Men of old, men of today
In the rich mythos of ancient legends surrounding humanity can be found many references to Men of Stone, Men of Iron and Men of Gold. Exactly who or what these 'men' were remains the subject of great controversy in learned circles.
Men of Stone/Stone Men: The 'Men of Stone' is a reference to the hardy breed of 'mundane' humans that set out from Old Earth to settle in colonies around distant stars before the advent of the Warp Drive. Supported by extensive surveys of their target systems and preceded by robotic fleets that would start the laborious terraforming and housing process, these brave souls would set across the void in great ark-ships, taking decades or centuries to reach their targets. Armed with the STC system these colonists would develop their new homes in total isolation, secure in the knowledge that whatever their new home could throw at them, they would prevail.
Men of Iron/Iron Men: The 'Men of Iron' is a reference to the artificial life (whatever shape it might have taken take) created by humanity to aid them in their conquest of the galaxy. The Men of Iron were instrumental in establishing and maintaining humanity's first galactic empire, with Iron Men eventually replaced human labor in most areas. Indeed they became (together with the Warp Drive, Navigator Gene, and STC system) one of the key components that allowed humanity to spread across the stars with unprecedented speed, colonizing millions upon millions of worlds. This expansion – the Great Diaspora – led to the Golden Age of Humanity, a time when the teeming trillions of men wanted for nothing and believed none would ever challenge their rule.
Men of Gold/Golden Men: The 'Men of Gold' is a reference to the strain of human mutants now collectively known as 'psykers'. While psykers and psychic powers had existed for quite some time and was scientifically proven to exist, the sudden emergence of masses of potent psykers was something altogether new. While some sort of experimentation or conspiracy may have been involved, it is far more likely that the staggering total human population made it far more likely for mutation to occur naturally. Great Warp-storms made travel and communication all but impossible, effectively isolating humanity's colonies and putting an end to the first empire. And then came the daemons...
Oddly enough there are myths placing the Golden Men in Terra's ancient past, having them coincide with the Stone Men, or even predate them. Those that subscribe to the ridiculous (and heretical) line of though think that great psykers might have lived within humanity for quite some time, guiding and nurturing the race. Some even place the Holy Emperor as one such Man of Gold (needless to say few outside the Inquisition can freely hold to such ideas). Of course, those same persons are frequently also firm believers of any other wild tale – the humans and eldar are genetically related, that he race of man was created by xenos through the manipulation of apes, that the Emperor passed away long time ago, that the cosmos is a big golden pancake. That sort of thing.
Once again the true lesson is this; the 'truth' (whatever that means, as if truth was something objective) can never be known, and even if it could it doesn't matter. Avoid wasting time on useless philosophy when the Emperor, the Imperium and Mankind has need of your dutiful service! Waste not your life in doubt when the real truth has already been given to you – you are a man, and it is better to die serving the Emperor than to live for yourself.