text: Who told you there was no benefit?  A and B bloodtypes refer to the types of glycoproteins on the outsides of cells.  These gylocoproteins serve the function of acting as immunological recognition factors--they help the immune system differentiate "self" from "non-self."  Though the specific structure of such chemicals is largely arbitrary, it is not true to say that the existence of such glycoproteins is arbitrary.  As an analogy, there is no benefit to the *specific* combination of numbers that is in my social security number.  It's a random sequence, but a registry at the federal government allows the government to use that number to recognize my unique identity.  Hence the sequence of digits may be arbitrary, but the existence of the number is far from arbitrary.  I'd have a harder time getting through life in the US without one.\n\nLikewise, there may be not much evoultionary differences in today's society for the glycoproteins on a specific cell type, but if you had no such glycoproteins at all, you'd be unable to have a functioning immune system.  Evolutionary selective factors might not be so strong as to eliminate everyone with a particular cell left unlabeled by glycoproteins, but there is an evolutionarily important reason for having glycoproteins in general.\n\nMy blue eyes are such an anomaly comparable to the O blood type--the gene for making some modestly useful protein (likely more useful in the past than it is today) is not functional, hence a certain anamolous differnence exists between me and most people.  There is a slight advantage to having brown eyes, in that blue eyes are more sensitive to UV light (less important for my northern ancestors than for people from equatorial regions).  Likewise, it has been proposed (though not definitively concluded) that the A blood group had better chances of surviving the bubonic plague back in the times that it swept across Europe, leaving a greater prevalence of the A blood type in areas where the bubonic plague hit hardest.