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Human blood types most likely came to exist to fend off infectious diseases. The incompatibility of some blood types, however, is just an accident of evolution.

There are four main blood types. Blood type A is the most ancient, and it existed before the human species evolved from its hominid ancestors. Type B is thought to have originated some 3.5 million years ago, from a genetic mutation that modified one of the sugars that sit on the surface of red blood cells. Starting about 2.5 million years ago, mutations occurred that rendered that sugar gene inactive, creating type O, which has neither the A or B version of the sugar. And then there is AB, which is covered with both A and B sugars.

These sugars are what makes some blood types incompatible: if blood from a type-A donor were given to a person with type-B, the recipient's immune system would recognize the foreign sugars as an invader and cue an attack. The resulting immune reaction can kill. Type O-negative blood is known as the "universal donor" because it lacks the molecules that would provoke that reaction (the "negative" means it lacks another type of surface molecule, known as the Rh antigen). [Is It Safe to Drink Blood? ]

But incompatibility is not part of the reason humans have blood types, says Harvey Klein, chief of transfusion medicine at the National Institutes of Health Clinical Center. "Blood transfusion is a recent phenomenon (hundreds of years, not millions), and therefore had nothing to do with the evolution of blood groups," he said.

The evolutionary cause, or at least one of them, appears to be disease. For example, malaria appears to be the main selective force behind type O, according to Christine Cserti-Gazdewich, a hematologist at Toronto General Hospital. Type O is more prevalent in Africa and other parts of the world that have high burdens of malaria, suggesting that blood type carries some sort of evolutionary advantage. [The 6 Craziest Animal Experiments ]

In this particular case, the advantage appears to be that cells infected with malaria don't stick well to type-O or type-B blood cells, Cserti-Gazdewich said. Malaria-infected blood cells are more likely to stick to cells with the A sugar and to form clumps known as "rosettes," which can be deadly when they form in vital organs, such as the brain. As a result, people with type O get less sick when they're infected with malaria, according to a 2007 study in the Proceedings of the National Academy of Sciences.

On the other hand, people with type-O may be more prone to other diseases. For example, they are known to be more susceptible to Helicobacter pylori, a bacterium that causes ulcers, Klein said. But research hasn't yet shown whether that or some other disease explains why humans still have blood types.