But we know that God can neither deceive nor be deceived, and it seems reasonable to conclude that "Adam" must refer to an individual person, as Genesis clearly records, and not to a group of people.
Against this, it can be argued that God selected one person among many human beings to make the choice on behalf of others. Or that a group of early human beings could have delegated authority to one leader. But this idea is totally dependent on every person in the group granting stupendous decision-making powers to this one person. Each could not afford the slightest doubt; total faith in the leader would be required. If only one person baulked at the idea, this concept is flawed. And God would have had to leave open the possibility of disagreement, a position considered above to be untenable.
One problem for such versions of polygenism is that Original Sin would not be transmitted through generation to the descendants of the sinless others in the group, but rather by the direct action of God. And there are other conceptual problems:
What about Eve - was she "the mother of all the living" (Gen. 3:20), or only one of the group? Where did the group come from in the first place, since Evolution is effectively ruled out by modern findings in biochemistry and genetics? What Scripture passages can be cited in support of the concept? What clue or hint is there in Genesis or Tradition that the single couple - Adam and Eve - were in fact "many first parents"?
The most satisfactory solution to the above problem facing polygenism is that only Adam could have made the choice on behalf of mankind. Eve was involved, of course, in the shaping of Adam's choice, but the responsibility was his alone. The wording of Genesis points strongly in this direction, and the wording of Humani Generis confirms it (Pope Pius XII wrote about "an individual Adam").