system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] What do we know about the evolution of of the creatures of Arrakis-- especially the Shai-Hulud(sandworms)?
gpt
It's truly unknown where the worms came from. We know from geological and ecological surveys by Pardot Kynes and his son, Liet-Kynes, that the worms had to have been introduced to Arrakis from another source. Water ran freely over the planet in the recent past (geologically speaking) until the introduction of the "sandtrout": the first life phase of the sand worm. The sand trout trap water, and once enough existed on Arrakis to take enough water out of the system, the sandtrout merged to create the first sandworms on Arrakis.

The ecological transformation of the planet started by Paul Muad'Dib and completed by his son, Leto II gradually destroyed all of the sandworms, as they cannot survive with too high a water concentration in the air. By now the sandworms have returned to Arrakis, as the sandtrout fused with the legendary God Emperor Leto II restarted the process of trapping water that began when the sandtrout first came to Arrakis.

Most of the animal life on Arrakis adapted from Terran creatures, such as the blood-drinking birds of prey, and muad'dib (the small mice from which the first Atreides Emperor took his Fremen name).