The Maze of Memory hints that the dungeons were populated by abducted life from others worlds, including various periods in human history. Indeed, the player characters (called prisoners) were snatched from their lives and stripped of memory, recovering them only through great effort. The necessary mystery is best served by leaving these details to the game master (or jailer)...
Even so, the rules provide multiple creatures (and races) of probably extraterrestrial origin, and there's more arriving every day, including the enigmatic BONE CHILDREN:
#AK: 1/weaponry AR: 8+ (hide) DE: by weapon ME: armor worn SL: 1d8+2
The BONE CHILDREN are archaic humans, probably neanderthals, abducted individually or en masse, perhaps in some earlier episode per the jailer. They have long-since adjusted to their unusual environment which in many ways was not that different, establishing tribal colonies in vast natural caverns. They wear crude hides, fighting with primitive stone weapons and possessing great physical strength (assume 1-3 ranks of the strong memory).
Interestingly, and perhaps resulting from their more primitive brains, bone children are not subject to the mysterious translator, speaking a native tongue little changed over time. Prisoners attempting to negotiate must resort to gestures and other creative strategies, noting that as a race they are shy, shunning the ghul and fearing the ithlak and ugliniak as terrifying spirits.
Once again, the desired level of mystery is best maintained by leaving these questions to the jailer, who is free to interpret the presence of ravenous phogs and deadly yorls however they see fit, because ultimately, in only matters that they're THERE, particularly in a one-off session. But for ongoing campaigns, the individual jailer has a free hand to answer these and other pressing questions!