Thanks Christy for your interest.
The declaration that brings the original sin in relation with the issue of “polygenism” is formulated by Pope Pius XII in his Letter Encyclical _Humani generis_ (1950) as follows:
For these reasons the Teaching Authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions, on the part of men experienced in both fields, take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter - for the Catholic faith obliges us to hold that souls are immediately created by God.
When, however, there is question of another conjectural opinion, namely polygenism, the children of the Church by no means enjoy such liberty. For the faithful cannot embrace that opinion which maintains that either after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parent of all, or that Adam represents a certain number of first parents. Now it is in no way apparent how such an opinion can be reconciled with that which the sources of revealed truth and the documents of the Teaching Authority of the Church propose with regard to original sin, which proceeds from a sin actually committed by an individual Adam and which, through generation, is passed on to all and is in everyone as his own [Cfr. Rom., V, 12-19; Conc. Trid., sess, V, can. 1-4].
In this declaration the exclusion of “polygenism” is formulated very carefully under the condition that there is no way of reconciling “such an opinion” with the Teaching of the Church regarding original sin. If this condition is not fulfilled, then doors remain open. This was stressed by Joseph Ratzinger (today Pope emeritus Benedict XVI) in his Münster Lectures as earlier as 1964: “With this text a door is in principle quite clearly opened”; what is important for the Church is not the claim of the hominization in one couple but the claim that all human beings became guilty in their original state; “monogenism is assumed only in function of this theological statement”.
Accordingly, the only thing that the papal Encyclical says “Catholic faithful cannot deny in any way” is the “Teaching of the Church with regard to original sin” as stated in the Canones 1-4, 5th Session of the Council of Trent, that is, that “original sin” means a fallen state (peccatum originale originatum) and because of it all human persons require salvation, and to save us the Second Person of the Holy Trinity became a man, Jesus Christ. But the Magisterium does not state that the genetic origin of all humans from a primal couple (“monogenism”) is essential for “the state of original sin” and leaves open the possibility of “polygenic” origin if this is supported by science.
The final outcome of Humani generis is that one should not deduce original sin and Redemption from ‘Adam and Eve’, but resort to the primeval single couple only if there is no other way of explaining how the first personal sin “is passed on to all”. Joseph Ratzinger/Benedict XVI himself has suggested that other explanations are possible and proposed his “relational damage” model (1986, 1995, 2014). This proposal is quite close to the Homo divinus model as proposed by Denis Alexander, Sam Berry, and Graeme Finlay. My own explanation being discussed in this forum further develops these models to avoid lateral transmission of sin through sort of “spiritual contamination” from sinners to non-sinners.
In my view these facts regarding Humani generis may help to correct misinterpretations as those of authors like Jerry Coyne.