In this blog post, we will explore the concept of evolution and its biological significance, focusing on the question: “Is evolution progress, or simply adaptation?”
It would not be an exaggeration to say that Charles Darwin’s theory of evolution revolutionized the way we view living organisms. However, unlike how we instinctively think of Charles Darwin when using the term “evolution” in a biological context, Darwin did not use the word “evolution” when he first wrote *On the Origin of Species*. It seems he wanted to avoid the word conveying a sense of “progress,” which might lead readers to believe that life is changing toward some specific goal. Even today, however, questions about whether evolution is truly progress continue to be raised. Viewing evolution as progress carries the risk of leading to eugenics or species supremacy. Let us, then, argue why, biologically speaking, evolution is not progress.
The idea of viewing evolution as progress is rooted in the Western view of nature known as the “Great Chain of Being,” a concept that has persisted since the time of Aristotle. This view posited that there are distinct stages in life: simple, lower animals at the bottom of a “great ladder,” with higher animals above them, culminating in humans and, ultimately, God. Furthermore, because the word “evolution” is widely used in everyday language to mean “gradual development,” people tend to assume that biological evolution also moves in a specific direction—and, in extreme cases, toward perfection—that is, that it is a process of progress. However, the biological definition of “evolution” refers to the process by which the distribution of traits within a specific population changes as variations suited to the environment are passed on to the next generation. This definition—that evolution occurs as individuals with variations suited to the environment, or advantageous for survival, survive—is based on Darwin’s theory of natural selection. This definition itself does not imply any directionality in evolution. To truly understand that evolution is not progress, we need to examine the entire evolutionary process as a whole. If evolution were progress, then when comparing humans—a vertebrate that has undergone a much longer evolutionary process since diverging from a common ancestor with the invertebrate squid—to the squid, humans should be in a more “perfect” state than squids.
However, when comparing the eyes of squids and humans, unlike squids, where the optic nerves are distributed behind a smooth retina, the human eye has optic nerves and blood vessels distributed in front of the retina, which pass through a hole in the retina to connect to the brain. This hole is the “blind spot,” which creates a blind spot in a person’s field of vision. This occurs because the structure of the human eye evolved from a form where the optic nerves were spread out over the retina to one where they are curled into a circle. To compensate for the shadow cast by the optic nerves and blood vessels that cover the area in front of the retina where images are formed, the entire eye vibrates minutely, which is why human eyes tire easily. Thus, the more evolved human eye does not possess a more perfect structure than that of a squid. This is because evolution is not a process leading toward perfection, but rather a process in which slightly better variations are selected based on the environment in which living organisms exist at a given time. In other words, at the time such a variation was selected, it was the best possible change and provided a survival advantage over other individuals; it was not a process leading toward the ultimate structure. A similar example is the fact that the human windpipe and digestive tract do not follow separate paths but share overlapping sections. While this structure is highly inefficient—as food entering the windpipe can easily lead to death—it was simply the best option available to organisms when lung respiration first emerged during the evolutionary process. It was not a process designed to create a windpipe distinct from the digestive tract, which we now consider a perfect structure.
Those who claim that evolution is progress base their argument on the fact that living organisms today have become increasingly structurally complex compared to when life first emerged. Of course, if we line up the fossil evidence, we can intuitively sense a sense of progress as we observe the long and majestic evolutionary process of life: from the simplest prokaryotic cells, through eukaryotic cells, to the emergence of multicellular organisms, the rise of invertebrates and vertebrates, and the appearance of life forms with exceptional intelligence, such as humans. However, the claim that an increase in structural complexity constitutes progress is based on the premise that organisms with higher structural complexity are “better”—that is, they possess structures that are more advantageous for survival. If this premise were true, nature should have selected organisms with structures better suited to their environment while eliminating those that were not. Yet the existence of bacteria—single-celled prokaryotes with very simple structures that are thriving even at this very moment—refutes this claim. If an increase in structural complexity were indeed progress, bacteria should have been gradually and progressively driven to extinction by eukaryotes from the moment they appeared. Yet, even today, an incredibly diverse array of bacteria thrives among higher vertebrates—organisms with structures far more complex than their own—and even infects and kills them while living openly. When comparing structural complexity, it is true that complex organisms like extant vertebrates evolved from organisms with simpler structures. However, this is merely the result of adaptation and natural selection; nature does not select with any specific goal or purpose in mind. The claim that evolution occurs in a direction that increases structural complexity can also be criticized based on the existence of viruses. Viruses possess only a protein shell and nucleic acid—a structure even simpler than that of bacteria. The scientists who first isolated viruses even speculated that, due to this simple structure, viruses might be the first life forms. However, since viruses cannot reproduce on their own and can only multiply through the metabolism of host cells, they are not the first life forms. In other words, it can be speculated that viruses adopted a simpler structure by choosing parasitism as a strategy through evolution. Evolution does not occur solely in the direction of increasing structural complexity.
The proposition discovered by Haeckel, “ontogeny recapitulates phylogeny,” is also sometimes cited as evidence that structural complexity increased through evolution and that this constitutes proof that evolution is progressive. The argument is that the embryos of vertebrates in their early developmental stages have relatively simple structures—even mammalian embryos possess gills at this stage—and the process by which increasingly complex organs emerge and the embryo finally develops into an individual resembles the phylogenetic evolutionary process by which today’s complex vertebrates arose from simpler life forms. However, the repetitive process of evolution observed in individual development does not necessarily serve as evidence that evolution is progressive. The increase in structural complexity in living organisms is due to their subjection to physicochemical laws. There exists a minimal, simple form that can be called a living organism, and since anything below that point is not considered a living organism, the structure can only either become more complex or maintain its current form as a result of evolution. Stephen J. Gould explains this using the “drunkard model.” Imagine a drunkard stumbling out of a bar, swaying to the right or left, with a ditch on the right and the bar’s wall on the left. Although the probability of the drunkard swaying to the right or left is equal, once he hits the bar’s wall, he can no longer move to the left; consequently, he ends up moving toward the ditch on the right and falls into it. Looking only at the result, it appears as though the drunkard moved with a directional bias toward the right, but this is an accidental outcome that arose inevitably due to the presence of the left wall. Heckel’s developmental argument can be refuted in a similar manner. Since embryos in the early stages of development are highly unstable, any mutation occurring during this phase would easily lead to the embryo’s death; thus, evolution has inevitably proceeded by adding new stages.
Regarding why the evolutionary process, which appears to be progressive, is not actually progress, it has been clarified that the evolutionary process is merely a selection of the best available conditions at the time, and when viewed in hindsight, it does not represent a change toward a perfect structure. Furthermore, the increase in structural complexity is a natural result of the passage of time and physicochemical laws, not a process with a deliberate direction. Nature does not make choices based on will. The fact that life appears to be progressing in certain respects is merely a coincidence, and the structures and functions of the diverse organisms that exist today are simply the result of natural selection. Hitler and eugenicists, who were devotees of evolutionary theory, viewed evolution as progress and believed that organisms possessing certain traits were superior to those without them, thereby inflicting indelible wounds on many people. As such, because certain definitions in science can have a significant impact on society, they must be approached with caution and precision. The definition of the relationship between evolution and progress is no exception.
