The First Propagator was the leader of the Archaeons.
Circa 450 million BC, it greeted the First Doctor, Susan and Stoyn when they arrived in one of the Archaeons' vivariums beneath the surface of the Moon. It communicated to them by forming a reflection of Stoyn on its glassy surface and mimicking Stoyn's voice. It explained to the Doctor that the Archaeons were seeding the planetary body above them with lightning, which the Doctor determined meant that the Archaeons were nurturing the life that would evolve there in a controlled environment.
After the Archaeons accidentally created a stasis field around them and the three Time Lords, they were frozen in time until millions of years later, when the humans evolved, named the planet "the Earth", and colonised the Moon. A human expedition broke through the field, releasing everyone from being frozen in time.
The First Propagator considered their experiments in seeding the world a failure, because their "garden" had no formal beauty or order to it and the humans had "run rampant". The Archaeons blamed the Doctor for this, and forced the lunar rover that had found him to return to their cavern under the surface of the Moon. The cannon was set to "purge" the human life on the Earth and the Moon, but the humans struck back with missiles. During this attack, the Doctor and Susan escaped. The missile strike breached the atmosphere of the Archaeons' cavern, destroying their weaponry; the First Propagator was pulled outside. (AUDIO: The Beginning)