At the time period the Oldcron meddling was supposed to have happened (sometime around the Cretacious) mankind's ancestors were tree dwelling shrew like creatures that ate bugs and dinosaur eggs. Supposedly the Necrons shoved the Pariah gene in them to serve as a future bioweapon, which is an incredibly stupid plan in the first place considering how long it would take to get an actual usable creature out of it so why bother when you're in the middle of the war that you want to use them in and won't get them until long after the war ends.
It's one of the dumber things about the Oldcrons that I'm glad got retconned away. I actually much prefer their new stance on biology, with some of them experimenting to try and turn Necrons back into living beings rather than the other way around.
Nah, it made sense in oldcron fluff and retconning it away didn't really then explain why pariahs were still around, which was a step back in fluff :<
The C'Tan won the war, GG, Empyrean is fucked and souls are all being harvested. Despite winning the war, their objective to gain total dominance over the milky way kinda got fucked with the advent of the warp turning into the warp everyone in 40k knows and loves, ending up killing the same food source the C'Tan needed to survive. The C'Tan and their Necrons were playing the long game, where they had to wait a loooooooooooooooooooooong time to deploy weapons that would deliver final victory versus the warp. Sleepy crons operate on another strategic timeframe entirely, eternity is not an abstract concept to the material gods, and millions of years is barely anything and is pretty much their modus operandi. Build pylons, tombs and warships that will only be used much, much later, when the time is right to harvest once more