The Timeless Child has put the "who?" back in Doctor Who. After decades' worth of stories gradually revealing more and more about the Time Lord's history, fans have a pretty clear idea of whom the Doctor had been before starting their legendary travels through time and space. However, Season 12's "The Timeless Children" threw the character's origins back into mystery. For years, the Doctor had been a rogue Time Lord, who eventually became the last of the Time Lords. "The Timeless Children" dropped the shocking revelation that the Doctor was in fact the Timeless Child and the first of the Time Lords, whose true origins were unknown.
Source：https://www.cbr.com/doctor-who-river-song-timeless-child-origin/
