineer and an architect, focusing mostly on biological engineering in an effort to prolong the lives of upper class Gallifreyans, fulfilling his wish of living longer, seeing everything the universe had to offer, and being able to rule Gallifrey as a God-like figure for eons. This origin myth poses that Rassilon developed a third strand of DNA that he added to the Time Lords' physiology. The new triple-helix DNA enabled Time Lords to regenerate into a new form, meaning this myth is less of a religious origin for Time Lords, but one based solely on science.
Breeding Looms
Before the rise of the Time Lords on Gallifrey, the planet was ruled by a matriarchal priesthood known as the Pythia, though as Rassilon rose through the ranks, forces loyal to him began a monumental war with the Pythia. When the Pythia were defeated and banished from Gallifrey, they cursed the Gallifreyan population with sterility, so Rassilon developed the breeding looms to continue to advance the species. Some theorized that the extensive use of the looms gave Gallifreyans the ability to regenerate, and the fact that only the Great Houses had access to looms meant that only the elite would develop the incredible ability. Over generations, the Time Lords would rise to power thanks to their regenerative ability, while the Pythia were all but forgotten, going on to become the Sisterhood of Karn, who would one day trigger the Eighth Doctor's regeneration.
Natural Evolution
Perhaps the most simple of the origin of the regeneration ability would be simply deeming it as a natural ability ingrained in the Gallifreyan population. There were s of Omega regenerating into a muscular figure long before he and the other Founders of Gallifrey had established the Time Lords. This explanation of regeneration would mean that not just the upper echelons of Gallifrey would be able to change their forms, but the entire population of Gallifrey would have the exact same ability. There has been very little evidence of this ability being present in lower-class Gallifreyans throughout the rich history of Doctor Who, and now with the reveal of the Timeless Child, it seems likely that this simple reasoning for the ability has been debunked.
Untempered Schism
The Untempered Schism was featured for the first time in Doctor Who season 3, episode 12, "The Sound of Drums," with the Tenth Doctor describing it as "a gap in the fabric of reality from which can be seen the whole of the vortex." The Doctor has theorized multiple times that exposure to the Untempered Schism and the full force of the time vortex over billions of years could be one of the largest contributing factors to the Time Lords' ability to regenerate since Gallifreyan children who had the prospect of becoming Time Lords would be initiated by staring into the Untempered Schism. Interestingly, this is what contributed to the Master's insanity, as well as indirectly River Song's ability to regenerate, since she was conceived on board the TARDIS while in flight in the time vortex.
Eye Of Harmony
Similar to exposure to the Untempered Schism, one theory suggests that Rassilon's engineering of the Eye of Harmony caused him to become the first Gallifreyan with the ability to regenerate before it was then disseminated through the population. Rassilon suspended time around an exploding star in order to tap into the energy it created, using this energy to harness the power of time travel. While working on the Eye, the elemental forces caused the death of his original body, but granted him a set of 12 regenerations. When he rose to power, he allowed a select few of the Gallifreyan upper-class to gaze upon the Eye of Harmony and gain this ability too. Though the Eye had been thought to be lost at the end of the Last Great Time War, it was revealed during the 1996 TV movie and the Eleventh Doctor story, "Journey to the Centre of the TARDIS," that the Eye was in fact held within the core of the Doctor's TARDIS.
Yssgaroth Injections
While investigating a race of vampires known as the Mal'akh, 19th-century adventurer Richard Francis Burton noticed similarities in their healing abilities and the regenerative abilities of the Time Lords. The Mal'akh were descended from the Yssgaroth, great vampires that had started a war with Gallifrey during the Dark Times. Burton theorized that early Gallifreyans had obtained their regenerative abilities by purposefully injecting themselves with the essence of the Yssgaroth, enabling their bodies to rapidly heal, with the complete change of their forms being an extension of this. In the 2021 short story, Out of the Box, it was noted that the vampires were motivated by their constant search for the "Child-That-Was-Taken", perhaps pointing to their knowledge of the existence of the Timeless Child.
The Timeless Child
Though many theories had been posed throughout Doctor Who's history on the origin of regeneration, a direct answer was finally given during season 12's climactic episode, "The Timeless Children." The story goes that a Shobogan (the indigenous race of Gallifrey), named Tecteun, found a lost child and adopted them. The child displayed an ability to regenerate an indefinite number of times, surprising Tecteun since regeneration had not been introduced to Gallifrey before then. After experimenting on the child, Tecteun isolated the regeneration ability, tested it on themselves, and then gave the power to other Gallifreyans within the Citadel, founding the first true generation of Time Lords. There is still some debate over whether the limit of 12 regenerations for Time Lords was discovered in the genetic code of the Timeless Child, or if the limit was created by Tecteun.
The Doctor was revealed to be the Timeless Child, with the Time Lords closely watching over them throughout their life, wiping their memory after each set of 12 regenerations to uphold their belief that they were just a normal Time Lord. This reveal not only makes the Doctor seem like some sort of Godly figure in Gallifreyan history, but also means that the Doctor themselves was in fact one of the Founders of Time Lord society, albeit unbeknownst to themselves. The legend of the Timeless Child survived for billions of years, though many disregarded the evidence, with people instead being prone to believing Rassilon to be the engineer behind regeneration. Since the truth is now out there, future seasons of Doctor Who will surely deal with the fallout of this storyline.