I believe that many years ago when human were brought to this planet, they are REAL intelligent species. But as time passes, due to the strong gravity of the earth, human slowly became weak. Their appearance begun to changed. They started expressing emotion which the other aliens could not understand.
After some time, the aliens who brought human here finally realized that earth is not suitable for human after all. And so they took the still "normal" humans away from this planet and left rest who are too late to be save or the "insanes" on this planet to eventually die off. However they left a few guardians behind to look after them and monitor their progress. Those guardians eventually too begun to became abnormal. They taught the human about technology. They convinced the humans that they are the creator of them or god and so the human started worshpping them.
Not long after that, those guardians died off, leaving behind insane humans with limited technology and knowledge.