Theological attention to the Catholic doctrine of original sin has a history that extends from the letters of Saint Paul through the Council of Trent and Pius XII’s 1950 encyclical, Humani generis. The doctrine has traditionally been articulated through the Genesis narrative of Adam and Eve as the first human beings from whom all others descend, an account known as monogenism. In the course of the nineteenth century, scientific research into human origins increasingly relied upon polygenism, the descent of humanity (...) from non-human ancestors through a transitional population. Subsequent Catholic engagement with evolution included resistance to polygenism from the Vatican due to a perceived conflict with the doctrine of original sin. Humani generis included a prohibition that remains in place today in spite of widespread de facto acceptance of polygenism among theologians. Understanding the origin and persistence of this disparity stands to benefit from comparison to a corresponding ambivalence toward the sixteenth century Copernican hypothesis of a moving earth, only conclusively resolved in 1992. In Part I of this essay I introduce this historical comparison and describe the origins of monogenism and polygenism terminology in nineteenth century debate over the unity of the human race. I then describe the conceptual changes that transpired during the first half of the twentieth century and the resulting role of polygenism in the nouvelle théologie of the decade prior to Humani generis. Subsequent developments and implications follow in Part II. (shrink)