Asked by: Barry Eade, by email
Because group O blood can be received by anyone in a blood transfusion, it was originally thought that this was the ancestral blood type, but more recent work suggests that there are several different mutations that can deactivate the A or B genes to turn them into O. This makes a firm evolutionary timeline much harder to establish and there are several competing theories that contradict each other.
What they agree on is that AB is the most recent and didn’t occur until the 16th Century when group A populations from Europe and group B populations from Asia began to mix. The other blood groups are tens of thousands of years old with B being more recent than A. The oldest group is either group A or one of the forms of group O.