Unfortunately there is little understanding of how and why such blood types evolved in the first place. These blood molecules stand as a reminder that we still have a lot to learn about human biology 🙂
To summarise, different blood types developed over 20 million years ago in primates as a way to save the entire species from certain diseases which can latch on to some types of blood: those apes with a different blood type didn’t get the disease, so the species as a whole wasn’t wiped out in a plague.
Even today, lots of primates share our blood types – with some differences. For example, Gorillas only have type B blood, whereas Chimpanzees only have types A and O! Fascinating stuff – I’m glad you asked the question