A friend of mine told me about an interesting paper that says that the human type A gene was created by recombination of the type B and O genes.
Takashi Kitano, Antoine Blancher, Naruya Saitou. The Functional A Allele Was Resurrected via Recombination in the Human ABO Blood Group Gene.
Molecular Biology and Evolution, Volume 29, Issue 7, July 2012, Pages 1791–1796, https://doi.org/10.1093/molbev/mss021
Functional A and B alleles are distinguished at two critical sites in exon 7 of the human ABO blood group gene. The most frequent nonfunctional O alleles have one-base deletion in exon 6 producing a frameshift, and it has the A type signature in two critical sites in exon 7. Previous studies indicated that B and O alleles were derived from A allele in human lineage. In this study, we conducted a phylogenetic network analysis using six representative haplotypes: A101, A201, B101, O01, O02, and O09. The result indicated that the A allele, possibly once extinct in the human lineage a long time ago, was resurrected by a recombination between B and O alleles less than 300,000 years ago.