>But it's always hard to infer "why" I thought I read a while ago that there's a theory that different blood types started out with O-, and evolved as a way to

>But it's always hard to infer "why" I thought I read a while ago that there's a theory that different blood types started out with O-, and evolved as a way to be more resistant to certain diseases, but that there were downsides to the other types (certain health problems or less efficient use of energy, I forget now).