Genestealers were the first Tyranid species to be encountered by the Imperium. But before the Imperium had knowledge of the Tyranids as a species, they believed Genestealers were a separate species indigenous to the moons of Ymgarl which spread to other planets by infiltrating passing cargo ships and derelict Space Hulks.[1a] It is believed Genestealers were introduced to the moons of Ymgarl in M35 from Hive Fleet Tiamet in the Tiamet System, carried inside the Imperial vessels that were sent to destroy the system.[2a] In 680.M41, the first Genestealer Cult was encountered by the Imperium on Ghosar Quintus.
When the Battle for Macragge with Hive Fleet Behemoth took place, Technomagi spent years classifying the left-behind bodies, including Genestealers used as Tyranid shock troops. Further investigation confirmed that all Genestealers were in fact Tyranids.[1a]
The truth is that infiltrating Genestealers had been working their way into the galaxy for thousands of years prior to the invasion of the Hive Fleets proper.