Board Thread:Warhammer 40k General Discussion/@comment-9558536-20130517084538/@comment-3258762-20130520130357

There were human psykers, the mutation was first discovered in M3

But it doesn't matter if you're a psyker or not, unless you're a blank you have a psychic resonance. And there has been psykers before M3, the shamans who gave birth to the Emperor were psykers. Just because there were no recorded psykers doesn't mean there weren't anything. In fact, all humans except blanks can be considered psykers. Not that they can use their powers, but they have an influence on the Warp and the Warp has an influence on them.

As for full of sentient life: Considering how calm everything was when it was only the Old Ones and the Necrontyr, it is fully possible that, at the time, there simply were no other sentient life with a psychic resonance in the galaxy than the Humans, the Orks and the Eldar