Rh negative factor in human blood

I am sorta in a rush so I will get strait to the point. I have searched for a long time but have yet to find anyone who can give any info on where the Rh negative factor that can appear in human blood cam from. It's obvious that Rh+ originated from our long time ancestors, monkeys, hence Rh for Rhesus Monkey from which the discovery of Rh factor originated from. But, then where did the negative come from? I am very interested in this, as I myself am a O- blood type and many people tell me that my blood type is considered, "pure blood". I don't know about that, but I simply want any answeres on where the Rh- may have developed from.
-Thanks!!!