Mauvril was a Tractite.
When the Earth Empire purged Tractis after a rebellion, Mauvril was one of the survivors. She escaped and found a Time Tree. She used it to travel back to Earth's prehistory to prevent humans from ever evolving.
The alternate timeline created was unstable, drawing the Eighth Doctor's attention. He tried to talk Mauvril and the other Tractites into letting him take them to another planet to start anew. However, fighting broke out and Jo Grant destroyed the Tractites and their Paratractite settlement with a laser cannon. (PROSE: Genocide)