In 1975, Harvard biologist E. O. Wilson created a firestorm when, in his book Sociobiology: The New Synthesis, he argued that human nature might be explainable in evolutionary terms. Centuries earlier, however, a leading Christian scholar was already applying many key evolutionary principles to the understanding of man.
Thomas Aquinas (1225-1274) was the foremost Christian scholar of the High Middle Ages and is today regarded as a "doctor" of the Catholic Church. Working six centuries before Darwin, he obviously was not an evolutionist. His major project was the Christianizing of Aristotelian philosophy. As an ardent Aristotelian (enough so that some of his teachings were condemned by the Bishop of Paris in 1277), Aquinas assumed that species were fixed and unchangeable, an idea incompatible with evolution. But Aquinas was the star student of Albert the Great, an enthusiastic Medieval naturalist. Albert assiduously observed the Dominican Order's policy of walking, not riding, when traveling. Ostensibly this was to emphasize the Order's commitment to poverty, but for Albert it was an opportunity to more closely observe nature's minutiae. Under Albert and Aristotle's mentorship, Aquinas acquired a deep appreciation for nature's continuity, which he understood as reflecting purposeful design rather than common descent.
Aquinas had no doubt that humans were specially created by God. However, he was also convinced that they were created out of the same basic materials used for all creatures and were therefore connected to all of nature. In his Summa Theologica he writes:
"But it was fitting that the human body should be made of the four elements, that man might have something in common with the inferior bodies, as being something between spiritual and corporeal substances." (ST P1 Q91 A1)
Aquinas had no qualms about calling humans animals:
"Socrates and Plato ... have the same human species; others differ specifically but are generically the same, as man and ass have the same genus animal." (De Principiis Naturae 45)
Following Aristotle, Aquinas rejected the strict dualism of the Augustinian/neo-Platonic philosophy dominant at the time. No, Aquinas was not a materialist neuroscientist, but he understood the intimate interdependence of mind and body. For Aquinas, different bodies meant different levels of intelligence: "...because some men have bodies of better disposition, their souls have greater power of understanding." (ST P1 Q85 A7)
Aquinas is most famous for his Summa Theologica, much of which is considered authoritative in Catholic theology. Less known is another great summa, Summa Contra Gentiles, where he sought to persuade non-believers using purely rational arguments for Christian doctrine. It is here that we find a naturalistic discussion of marriage.
"We observe that in those animals, dogs for instance, in which the female by herself suffices for the rearing of the offspring, the male and female stay no time together after the performance of the sexual act. But in all animals in which the female by herself does not suffice for the rearing of the offspring, male and female dwell together after the sexual act so long as is necessary for the rearing and training of the offspring. This appears in birds, whose young are incapable of finding their own food immediately after they are hatched. ... Hence, whereas it is necessary in all animals for the male to stand by the female for such a time as the father's concurrence is requisite for bringing up the progeny, it is natural for man to be tied to the society of one fixed woman for a long period, not a short one." (SCG B3 Q122)
The ideas expressed above are familiar to evolutionists as part of parental investment theory -- male/female pair-bonding is more likely to emerge where offspring are highly dependent.
Aquinas also anticipated another core evolutionary concept: paternity certainty. Males find an evolutionary advantage in long-term pair bonding because it helps to insure that offspring possess their genes. Without this assurance, males are unlikely to provision or protect the offspring. Thus, monogamy serves the genetic interests of both males and females. Females and their offspring receive resources and protection from the male (paternal investment), while males gain assurance of a genetic legacy (paternity certainty).
"...every animal desires free enjoyment of pleasure of sexual union as of eating: which freedom is impeded by there being either several males to one female, or the other way about ... But in men there is a special reason, inasmuch as man naturally desires to be sure of his own offspring ... The reason why a wife is not allowed more than one husband at a time is because otherwise paternity would be uncertain." (SCG B3 Q124)
Note how Aquinas' discussion also alludes to another important evolutionary precept: male mate competition. Aquinas goes on to describe how monogamy benefits women by reducing the female competition inherent in polygynous households, thereby insuring the concentration of emotional and material resources on a single female mate.
"For among men that keep many wives the wives are counted as menial. For one man having several wives there arises discord at the domestic hearth..." (SCG B3 Q124)
Along with anticipating many key concepts in evolutionary psychology, Aquinas also understood that humans possessed a natural moral sense. Some believers today foolishly try to argue that without religion there is no morality. Aquinas would have scoffed at such simple-mindedness. Synderesis, as Aquinas called it, was the natural human inclination toward right behavior.
"Wherefore the first practical principles, bestowed on us by nature, do not belong to special power, but to a special natural habit which we call synderesis. Whence 'synderesis' is said to incite the good, and to murmur at evil, inasmuch as through first principles we proceed to discover, and judge of what we have discovered. It is therefore clear that 'synderesis' is not a power, but a natural habit." (ST P1 Q79 A12)
In contrast to Augustine, Aquinas did not see human nature as inherently depraved. Instead, his view was generally more positive. We are, as Aristotle had argued, naturally social animals who seek to get along in society. Divine grace did not radically alter human nature, it perfected it.
If he were alive today would Aquinas be an evolutionist? His writings suggest a mind already resonating with many evolutionary concepts. My sense is that Aquinas, like Aristotle and Albert before him, was just too curious and too smart not be at the intellectual vanguard wrestling with exciting new knowledge. Limping weakly behind with whiny unimaginative creationists would have been far too boring for a mind such as his.