The Tau have made their clock fast indeed.

But those who abandon adaptation for tradition, those who sacrifice sober judgement for faith, those who only make decisions if they are forced into them by circumstances outside their minds,

break their clock in half.

For all it's power, for all it's reach and importance and for all the billions of sharp minds behind it, there's a sense in which Imperium of Man isn't a faction, but a piece of scenery. 

...

Earth is ancient. For a third of the Universe's existence, it stood firm.

When the Galaxy was aflame with war, it haven't been entirely unused or untouched. But it survived the devastation without much overt change.

Man was created, and rose from the dust-fringed edges of the cooling mud. To love, to conquer, to kill - AND HE WILL.

When a race with a mind structure of significant Warp sensetivity reaches a level of sentience and sophistication high enough to develop proper psykers, the race encounters a deadly test. Because if a psyker was to stare for too long at the Abyss, the Abyss will stare back. A single minor daemon summoned and not stopped in time can and will corrupt people into the service of Chaos, who will then summon more daemons through rituals no longer requiring a psyker; the process will repeat until the new race falls, and if it halts partway by inordinate amounts of resistance, the already empowered corrupted faction has the option of summoning a Greater Daemon, each far more dangerous than most monoplanetary civilizations to ever exist.

In the ancient times far before Slaanesh or indeed before Tzeentch, summoning demons fas far less of a trivial task, as they were both less numerous and less powerful. But worlds can fall to more subtle influences of Chaos than that. Cults devoted to entirely unrelated things twist over time into the worship of the Ruinous Powers. Metaphors of inspired poetry and whispers of dreams slowly, slowly bend cultures towards ruin. With time, water cuts the stone.

Some worlds are lucky enough to contain Warp-suppressing Necron pylons or other artifacts of similiar function. Other lucky worlds are located in areas of space where the boundary between Warp and realspace is unusually thick. Some young species have been forewarned by their more knowledgable neighbours, often while being conquered by them.

But the unlucky worlds with normal conditions usually only have a shot at long-term independence given strong traditions of both witch-hunting and cultural stagnation, which have to begin and become universal before the true dangers of witchcraft and cultural drift become apparent.

But there is time before one of such unfortunate changes happen.

That is the time in which free users of the powers of Warp may walk peacefully among their people and practice their craft.

Education isn't yet even invented.

There aren't any holy texts, there aren't many texts at all.

Shamanism is sort of an oral tradition.

Sort of. There are methods of communication available to shamans that are not available to humankind at large. The shamans find each other in dreams and visions.

And it isn't the only thing that they see in their visions.

Vast, terrifying powers on the horizons of perception.

A great horror of violence and a great horror of decay, looming over the future of their race.

Beckoning.

Aah! 

Well, what can a man do, before gods?

A man who knows nothing of the workings of the cosmos, unaware even of the shape of their world?

Cower in fear and wait for an opportunity, that's all.

Isn't that just so.

But then, someone stumbles upon an interesting ruin.

A very interesting ruin, and a very ancient one.

And within it they find weird things.

And tell about them to their shaman - one of the 30 or so genuine shamans in the world.

It's certainty unearthly, and ungodly too.

It's full of things that defy the reason of a tribesman.

And yet it also has some... familiar? Intuitive things within it.

And the intuitive thing is a powerful thing, too.

It is an opportunity.

And if a different sort of a person would have found it, the history of Earth would have had a very different path.

But the first shaman studying it had no interest in anything like world conquest.

Instead,

they were terrified.

And how terrifying was the fact that the ruin with such a wonder was empty of people!

So, one night, the terrified shaman have called the other shamans of the world to convene.

And this was before the planes, before the roads, and even before the widespread knowledge of points of sail that let boats travel against the wind.

But the caller have lent the shamans the knowledge of instantaneous travel, something far, far beyond the means of any psyker of the time to discover on their own.

And the 28 geniuine shamans living at the time, all without exception, came to the summoner.

And for a while, they talked, or rather convened in ways the shamans can, for they had no one language between them to talk.

They communicated. And it was no polite conversation, no small talk.

But they came to a decision, and they joined their hands together, and they changed the course of history, and they died. 

And from that point, there were no more genuine shamans.

But there's something else instead.

An unusual man is born, and power courses through his veins.

He is by loving parents, if not unusually so for the standards of the time.

Also not unusually for the standards of his time, his uncle murders his father to take his property, and there is no force that would stop or avenge him. The boy is furious and distraught, and the murderer dies of a heart attack.

Later, he who will call himself the Emperor of Mankind, would say that this event first sent him onto the path of seeking to establish law; but that is a simplification that skips so many thousands of years and so many details that it might as well be a lie. The kid doesn't know what law is, he is illiterate and barely understands tradition; for now, he doesn't quite understand what he's done and why and if he in fact did that.