In the beginning of Humani Generis, there is an interesting comment about divine revelation being considered as “morally necessary”.
To me, that appears to be why affirming the origin of the human species as beginning with the first human man is so important in paragraphs 35, 36, 37.
From Humani Generis w2.vatican.va/content/pius-xii/en/encyclicals/documents/hf_p-xii_enc_12081950_humani-generis.html
Greetings and Apostolic Benediction[/LEFT]
[LEFT]Disagreement and error among men on moral and religious matters have always been a cause of profound sorrow to all good men, but above all to the true and loyal sons of the Church, especially today, when we see the principles of Christian culture being attacked on all sides.[/LEFT]
[LEFT]3. It is for this reason that divine revelation must be considered morally necessary so that those religious and moral truths which are not of their nature beyond the reach of reason in the present condition of the human race, may be known by all men readily with a firm certainty and with freedom from all error.[/LEFT]
[LEFT]4. Furthermore the human intelligence sometimes experiences difficulties in forming a judgment about the credibility of the Catholic faith, notwithstanding the many wonderful external signs God has given, which are sufficient to prove with certitude by the natural light of reason alone the divine origin of the Christian religion. For man can, whether from prejudice or passion or bad faith, refuse and resist not only the evidence of the external proofs that are available, but also the impulses of actual grace.[/LEFT]
Quick comment about paragraph 4. Please read chapter 14, Gospel of John.