The problem of the origin of life has given rise in the last centuries to a debate between the Mechanicists and the Vitalists. Although the problem was not confronted directly and explicitly by St. Thomas, he would have chosen the alternative of creation by God, instead of merely chance, says Mondin.²⁴ Therefore, St. Thomas would incline to say that God is involved in the origin of life.

Although St. Thomas affirms the absolute primacy of God as the principle cause of everything produced by nature, does St. Thomas affirm that God works through secondary causes too? In his argument against St. Augustine and Avicenna,²⁵ St. Thomas asserts that not just God and spirits are efficient causes, but there are secondary causes in nature. St. Thomas affirms there are secondary causes in nature.²⁶ As proof of secondary causes, St. Thomas has three arguments (Aquinas Summa Theologiae 1. 105. 5).²⁷ First, without secondary causes there would be no connection for creatures between their causation and the effect; creatures would be impotent and their powers in vain. Second, every being exists through its operations, so that without secondary causality, creatures existence would be imperiled. Third, less perfect things are ordered to more perfect: matter is ordered to form as the first act, and matter is ordered to operation as the second act, in such a way that operation is the goal of created things. Therefore, St. Thomas confers upon secondary causes the full share of being and efficacy to which they are due. In the real world, the nature of the effect is similar to the nature of the cause, so that warmth does not chill, and humans generate humans. So the existence of natural laws suppose that God created beings endowed with causality.²⁸ How can the same effect be produced by two different causes (God and the natural agent) at the same time? These causes are at the same time, but not under the same relation, e.g., a workman uses an axe to cut wood, and both are causes. The analogy applies to God, but God’s influence on the secondary cause penetrates more deeply, so that when God grants existence, God grants form, movement, and efficacy.²⁹ Thus the existence of secondary causes points to no lack of power in God, but to the immensity of God’s goodness (confer: Aquinas Summa Contra Gentiles 1. 13).³⁰ Philosophically, secondary causality in creatures is an affirmation of the principle of causality which is fundamental to classical metaphysics and especially to Thomistic metaphysics. The principle of causality regulates the relationship between cause and effect according to the definition of Aristotle, which St. Thomas made his own, declaring, "Everything that is moved is moved by another," or in another way, "Everything that happens presupposes a principle that produces it."³¹

Does St. Thomas teach that lower creatures, "from the lifeless" as Aristotle says, are in service of higher creatures? Yes, he does, and so reprises Aristotle who taught that "nature proceeds little by little from things lifeless to animal life" and "there is observed in plants a continuous scale of ascent toward the animal."³² This observation of natural ascent is helpful to understand the evolutionary progress from lifeless to life.³³ St. Thomas notes, "...less noble creatures are in the service of the more noble...Further, every creature is in the service of the perfection of the universe...Finally, the totality of the universe with all its parts is ordered to God as its goal" (Aquinas Summa Theologiae 1. 65. 2).³⁴

On the hypothesis that living things originally evolved from non-living matter, the "urge to live’ must be located in matter itself. St. Thomas called this "urge to live" matter’s appetite for the most perfect actuality attainable. Mondin and Benignus cite the classic text of St. Thomas in the Summa Contra Gentiles to establish this point.³⁵