By studying the blood group of different populations around the world, backed up by important work done in the laboratory, scientists first thought that primitive Man was type O and Rhesus + - two of the best known systems. But now it is thought that the A, B, O systems all originate from a single system, although it is difficult to know exactly how. How can we take a blood sample from a skeleton which is some tens of thousands of years old! However, it is thought to have resembled group O.