Original palimpsest universe

According to some accounts, before the invention of time travel made it the original palimpsest universe, the universe existed as a single linear timeline later remembered as a golden age. (PROSE: The Infinity Doctors, Mr Saldaamir) Nevertheless, the Third Doctor once declared "there never was a golden age" of history, insisting that it was merely nostalgia blinding people to assume such past ages were perfect. (TV: Invasion of the Dinosaurs)

A Faction Paradox member speculated that in the original version of history, the Doctor came from a planet in the 49th century, fleeing the Enemy who had overrun his home. (PROSE: Unnatural History)

Origins
Although accounts dealing directly with the "golden age" seemed to accept the premise that it was the earliest state of reality, (PROSE: Mr Saldaamir, The Infinity Doctors) other, linked accounts showed that in the Dark Times the universe was originally carnivalesque and irrational, with no fixed laws of physics. Rassilon and the other Founders of Gallifrey tore down this version of reality and compressed it into the perfect, linear structure of history, a concept they had invented, using the anchoring of the thread. Everything which did not fit in this view of reality was instantly and retroactively erased, (PROSE: Christmas on a Rational Planet, The Book of the War, etc.) except for a few survivors hidden in alter-time states, such as Cernunnos. (PROSE: Cobweb and Ivory)

Golden age
For the first several billion years of linear existence, time existed as an elegant parabola, a single harmonious note, a continual forwards progression. For the many immortal elder races living in this, (PROSE: Mr Saldaamir) including the Gallifreyans, (PROSE: The Infinity Doctors) the singular future was thus easily predicted due to it inherently following from the past.

As a whole, the universe was elegant and beautiful, with everything happening for a reason. It was a time of heroism, exploration, and discovery, (PROSE: Mr Saldaamir) with Gallifrey having a fruitful Time of Empire. (PROSE: The Infinity Doctors)

Destruction
While time travel had long been known to be theoretically possible, (PROSE: Mr Saldaamir) it was only in the age of Rassilon, Omega, and the other that it was realized. The Gallifreyan Empire had swiftly fallen at this time, leaving behind an infertility curse which caused the Triumvirate to invent looms, creating a generational gap between the Elders of Gallifrey who remembered the golden age and the Loomborn who longed to create a new future.

The Triumvirate led an expedition to Qqaba, (PROSE: The Infinity Doctors) the final star in the universe capable of being collapsed into the very specific type of supermassive black hole capable of powering time machines. (PROSE: The Infinity Doctors, Mr Saldaamir) Doing just this, they were doused in Time Energy as the star exploded. At the centre of this, Omega briefly saw the universe's singular harmonious timeline, but then as the star became a singularity space and time were shattered, with a network of cracks appearing across them. (PROSE: The Infinity Doctors) With a time machine power source harnessed, the timeline "a squiggle, with branches off and crossing out and gaps and bits which repeated each other, or cancelled each other out. Suddenly nothing made sense, except on a very, very local level." (PROSE: Mr Saldaamir)

Thus, the universe became a subjective mess of paradox and contradiction, time became relative, (PROSE: The Infinity Doctors) an ocean of possibilities only solidified via the Observer Effect. (PROSE: The Gallifrey Chronicles) From his vantage point of the anti-matter universe, Omega saw the original timeline overwritten and replaced with an infinity of palimpsest pasts and futures. (PROSE: The Infinity Doctors)

This had been predicted in the Book of Prophecy, (PROSE: The Infinity Doctors) but these and other predictions were made with a linear view of time which could not fully understand the consequences. In the six days following the fracturing, the elder races tended to either go mad, leave reality, get killed, and/or regress, (PROSE: Mr Saldaamir) leading to a generation in which the Time Wars were fought. (PROSE: The Infinity Doctors, Mr Saldaamir)

Legacy
As with all palimpsest universes, traces of this version of history remained in the patchwork of time, contributing to its contradictoriness. (PROSE: The Infinity Doctors) The average person had a vague memory of the original golden age, a faint idea of a time when things were simple. (PROSE: Mr Saldaamir) The eternal return which structured Gallifrey's stagnating society meant that each Time Lord generation had aspirations towards a golden age subconsciously approximating the spirit from Gallifrey's ancient past. (PROSE: The Infinity Doctors) Nevertheless, the Doctor came to the conclusion that the idea of there once being a "golden age" was an "illusion". As argued by the Third Doctor, every era of history had its issues, so to declare one a "golden age" was false. (TV: Invasion of the Dinosaurs)

Traumatised by the golden age's end, many of the elder races persisted into future versions of time in more hostile forms, including an ancient evil which drained planets of life and a "chap dressed as a Chinese mandarin" obsessed with toys and games. Mr Saldaamir remained the one true survivor of the original timeline, outliving the temporal erasure of everything he once knew and living immortality through history without any time travel. Above all, he sought to protect the existence of the timeline and prevent its further fracturing. (PROSE: Mr Saldaamir)

Behind the scenes
In the original Doctor Who pilot episode, Susan Foreman said she was born in the 49th century. This line was not carried over into An Unearthly Child, as it was decided that the Doctor and Susan's home would not be attributed to a specified time. This was referenced in Unnatural History and Escape Velocity, as well as Deadline.