The most important ability of the Marshmen was their ability to adapt quickly to new circumstances. These changes could be from a mental point of view or a biological point of view. They could develop new plans and learn quickly, usually by copying other people.
The biological changes were much slower, which meant they could become a new species in thousands of years. Because of those quick changes, they were closely related to the Alzarian spider (their ancestors) and the Alzarians (their descendants). Despite how close they were genetically, the Marshmen still feared the Alzarian spider and hated the Alzarians.
The Marshmen were responsible for an attack upon the original crew of a Terradonian Starliner that had landed on Alzarius. They quickly adapted into the form best suited to operate the Starliner, the form of the original crew. Over time they learnt to read and repair the ship, these events were documented in the System files.
Some generations later, another group of Marshmen arose from the swamp as Mistfall returned. They attacked the Starliner containing the original Marshmen (unknown to them they were their decendants).
They quickly over ran the whole ship, and killed members of the crew. They were forced out when the ship was flooded with rich oxygen that inhibited their ability to adapt. The original crew of Marshmen then left Alzarius in the Starliner.