Why don’t all humans have the same blood type?

Our distant ancestors used to, but then mutations created different antigens on the outside of red blood cells, leading to the different blood groups. The first of these is thought to have occurred before chimpanzees and humans diverged, about 13 million years ago, creating blood group A; others are more recent. There are now 30 blood groups recognised and their proportions differ in different ethnic groups. Answered by Susan Blackmore