The Alzarians were nearly identical to humans, though they could heal extremely quickly. (TV: Full Circle, The Visitation; AUDIO: Iterations of I) Their hearts were in a different location than in a human. (AUDIO: The Darkening Eye)
The Alzarians lived in a simple society with three Deciders as leaders, with one of these taking on the role of head decider. There was also the rank of Elite for those who were considered intelligent. Romana II classified it as a "type D oligarchy".
They used the System Files to repair the ship, but the files relating to take off were damaged when the ship crashed. Therefore they worked meaninglessly under false pretences they would one day leave for Terrandon. The Deciders allowed their people believe they were to leave one day as they thought it was better to have some hope.
A number of Alzarians, calling themselves Outlers, recognised the futility of working on the starliner and left to pursue a rugged life where they operated from a cave and stole riverfruit for sustenance. (TV: Full Circle)
The Alzarians developed a knowledge of technology through the Terradonian System files. They were capable of repairing the Starliner to the point where they could take apart and rebuild it. They were however, unable to pilot it. Only through the influence of the Fourth Doctor did they come to have greater technological understanding. (TV: Full Circle)
A ship from Terradon crashed on Alzarius, a planet of hyper-mutating creatures. The Terradonians were killed and replaced by the humanoid Alzarians, the Marshmen, who were unable to make the Starliner take off.
The evolving society lived in fear of Mistfall, a period every 50 years or so, when the influence of another planet took Alzarius away from the sun's warmth. This cooling process acted as a "trigger" to the marsh creatures to emerge onto the land. During this period the people, ignorant of their true heritage, shut themselves away in the starliner.
The Alzarians believed they were Terradonians, and that they had been stranded there for forty generations, but were gradually mending their ship. In actual fact, the ship was completely operational and successions of advanced Alzarians had lived around the Starliner for 4000 generations, approximately 140,000 years. Eventually, with the help of the Fourth Doctor, the Alzarians left Alzarius to colonise a new world, which they called New Alzarius. (TV: Full Circle, AUDIO: Mistfall)