When I look at Humani Generis paras 36 & 37, I can clearly see the author trying to leave the door open for an ‘evolustic’ explanation of human origins, whilst simultaneously strenuously insisting that the doctrine of Original Sin is not up for negotiation/dilution. I think it is very significant/fortunate that he avoided the slightest mention Eve. He (rightly) put all the stress on Adam, since Adam alone is relevant for the transmission of Original Sin.
If you look at it practically, it is highly unlikely that Adam, especially in his fallen state, remained faithful to Eve once they were out of Eden (he blamed her for his downfall, remember?). It is quite likely that both of them led promiscuous lives thereafter, under the “slavery of sin”. Whom would they have mated with? Mates could have come from their contemporary group (in an evolustic scenario) or from other humanoid animals then existing.
If Eve indeed had any progeny from other mates, we can safely assume that God let/arranged for those lineages to die out. The same can be said of progeny (if any) of contemporary human couples (if any) existing.
Though we do not know how long Adam lived, I tend to think that God gave him a generously long fertile life (even running into a millennium plus) so that he could have a very, very large number of mates and therefore a humongous progeny. All these progeny and their lineages would carry the stain of Original Sin.
My prime reason for constructing this scenario is that I want to keep Adam as the earliest common ancestor of the human race whist simultaneously ensuring a very large initial gene pool. This is very crucial in order to account for the vast diversity in the human gene pool seen today. Two of the scientific objections to the ‘Adam and Eve first couple’ story is that (a) the diverse human gene pool of today could not possibly have stemmed from a single intial couple, and (b) at no point in history did the human population fall below the tens of thousands. By allowing Adam a large number of mates (running into 10,000+ in the course of a millenium of prolific sexual intercourse), I hope to tide over both the objections. I’m not sure how comfortable genetic scientists would be with such a proposition.
I don’t think that such a stand has the colour of Polygenism as proscribed in HG37, because here Adam means a real and unique individual and not a group of ‘Adams’. This is not to deny the existence of Eve. She would have really existed and played her part in the Fall, but after that she becomes irrelevant.