Warhammer TT rules (3rd Ed.) contains the 'Journal of Keeper Cripias' which talks cryptically about the early history of the emperor and the Imperium.
In it he refers to 'Gold Men' (seemingly Humanity prior to the Dark Age of Technology), 'Stone Men' (those who lived through the DAoT), and 'Iron Men' (who apparently served/rebelled against the sone men).
Any thoughts on what these might be and how they relate to the current Imperium?