As documented in Part I, monogenism, the descent of all human beings from Adam and Eve, was closely linked to the Catholic doctrine of original sin throughout the first half of the twentieth century. Theological reservations about polygenism, the more scientifically supported account of human origins through a transitional population, was brought to a head by Pius XII’s 1950 encyclical Humani generis. Although the encyclical allowed discussion of human evolution, polygenism was prohibited because “It does not appear how such a (...) view can be reconciled with the doctrine of original sin.” Nevertheless, during and after Vatican II de facto theological acceptance of polygenism became commonplace even though the prohibition was not withdrawn. While some modern conceptions of original sin make a reliance upon monogenism unnecessary, another school of thought emphasizes a spiritual rather than a biological understanding of the concept. It remains to be seen whether polygenism can be unanimously accepted as a scientific phenomenon that is not theologically threatening. Reflection on the Vatican’s long period of ambivalence about the motion of the earth suggests that a similar prospect can be foreseen for polygenism even though it is subject to development in a manner that geostasis was not. (shrink)