Why do humans have different blood types?

The answer

The ABO blood group system is determined by specific sugar molecules (antigens) attached to the surface of red blood cells. Type A blood has N-acetylgalactosamine antigens, type B has galactose antigens, type AB has both, and type O has neither. These antigens are produced by glycosyltransferase enzymes encoded by the ABO gene on chromosome 9. The immune system produces antibodies against whichever antigens are absent from a person's own cells, making blood type matching critical for safe transfusions. The leading theory for why different blood types evolved involves disease resistance: type O individuals show some resistance to severe malaria, while type A and B individuals may have advantages against certain bacterial infections and cholera. The ABO polymorphism is remarkably ancient, predating the divergence of humans and other great apes by millions of years, suggesting strong balancing selection maintaining the different types.