The Thals were a spacefaring humanoid species that originated from the planet Skaro. According to the Doctor, they were known throughout history as one of the most peace-loving peoples in the galaxy. (DW: "Planet of the Daleks")
A neutronic war between the Thals and the forefathers of the Daleks took place. Skaro, a great world full of art and ideas and invention, was destroyed in one day. Most of the Thals perished in the war, and the survivors mutated. The Doctor speculated that the mutation "came full circle" and refined itself into their later form.
Thal society changed over the centuries following the neutron war; the once famous warrior race of the Thals became farmers. The survivors managed to cultivate small plots of land, but were reliant on a great rainfall that only happened every four or five years. When the rain was two years overdue, all the crops were ruined. The whole Thal race, under the leadership of Temmosus, left the plateau they inhabited to go in search of food.