|"What's the matter? The CIA got you pushing too many pencils?"
An unknown type of worm was the only known indigenous species that inhabited LV-223.
The worms were discovered by Millburn, a biologist who participated in the USCSS Prometheus voyage to search for the origins of humanity and possible extraterrestrial life on the moon LV-223. During the Prometheus crew's expedition within the Engineer Temple, Millburn examined the structure's ground and managed to catch a live specimen of the worms, much to his enthusiasm as they've never discovered any other life larger than bacteria before.
The crew ventured further within the structure and found a large room filled with Steatite Ampules. When the crew entered the room the Ampules started leaking Chemical A0-3959X.91 – 15, that were stored within the Ampules. The worms were eventually exposed to A0-3959X.91 – 15, which resulted in the worms transforming into Hammerpedes.
Behind the scenes
No explanation was given for where the worms came from in the theatrical cut of Prometheus: the camera simply shows them on the ground when the black liquid is released from its vases. This led to some fan confusion over whether it was an indigenous lifeform, or if perhaps it was tracked in by the expedition, i.e. it started out as simple bacteria on their boots, and was already mutating into worms from exposure to the black liquid.
The Blu-ray of Prometheus, however, includes a deleted scene in which the team finds the worms in a different part of the temple, confirming that they were already present on the planet (though whether they are indigenous, or escaped "lab rats" for the Engineers, is unclear). The expedition is excited to find them, because even though they are only worms, they state that no extraterrestrial life forms more advanced than bacteria have ever been discovered. This scene helps explain why Millburn was so excited when he later saw the Hammerpede and immediately approached it without caution: even finding simple alien worms was a historic scientific discovery, and Millburn was awestruck at something as complex as an apparent vertebrate.