Notwithstanding the imperfections that living beings may exhibit, the functional organizing principle of life is so striking and pervasive that observers in past centuries, such as Aquinas, felt compelled to explain it in terms of a divine Creator/Designer. The effects of the principle are so ubiquitous that any proper understanding of the existence and nature of life must account for it. This author contends that the functional principle can be best explained within the framework provided by evolutionary theory, where organisms become adapted to fulfill their needs successfully within their given environment by a process of natural selection.

According to the theory as originally proposed by Darwin and early geneticists, minor alterations arise in individual representatives of a life-form purely by accident, as a result of mutations or random crossing of genes from separate parents. Individuals then pass these alterations to their offspring, unless the alterations tend to cause the individual to fail to survive and reproduce. Over a period of time, therefore, only those alterations that are favorable to or at least compatible with survival and reproduction remain in the species. Consequently, the species itself tends to evolve, over thousands or millions of years, so as to become better adapted to survival within its environment. Even though evolution arises from a series of random changes, it thus develops a definite pattern of adaptation over time—which, to the uninformed observer, might seem to reflect some deliberate design, as Aquinas thought.