the Timeless Child, a cosmic being who predates Time Lord society and who became the base genetic code for the entire Time Lord race.
This was revealed in the Doctor Who season 12 finale, "The Timeless Children," written by Chris Chibnall himself. It apparently builds upon an idea Chibnall first came up with back in the '80s, when he was a teenager glued to the TV screen watching the dying days of the original series. As such, it's hardly a surprise the retcon has been heavily criticized for above the 2005 relaunch. It doesn't help that the episode wasn't particularly well-written, employing a "tell-not-show" approach as the Master literally gave the Doctor a lecture inside the Matrix. The retcon has naturally proved incredibly divisive.
In all the heat and fury over the Timeless Child debate, however, fans have mostly forgotten the Doctor's established history was itself a clumsy retcon. When Doctor Who launched in 1963, the show's creators had only the vaguest idea what the Doctor's backstory was; William Hartnell's First Doctor gave a brief explanation in the first episode "An Unearthly Child," and it doesn't exactly gel well with later ideas. "I tolerate this century," the Doctor furiously insisted when speaking to schoolteachers Ian and Barbara, "but I don't enjoy it. Have you ever thought what it's like to be wanderers in the fourth dimension? Have you? To be exiles? Susan and I are cut off from our own planet, without friends or protection. But one day we shall get back. Yes, one day. One day." These first stories portrayed the Doctor as an inventor who had created the TARDIS, a device that had actually been named by his granddaughter Susan. They were presented as explorers who had gotten cut off from their own people, and were desperately attempting to figure out how to get back. The last words in this diatribe - "Yes, one day. One day." - were spoken with a quiet intensity, with Hartnell clearly believing this was key to the Doctor's entire character.
The backstory began to change when the Doctor and his friends encountered the Meddling Monk, another member of the Doctor's race, who possessed a TARDIS of his own. But the Time Lords themselves wouldn't make their appearance until 1969's "The War Games," the final story of the Patrick Troughton era. Troughton's Doctor finally encountered a problem too great for him to handle on his own - a crisis encouraged by another Renegade Time Lord - and he was forced to summon his own people. The Doctor's companions learned he came from a race who had sworn not to interfere in the affairs of other beings, but the Doctor had rebelled against this. He insisted some corners of the universe had bred the most terrible things, evils that must be opposed. This redefined the relationship between the Doctor and the Time Lords, laying the foundation for everything that would follow over the next 50 years.
This retcon may have been widely accepted, but on critical examination it is clearly a very clumsy retcon. It blatantly contradicts everything the Doctor said about his origin in "An Unearthly Child," to the extent he and Susan can only have been lying to Ian and Barbara. Rather than a wanderer trying to get home, the Doctor became a runaway. Rather than the TARDIS' inventor, traveling with the girl who had named the ship, the Doctor was a thief who had stolen a battered old Type 40 TARDIS. Amusingly enough, then, the sheer clumsiness of the Timeless Child retcon - another revision to the Doctor's origin - is oddly in keeping with Doctor Who tradition.