Pre-Cambrian

The pre-Cambrian was an era of Earth history.

Lacuna felt a fear of the dark in Lacuna's cave, an instinct inherited from her ancestors who swam in pre-Cambrian oceans. (PROSE: Deceit)

The Seventh Doctor had a thermos of coffee he described as "Vintage pre-Cambrian." It was kept for emergencies in the TARDIS, albeit from an alternate timeline. (PROSE: Blood Heat)

Behind the scenes
Peter Darvill-Evans joked his went to university during the pre-Cambrian Era in his "A History of Errors and Falsifications" essay at the end of PROSE: Asylum.