One thing to keep in mind is that the A, B, and O blood types are also present in chimps and gorillas. And the plus-minus thing is known as the "Rh factor", where "Rh" is short for "rhesus monkey" (a kind of macaque monkey) where it was first identified.
So the blood sugars (in the case of ABO) and proteins (in the case of Rh) that cause these blood types cross species lines and are found in other primates. This is a strong piece of evidence of evolution, as there is no functional reason an A-type human should have a blood sugar in common with an A-type chimp but not a B-type human.
Not everything in evolution has to arise because of advantage. Something can arise that provides neither benefit nor harm, but it spreads into the population through what we call "genetic drift" ... just random spreading of a gene into the population due to random events such as migrations or natural disasters.
However, here's an *outstanding* article (as an audio-only radio broadcast on NPR) about how the ABO-type may be related to a response to malaria (!). It includes a really cool interview with a hematologist (blood expert) who gives a nice explanation of the ABO system:
Here are two good sites on the origins and evolution of the ABO blood types:
And here's more on the Rh factor (positive/negative):
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