From the Encyclical Humani Generis: "On Evolution and Monogenism"
Excerpts from the Encyclical Humani Generis
3895. It remains for Us to say something on the questions which, although they have to do with the disciplines which are customarily called “positive,” yet are more or less connected with the truths of Christian faith. Not a few insistently demand that the Catholic religion give as much consideration as possible to these disciplines. Surely, this is praiseworthy when it is a case of actually proven facts, but caution must be exercised when the question concerns “hypotheses,” although in some manner based on human knowledge, in which hypotheses doctrine is discussed which is contained in the Sacred Scriptures or in “tradition.” When such conjectural opinions are opposed directly or indirectly to the doctrine revealed by God, then their demand can in no way be admitted.
3896. Wherefore, the magisteriumof the Church does not forbid that the teaching of “evolution” be treated in accord with the present status of human disciplines and of theology, by investigations and disputations by learned men in both fields; insofar, of course, as the inquiry is concerned with the origin of the human body arising from already existing and living matter; as to the souls, the Catholic faith demands us to hold that they are immediately created by God. This must be done in such a way that the reasonings of both opinions, namely those in favor and those opposed to evolutionism, are weighed and judged with due seriousness, moderation, and temperance; and provided that all are ready to yield to the judgment of the Church, to which Christ has entrusted the duty of interpreting Sacred Scriptures authentically, and of preserving the dogmas of faith. Yet some with daring boldness transgress this freedom of discussion, acting as if the origin of the human body from previously existing and living matter, were already certain and demonstrated from certain already discovered indications, and deduced by reasoning, and as if there were nothing in the sources of divine revelation which demands the greatest moderation and caution in this thinking.
3897. When there is a question of another conjectural opinion, namely, of polygenism so-called, then the sons of the Church in no way enjoy such freedom. For the faithful in Christ cannot accept this view, which holds that either after Adam there existed men on this earth, who did not receive their origin by natural generation from him, the first parent of all; or that Adam signifies some kind of multitude of first parents; for it is by no means apparent how such an opinion can be reconciled with what the sources of revealed truth and the acts of the magisteriumof the Church teaches about original sin, which proceeds from a sin truly committed by one Adam, and which is transmitted to all by generation, and exists in each one as his own [cf. Rom 5:12-19; Concilium Tridentinum, Sessio V, canones 1-4 ].
Pius XII, Enc. Humani Generis (1950), English transl. from the original Latin source in H. Denzinger - P. Hünermann, Enchiridion Simbolorum, definitionum et declarationum de rebus fidei et morum (Freiburg i.B.: Herder, 1991), DH 3895-3897.