The emergence of different blood types in humans is thought to be the result of evolution over millions of years. Here's a breakdown of the key points:

Genetic mutations: The main driver is believed to be mutations in genes that control the proteins on the surface of red blood cells. These proteins, called antigens, determine a person's blood type.

Evolutionary advantage: The different blood types likely provided advantages in different environments. For instance, some studies suggest a link between blood types and resistance to diseases like malaria.

Timeframe: The exact timeline is still being researched, but estimates suggest the A and B antigens arose over 20 million years ago.

Here's an interesting fact: The four main blood types are A, B, AB, and O. Type O has no antigens, while A and B have one each, and AB has both. This variation plays a crucial role in blood transfusions, as mismatched types can trigger an immune response in the recipient.

(http://www.autoadmit.com/thread.php?thread_id=5543868&forum_id=2Reputation#47768458)