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Monoids were a one-eyed humanoid species who travelled on the Ark with the last of the humans.
Physically, Monoids were about the same height as humans, but their skin was dark and scaly. They all had mop-top hair, although some had lighter hair than others. Their feet were like flippers, which probably contributed to their slow movement. The most striking feature of the Monoids, though, was the single eye in the centre of their heads.
by the 57th Segment of Time they had no visible mouths, relying on sign language for communication, although they apparently ate and drank through a hole in the side of their necks. Instead of names, the Monoids of this era had numbers based on rank.
The Monoids arrived on Earth long before the Ark went into space. Their own planet was dying, and they offered their services in return for a place on the Ark. The humans accepted and welcomed them on the trip. While travelling, the Monoids fulfilled a near-slave role, carrying out menial tasks for the humans.
After several hundred years of travelling, the Ark neared its destination, Refusis II. When the Doctor, Steven and Dodo visited the ship (christened "the Ark" by Dodo), they brought with them a strain of the common cold virus, to which neither the Guardians nor the Monoids had any resistance. The virus spread rapidly and killed several Guardians and Monoids before the Doctor was able to devise a cure. Thinking all was well, the TARDIS crew left - but a mutation of the virus appeared, and apparently was more virulent to the human population than the Monoids. The fever sapped the humans' will, and the Monoids saw their opportunity to seize control of the Ark and end centuries of servitude. The Guardians had encouraged research into voice boxes for the Monoids, and had also developed heat prods which the Monoids could use as weapons. With these advantages, and the fact that the Guardians were unprepared for conflict, the Monoids managed to kill many of them and make prisoners of the rest, using heat prods to keep their captives in order. The Monoids completed the humans' statue by adorning it with a Monoid head instead of the intended human one. In this, they planted a bomb to be detonated when they abandoned the Ark and left the humans behind.
After Two did not complete communications from the surface of Refusis II, the Monoids began a full-scale landing. However, mutiny resulted in the deaths of several Monoids as fighting ensued. The bomb was ejected into space by the humans, saving the Ark. The surviving Monoids returned aboard. The Refusians welcomed both species to their planet if they were to become friends. Both agreed.