We have different blood types because of evolution. Blood type A seems to be the most ancient blood type. Type B is then thought to have arisen from a genetic mutation that modified one of the sugars that sits on the surface of red blood cells. So then there became blood types A, B, and AB. (Type AB red blood cells are covered in both A and B forms of the sugar.) Eventually, Type O came about by another mutation that caused the inactivation of this sugar gene.
Scientists believe that disease may be one of the causes for our blood types to evolve. In Africa there is a higher proportion of people with blood type O. Interestingly, there is a high rate of malaria in Africa too. It turns out that cells infected with malaria don’t stick well to type O or type B blood cells, but they do stick well to type A. When they stick to type A blood cells, they form clumps and these clumps can be really dangerous if they’re formed in the brain or other vital organs. Luckily, for those people in Africa with blood type O, malaria cannot stick quite as well to their blood cells, so they are less likely to get infected – they have a selective advantage.