I feel certain that Kabr was the first Exo. He was infected by the Vex code that would have made the Vex part of the very fabric of existence and before he succumbed to the infection that was transforming him he was thrown back in time, probably dead. Humans found his now fully transformed machine body and backwards engineered him and thus the Exo’s were born. It is stated that humans did not create the Exo but humans did build them.