Originally, the Shift was a Gabrielidean soldier in the Nth Platoon, sent by the Time Lords to attack a small automated enemy outpost on Simia KK98. Despite being protected by bulletproof armor, he was shot by an enemy combat satellite while on a recon sweep; the rest of his platoon abandoned him as dead before also being killed by the satellites. After this, the dying soldier was visited by a future incarnation of the Doctor, who was spectating the battle; though the Doctor couldn't help him, he told him stories and tried to give him hope. However, the soldier lapsed into unconsciousness, and the Doctor left.
Enemy agents, dressed in flowing robes with high collars that parodied the costumes of the High Council, rescued the soldier, removed his memories of being a Gabrielidean, turned him into a Shift, and sent him to Qixotl's auction for the Relic as their representative.
The Eighth Doctor was able to trap the Shift in his mind and trick the Celestis into marking it as their agent when they attempted to mark him. As a result, not only did the Doctor obtain the Relic by tricking the other attendees into thinking that the Relic was now a temporal paradox (as the Celestis had marked him now, rather than immediately before his death), but the Shift was driven mad as it was torn between its new status as an agent of the Celestis and its original role as an agent of the Enemy. (PROSE: Alien Bodies)
Because of this madness, the Shift was put in a zoo of ideas on Mictlan. It escaped the zoo by making one of the Celestis' servants think about it and pass it on to others as an idea, but it only escaped Mictlan by boarding one of the Great Houses' timeships while the realm was being attacked by the Memeovore. Together, the Mictlan refugees and the fugitive timeships found a safe refuge, either in the very early universe or another universe entirely. (PROSE: The Book of the War)