The second type of vulnerability stems from a quirk of history. Archaeologists dispute the timing and manner of Indians’ arrival in the Americas, but almost all researchers believe that the initial number of newcomers must have been small. Their gene pool was correspondingly restricted, which meant that Indian biochemistry was and is unusually homogeneous. More than nine out of ten Native Americans—and almost all South American Indians—have type O blood, for example, whereas Europeans are more evenly split between types O and A.
