Every human has blood. But not everyone's blood is the same. Some people are Type A. Others are B, AB, or O. There are also dozens of other blood-group systems hiding beneath those familiar letters. So why did evolution create all these differences in the first place? The answer takes us deep into humanity's past, where infections, parasites, natural selection, migration, and ancient population changes may have shaped the blood types humans carry today. Why is Type O so common in some populations? Why are certain blood types linked to protection—or greater vulnerability—to particular diseases? And how far back do these differences actually go? Your blood type isn't random. It's a tiny piece of evolutionary history that you carry with you.

Why Did Humans Develop Different Blood Types?

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