The Marshmen are a species of amphibianoid humanoid creatures native to the swampy world of Alzarius. Due to the highly malleable biosphere of the planet, the Marshmen can be generated from Alzarian Spiders which undergo a type of extremely fast evolution or mutation. Through the same process the Marshmen have given rise to the sapient Alzarians -- although the Alzarians are unaware of this, believing themselves to be descendants of a group of Terradonian who once crash-landed on the planet. When examining them under the microscope, the Doctor noticed that the cells of each Alzarian species looked exactly identical.
The Marshmen usually dwell in the swamps, only rising to the higher lands during Mistfall, which happens every fifty years or so. They are upright bipeds with light-brown leathery skin, five-fingered hands and fish-like facial features. Although possibly not fully sapient, these creatures are intelligent and extremely fast learners. Their bodies also have the ability (shared by other Alzarian species) to undergo rapid mutation in order to adapt to new environmental circumstances, such as developing the ability to breathe the poisonous gases released during Mistfall. Pure and concentrated oxygen, however, seems to be highly unpleasant for the Marshmen and hampers their adaptive abilities. It is thus a very efficient way to draw them out.
The local Alzarians greatly fear the Marshmen, unaware that they are their ancestors. Intriguingly, the Marshmen themselves appear to be as frightened by their own ancestors; the Alzarian Spiders; as the Alzarians are by them.
- Doctor Who season 18 - Full Circle (1980)