We Cannot Live Alongside Demons!

"Actually, no.  There were different switches that affected mages and different switches that affected humans." 

"These differences probably occurred over a very long period of time in the far distant past, where humans and mages got separated without contact with each other for a long while." 

"I don't know what caused the separation, but it's such ancient history that we will probably never know for sure."

"Any gene that was crucial for humans to survive was turned on.  Anything that was detrimental to survival was turned off." 

"Over time, this caused humans and mages to become slightly different, but not dissimilar enough to be two different species."

"The humans sure got the short end of the stick!"  Gabriel grumbled.