In his 1950 Encyclical, Humani Generis, Pope Pius XII cautioned about a number of ideological trends that undermined the Faith of the Church. Among these was a certain idea connected with the Theory of Evolution called polygenism. For the evolutionary idea to be accepted it would require not just two first human parents, but the transition from animal to man would require a multitude of men and women. In other words, it is a rejection of the belief that Adam and Eve were two real people from which the entire human race descended. The Pope strongly condemned acceptance of this idea saying, “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” (Humani Generis, 37).
On the surface, it appears to make little difference as to whether there was an actual Adam and Eve or whether mankind traces its roots to a multitude of first humans. Diving beneath the surface, we see that acceptance of polygenism threatens to undermine the foundations of the Christian faith. If polygenism is true, then the Christian faith is necessarily false.
Evolutionary theory applied to man does not only mean that man was made by blind forces but is ultimately an attempt for men to remake themselves. The creature becomes his own creator. No Adam and Eve means no Original Sin. No Original Sin, no need for Christ. If we were never “in Adam” then there would be no need to be “in Christ.” With a multitude of races at our beginning, there would be fallen and unfallen men living together and only those who are direct descendants of Adam need redemption. Evolution eventually weeds this out through natural selection, removing any distinction and Christ becomes entirely unnecessary. Even if this is a case of unintended consequences on the part of Darwin and his ideological descendants, we can be sure there is at least one highly intelligent person who revels in this idea.
In the mind of many Christians, this sets up a Catch-22. If we accept a literal Adam and Eve, then where did their grandchildren come from? To accept a belief in only first two parents means to accept that their children were incestuous in populating the earth. With no outsiders to marry, Cain, Abel, Seth and their unnamed sisters would have married each other. Rejecting a literal Adam and Eve seems to be better than accepting this morally repugnant option. Or is it?
Why Incest is Wrong
When asked why incest is wrong, most of us would say because the genes of those closely related by blood are so similar that it can result in offspring with serious genetic defects. Looked at properly however, this is a consequence of the wrong and not necessarily the reason why it is wrong. Whether we posit that because Eve was taken from the rib of Adam they were nearly genetically identical (making their act of intercourse genetically the same as fraternal twins) or that Eve was fashioned with a different genetic code than Adam, the important point to remember is that their genetic code would have had no mutations in it. After the Fall, their offspring may have had mutations in their DNA, but, if we accept the modern scientific explanation of these mutations as appearing at random, we should not expect identical mutations to occur in Adam and Eve’s offspring. Without the necessary doubling of mutations in the parents, we would not see the same effects that we see with inbreeding today. Once the gene pool has a sufficient number of these mutations present in it and the likelihood of some deleterious effect occurring on the rise, God issues a positive command that a man may not marry someone of close relation like his sister, aunt, or niece (Lev 18-20).
In short, the consequence of serious birth defects is a sign that incest is wrong, but is not what makes it wrong. In City of God (Book XV, Ch. 16) Augustine visits this question as to why Cain, for example, committed no wrong when he married his sister. We can borrow from his explanation to help us see past this intellectual obstacle.
The Augustinian Solution
First, he looks at the purpose of marriage and procreation and says something that most of us would not think of as a purpose today. Augustine see this as one of the goods of marriage—marriage multiplies relationships. In the past, especially in ruling families, marriage was viewed as a means to bring the families together, making them one. It brings strangers together and makes them a family. A woman’s brother becomes the man’s brother-in-law, her father, his father-in-law. Without the marriage of the man and woman, these men would not have entered into a familial relationship.
When closely related persons married, this good is lost. When siblings marry, their mother is both mother and mother-in-law. This was obviously unavoidable in the case of Cain and his sister, but, according to Augustine, is a reason to avoid close marriage.
Obviously, this would not be a precept of the natural law, but Augustine and St. Thomas both say that marriage between a parent and a child was always contrary to the natural law because of the relationship of parent and child could never be placed on the equal footing required for marriage. A child always owes their parents piety while spouses have no such obligation. This is why Noah curse Ham when he “saw his nakedness” (Gn 9:20-25), which is a Hebraic euphemism for sleeping with his mother.
While not a precept of the natural law, marriage between siblings and close blood relatives is still wrong because of our fallen human nature. For men and women to live closely together (like siblings do today or close blood relations such as cousins did in the past) with the potential for the relationship to become sexualized is a great temptation to lust and use. This is why it would be just as wrong for Greg and Marsha Brady to get married as it would be for two blood siblings. To make such a union illicit can serve to remove this temptation and makes it taboo. The fact that we initially recoil at the thought of Cain and his sister means that this taboo has had its intended consequence.
Removing incest as an obstacle to belief in two first parents goes a long way in helping us to see why polygenism must be false and why we should reject any form of it. Grandpa Adam and Grandma Eve, first parents and first grandparents.