According to Wormwood, Quarlangians are a proto race of early sapient humanoid life on earth that he was ordered to wipe out by the Annunaki for an unknown reason (he states they never asked this sort of thing from him since). His understanding of them describes them as being bulky, unusually intelligent, aquatic, and possessed of a native ability to use quantum mechanics on the common level. They were a highly advanced series of races (Pelimar, Ilsigor, and Karatoa tribes) who lived alongside primitive ancestors of modern man. Almost all traces of them have vanished since Wormwood wiped them from the face of the Earth by laying waste to the seas and oceans.
However, he alleges he’s heard word some might still exist in some capacity. ‘Timeghost’ DNA that some UnMen possess appears to be that of the Quarlangians themselves. This in itself is incredible, since Borland foraged the DNA unintentionally from battleground soil during WW1, implying that Quarlangian DNA does not degrade like common DNA does. If Timeghost UnMen have accurate DNA from these beings, this would also show Quarlangians had gills on their neck area.
The world of Pelangia may have some curious relation to the Quarlangians, if by the knowledge that the style of the words is extremely similar on an identifying level.
The only known still existing full blooded Quarlangian is The Dead Mariner, aka Eristes.
The Demiurge was the direct creator of the Quarlangian race (it is unclear if he means one specific tribe or the idea of them). He makes a mistake by allowing their Determination to unknowingly cross themselves over from Particle World to the new universe without any ability to return. This is why no one else knows where the Quarlangians came from.
After the Entity’s defeat, Shard becomes a First-of-the-First and creates a new universe where the Quarlangians can be restored, Sanctuary.