The aliens, light years away, that had sent the meteorite with every intention of hyper evolving creatures and artificially giving them intelligence, morality, ethics, human instincts, and other things humans don't think of as 'odd' or 'strange' or even really existing... much like fish who have no idea on the existence of water.
Except, mistakes happen, no matter how grand the civilization.
Instead, the animal it had MEANT to mutate into anthros would instead become strange (by Earth standards) creatures, with all the empathy and reasoning of insects (the very things the meteorite was intended to ignore). Little more than biological machines carrying out their pre-instsalled programming.
Thankfully, it would be centuries by the time the aliens learned of their bungle... good thing too, as the scientists assigned to the task would have likely been publicly executed for this goof-up instead of dying of old age... And by then a bloody revolution would have happened against the aliens' immortal elite.
Humans themselves meanwhile, would mutate into the very creatures the meteorite had intended the dumb beasts of Earth to become. The aliens' tech was so advanced, that any scans or disassembly of the rock would show it to be just that, a rock (like a caveman finding a copy of King James' bible, and seeing just a buncha weird leafs), and even smashed into little pieces would still carry out its function.
However, due to this being a bungle, it mean humans changed would suffer memory loss, their personal memories being rendered vague and cloudy, and their minds malleable.
And for those who found the rock... this presented a golden opportunity... and the minds of those changed reduced to easily-shapable clay... The lie they were upgraded animals rather than former humans was the perfect fiction to feed outsiders ...