A new study has found that blood group A and B alleles have been stable (albeit in likely dynamic equilibrium) in the primate family since... always.
The ABO histo-blood group, the critical determinant of transfusion incompatibility, was the first genetic polymorphism discovered in humans. Remarkably, ABO antigens are also polymorphic in many other primates, with the same two amino acid changes responsible for A and B specificity in all species sequenced to date. Whether this recurrence of A and B antigens is the result of an ancient polymorphism maintained across species or due to numerous, more recent instances of convergent evolution has been debated for decades, with a current consensus in support of convergent evolution. We show instead that genetic variation data in humans and gibbons as well as in Old World monkeys are inconsistent with a model of convergent evolution and support the hypothesis of an ancient, multiallelic polymorphism of which some alleles are shared by descent among species. These results demonstrate that the A and B blood groups result from a trans-species polymorphism among distantly related species and has remained under balancing selection for tens of millions of years—to date, the only such example in hominoids and Old World monkeys outside of the major histocompatibility complex.
Razib has some more details on the matter (being PPV, I haven't read it). Still he wonders what kind of disease or otherwise evolutionary pressure may have been so virulent as to keep the whole order of primates (or at the very least all simians) on our toes all these millions of years.
The answer may well be known already: it seems that type A blood protects against the plague, while type B protects against smallpox, both great historical killers of those without enough defenses. However they may favor other less important health problems like blood clots or cancer, enough to exert a mild pressure in favor of a return to the basic type zero ("O"), which would have evolved by loss of function once and again.
Whatever the case I find fascinating that these immune mechanisms may be so extremely persistent and I wonder if the bacterian mechanisms they confront may be more generic than just an specific disease.