Human blood types have deep evolutionary roots

ABO system may date back 20 million years or more

Chimps, gibbons and other primates are not just humans’ evolutionary cousins; a new analysis suggests they are also our blood brothers. The A, B and O blood types in people evolved at least 20 million years ago in a common ancestor of humans and other primates, new research suggests.

The analysis deepens a mystery surrounding the evolutionary history of the ABO blood system, and should prompt further research into why the different blood groups have persisted over time, Laure Ségurel of the University of Chicago and colleagues report online October 22 in the Proceedings of the National Academy of Sciences.

“Their evidence is rather convincing that this is a shared, very old capability that has remained throughout the divergence of the species,” says doctor and transfusion specialist Martin Olsson of Lund University in Sweden.

Different forms of a single blood type gene determine what types of molecules sit on your red blood cells: type A molecules, type B molecules, A and B together, or no intact surface molecules in the case of type O (O was originally called type C, then was changed to O for the German “ohne,” meaning “without”).

The A, B and O versions of the gene differ only slightly, and scientists have debated two scenarios to explain their evolution. One posits that the A version of the gene existed long ago, and the B and/or O versions later cropped up independently in several species (including humans, gorillas, baboons and chimps). Alternatively, all of those species may have inherited the A and B types from a single ancestor.

To get some bloody perspective on the matter, researchers led by Ségurel looked at a particular stretch of DNA in the blood type gene in humans, bonobos, chimpanzees, gorillas, orangutans and several species of monkey. Then the scientists compared that stretch of DNA across species on the larger primate family tree. The pattern they saw suggests that the A and B blood groups were around at least 20 million years ago, well before the chimp-human split, and probably as far back as the common ancestor of humans and old-world monkeys. Sections of DNA in the human A version, for example, more closely matched the A version that gibbons have than the human B version of the gene.

Exactly why evolution would favor a mix of blood types in so many species is a mystery. Depending on blood type, people are more or less susceptible to particular pathogens. Type O people, for example, are more susceptible to cholera and plague, while people with type A are more susceptible to smallpox. Blood group diversity may have been maintained for so long because each version was immunologically advantageous in certain times and places.

“That diversity may have led to protection against whatever might come your way,” says glycoimmunologist Brian Cobb of Case Western Reserve University in Cleveland.

People with type A are also more prone to dangerous blood clots, Olsson says. That’s a disadvantage in the modern world, but in the days when humans and their ancestors were having babies in caves and fighting predators without the option of an emergency room, such clotting may have been beneficial.

“When we couldn’t do transfusions or patch people together it may have been good to coagulate better,” he says. “If that mammoth or Siberian tiger got you, you wouldn’t want to bleed to death.”

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