This blog post explores the concept and meaning of evolution biologically, centered on the question: ‘Is evolution progress, or simply adaptation?’
It would not be an exaggeration to say that Charles Darwin’s assertion of the theory of evolution revolutionized how we view living organisms. However, unlike how we biologically associate the term ‘evolution’ with Charles Darwin today, Darwin himself did not use the word ‘evolution’ when he first wrote ‘On the Origin of Species’. He likely avoided it because the word carried connotations of ‘progress’, which might lead readers to believe life changes towards some goal. Yet even today, questions persist about whether evolution truly constitutes progress. Viewing evolution as progress carries the risk of leading to eugenics or species supremacy. So let us argue why, biologically speaking, evolution is not progress.
The notion of evolution as progress is rooted in the Western natural philosophy of the ‘great chain of being,’ a concept dating back to Aristotle. This view posited that life forms existed in a hierarchical ladder: simple, lower animals at the bottom, ascending through higher animals to humans, and ultimately to the divine. Furthermore, because the word ‘evolution’ is commonly used in everyday language to mean gradual development, it leads to the idea that biological evolution also moves in a specific direction—towards perfection, or progress, in extreme cases. However, ‘evolution’ in its biological definition refers to the change in the distribution of traits within a specific biological population, where variations suited to the environment are passed on to the next generation. This definition, based on Darwin’s theory of natural selection, states that evolution occurs as individuals possessing variations advantageous for survival thrive. This definition itself contains no notion of directionality in evolution. To truly understand that evolution is not progress, one must examine the entire evolutionary process as a whole. If evolution were progress, then comparing humans—a vertebrate that diverged from a common ancestor with the invertebrate squid and underwent more evolutionary steps—to squid, humans should be in a more perfect state than squid.
However, comparing the eyes of squid and humans reveals a difference: unlike squid, whose optic nerves are distributed behind the smooth retina, the human eye has its optic nerve and blood vessels distributed in front of the retina. These pass through a hole in the retina to connect to the brain. This hole is the ‘blind spot,’ which creates a portion of the human visual field that cannot be seen. This occurs because the human eye evolved from a structure where the optic nerves were initially spread over the retina, gradually curling inward. To compensate for the shadows cast by the optic nerves and blood vessels obscuring the front of the retina where images are formed, the entire eye vibrates minutely. This constant vibration is why human eyes tire easily. Thus, the more evolved human eye does not possess a more perfect structure than the squid’s eye. This is because evolution is not a process toward perfection, but rather the selection of slightly better mutations within the environment the organism inhabits at the time. In other words, at the time such a mutation was selected, it was the best change available and provided a survival advantage over other individuals; it was not a step toward the ultimate structure. A similar example is the human airway and digestive tract, which share overlapping sections rather than separate pathways. This structure is highly inefficient, as food entering the airway can lead to death. However, during the evolutionary process when pulmonary respiration emerged, it was the best method available to the organism at the time. It was not a process of creating a perfect airway distinct from the digestive tract, as we might consider ideal today.
Those who argue that evolution represents progress base their claim on the fact that existing life forms have become structurally more complex compared to when life first emerged. Certainly, when examining fossil evidence in sequence, one can intuitively perceive a sense of progress: witnessing the long and majestic evolutionary journey from the simplest prokaryotic cells, through eukaryotic cells, to the emergence of multicellular organisms; the rise of invertebrates and vertebrates; and finally, the appearance of life forms possessing remarkable intelligence, such as humans. However, the claim that increased structural complexity equates to progress rests on the premise that organisms with higher structural complexity possess superior structures, meaning they are better suited for survival. If this premise held true, nature should have selected organisms with structures better adapted to their environment, eliminating those that were not. Yet, the very existence of bacteria—single-celled prokaryotes with extremely simple structures that thrive vigorously even at this very moment—refutes this claim. If increased structural complexity equates to progress, bacteria should have been temporarily and gradually eliminated by eukaryotes upon their emergence. Yet, an immense variety of bacteria persist today, thriving alongside—and even infecting and killing—higher vertebrates possessing vastly more complex structures. Comparing structural complexity, complex organisms like extant vertebrates clearly evolved from simpler ones. Yet this is merely the result of adaptation and natural selection; nature selects without any goal or purpose. The claim that evolution occurs in the direction of increasing structural complexity can also be criticized based on the existence of viruses. Viruses possess an even simpler structure than bacteria, consisting only of a protein shell and nucleic acid. Scientists who first isolated viruses sometimes thought they might be the earliest life forms due to this simplicity. However, viruses cannot reproduce on their own and can only multiply through the metabolism of host cells; thus, they are not the earliest life forms. That is, we can speculate that viruses likely adopted a simpler structure through evolution by choosing a parasitic lifestyle. Evolution does not solely proceed in the direction of increasing structural complexity.
Some also argue that Haeckel’s proposition, “ontogeny recapitulates phylogeny,” demonstrates that structural complexity increased through evolution, serving as evidence that evolution is progress. The early embryos of vertebrates possess relatively simple structures. Even mammalian embryos at this stage have gills, and the process by which increasingly complex organs develop until a complete individual emerges resembles the phylogenetic evolutionary process by which today’s complex vertebrates arose from simpler organisms. However, the repetitive evolutionary process within individual development does not necessarily provide grounds for the claim that evolution is progress. The increase in structural complexity of life forms is due to their subordination to physical and chemical laws. Since the simplest form capable of being called life exists, and anything below that point is not life, the result of evolution can only be either an increase in complexity or the maintenance of that structure. Stephen J. Gould explains this using the Drunkard’s Model. Imagine a drunken reveler staggering out of a bar, swaying either right or left. To the right lies a ditch, and to the left is the bar’s wall. The probability of the reveler swaying right or left is equal. However, if the reveler collides with the bar wall, they can no longer move left. Consequently, they inevitably move toward the ditch on the right and fall in. Looking only at the outcome, it appears the drunkard moved with a directional bias toward the right. However, this is an accidental result forced by the existence of the left wall. Heckel’s claims about development can similarly be refuted. Early embryos are highly unstable; if mutations occur during the initial stages of development, the embryo easily dies. Consequently, evolution has proceeded by adding new stages out of necessity.
Regarding why the evolutionary process, which appears progressive, is not actually progress, it has been clarified that evolution is merely a process of selecting the best conditions available at the time, not a change toward a perfect structure when viewed over time. Furthermore, the increase in structural complexity is a natural consequence of the passage of time and physical-chemical laws, not the result of any intentional direction. Nature does not choose with will. The apparent progress of life in any aspect is merely coincidental; the structures and functions of the diverse organisms that exist today are simply the products of natural selection. Hitler and eugenicists, who were adherents of evolutionary theory, interpreted evolution as progress and believed organisms possessing certain traits were superior to those lacking them, inflicting indelible wounds on many people. Thus, because certain definitions in science can profoundly impact society, they must be approached with caution and precision. The definition of the relationship between evolution and progress is no exception.
