These changes are reflected in the local success or failure of each of the blood groups, which appear to have each had a moment of pre-eminence at a critical juncture in our history. The ascent of humans to the top of the food chain (the early advantage of blood group O), the change from hunter-gathering to a highly concentrated, urban environment and agriculturally-based diet (the ascent of blood group A), and the mingling and migration of the races from the African homeland to Europe and Asia ( the opportunity for blood groups B and AB).