Of course, everyone knows how the humans were discovered. A quiet little backwater once lay in the middle of the Iovar Confederation, in a region that fickle race had only just got up the patience to explore. The humans had a nice little empire there, and a few little vassal races to call their own. They thought they were the strongest, and the Iovar proved them wrong.
The subjugation of humankind took three decades. Three decades of hell. The humans fought bravely, but pointlessly. They were hopelessly outmatched, and completely without hope. The remainder of the Species Union watched as the Iovar enslaved them, world by world. The Confederation's actions were denounced, and sanctions leveled, but no-one stopped them. It would have taken a war to change their minds, and why would we risk that, for a mere hundred billion in a place nobody ever heard of?
For the next few centuries, the humans remained in slavery. They were physically unimpressive, with poor reflexes, poor strength, poor durability, just about every disadvantage it was possible to give a race. But they were smart, charismatic, naturally clever. The perfect diplomats. Within fifty years, a third of all Iovar merchants had a human liaison. It took ten more years for that figure to rise to 90%. The humans had taken to their role marvelously well. It took very little to keep them in line, and it came to be accepted by the Iovar, and then by others, that humans were simply natural slaves.
All the defiance of the subjugation was forgotten. The humans integrated fully with the Iovar, still in slavery, but trusted above all others. In only two hundred years, the entire race inserted itself in the public consciousness as the perfect companion. They would never betray a secret, never cheat, never lie for anyone but their master. Subservient to a fault, absolutely loyal, and easy to trust. Trust. Trust. It always came back to trust.