What is the explanation of people having blood types from an evolutionary perspective?
Researchers have puzzled over the selective explanation for the ABO polymorphism at least back to the 1950s. Multiple lines of more recent evidence, including the very old age of the ABO polymorphism and (perhaps) multiple convergent evolution events, suggest that the ABO polymorphism is maintained by balancing selection (E.g. Saitou and Yamamoto 1997, Segurel et al 2012). As best as I know, is no general consensus on what is driving balancing selection, though some researchers have argued that the selective agent is Plasmodium falciparum (e.g. Cserti and Dzik 2007).