Have you thought that certain groups have certain characteristics ? I am AB NEG,and there tend to be clusters of us, the Basque region being one.
We can usually carry a Positive blood group to full term as the first baby, but the next, unless a Negative baby, used to be lost as the antibodies were present.There are plenty of records of certain groups of people where the virgins were the ones sought by intermarrying or conquering ( positive) tribes as the second child was known to die. Gradually the Negs died out.There are diseases like Sickle Cell Anaemia and Thalassemia carried genetically, so maybe that missing group of people were the ones who got across the world but then died away, having been outbred. Analysing ancient skeletons from Jerusalem also showed a high negative percentage, and there is a cluster in Ethiopia.B positive is a predominant group in Asia, with A positive close.
Just a thought