George Williamson was a 19th century geologist.
Fitz Kreiner and Anji Kapoor met him at the Crystal Palace. Tired of being the Doctor's sidekick, Fitz decided to join Williamson and Paul Anderton's expedition to Siberia. This was not a surprise to the Doctor, as he had purchased Fitz's partial journal of the expedition in 1938, while he was living on Earth after he had destroyed Gallifrey. (PROSE: The Ancestor Cell, PROSE: History 101, PROSE: Camera Obscura)
In St Petersburg in 1894, the expedition members found out that Anderton had come down with appendicitis and had been replaced by Hanson Galloway. Later, while camping near an abandoned castle, Williamson killed Galloway in self-defence, but Fitz was suspected instead as he and Galloway had been fighting earlier.
After burying Galloway, the expedition found an opening to an alternate timeline. Williamson threw a stone into it, which broke a barrier separating the realities. Dinosaurs passed through the opening and attacked the expedition, killing one of the members.
The survivors went to the castle, with the dinosaurs in pursuit. The other members were killed, leaving only Fitz and Williamson alive. Finding a secret passage, they went beneath the castle. There, they found a cave with an ice replica of the TARDIS and what looked like fire inside the ice. When the dinosaurs finally caught up, Fitz threw a grenade he had, pushed Williamson out of the way, and ran toward the ice TARDIS in hopes it would shelter him. The explosion was greater than expected, heard all the way to Moscow.
In the early 20th century, the Doctor was taken to the Naryshkin Institute, which was housed in the castle. Thanks to his research, he found the ice cave and excavated Williamson's body from the ice. However, he was alive, but insubstantial and left an impression in the ice, like the TARDIS.
The Doctor figured out that the light was from an o-region. The explosion released some of the light, and for Williamson, time and space were switched. By simply walking, he could travel forward and back in time. One of those trips was back to the time of the dinosaurs, where one of them observing him resulted in the creation of the alternate timeline.
Maxwell Curtis, slowly turning into a black hole, travelled along the time envelope created by Williamson's travels. Sabbath extended it to before Event One, convincing Curtis that the release of the energy within him before the universe's creation would be safe. However, the Doctor realised it would have catastrophic consequences. He convinced Williamson to travel back to 1894 and cause his death definitively. He did so, and his ghostly image startled his younger self so he did not seek shelter from the explosion. Williamson's death meant he did not travel in time, create the time envelope or the alternate reality. Curtis, already in the envelope, was killed as it collapsed, the energy within him safely distributed across time. (PROSE: Time Zero)