In this blog post, we’ll explore the scientific debate and various perspectives on whether life is evolving toward increasing complexity or has simply diversified into various forms while adapting to the environment.
Bernard Werber’s ‘The Tree’ features a short story about how humans might evolve in the future. The story imagines that, in an effort to reduce the time spent forming relationships, humans become creatures possessing both male and female characteristics, eventually evolving into massive, immobile, tree-like life forms. Reflections on the direction of human evolution are a recurring theme in literature, art, and science. So, does a directionality of evolution exist for all living things? Do living organisms become more complex and “progress” as time passes?
In ‘Darwin’s Table’, Stephen Jay Gould and Richard Dawkins debate whether the history of life has progressed in a specific direction through evolution. Gould argues that evolution cannot be equated with progress. He explains that the very act of linking the concept of “progress” to “evolution” is rooted in social influences. For example, he argues that the competition-centered values of the Victorian era, at the time Darwin’s theory of evolution was published, influenced how the theory was culturally received.
In response, Dawkins counters that science is a discipline that explains actual phenomena in the natural world, and that natural selection operates according to the same principles regardless of social context. In other words, he maintains that natural selection influences the evolution of organisms independently of the values of human society.
The oldest traces of life known to date are estimated to be microbial activity from approximately 3.5 to 3.7 billion years ago, and it is generally accepted that early life forms were prokaryotes similar to bacteria. Comparing these simple early life forms to humans today reveals that life has taken on far more diverse and complex forms. Dawkins views this fact as supporting the intuitive judgment that life has increased in complexity over time.
Gould, on the other hand, argues that evolution is a process of increasing diversity, not increasing complexity. He points out that even today, most organisms retain relatively simple forms, such as bacteria and archaea, and that the increase in complexity observed in certain lineages does not imply a universal directionality for life as a whole. In other words, he argues that it is difficult to conclude that average complexity has steadily increased.
I would first like to examine the question: “Has the history of life evolved toward greater complexity?” While Dawkins cites the fact that humans are far more complex organisms than the bacteria of the past as evidence, what matters is whether the majority of organisms—not just a few—have collectively shown a trend toward increasing complexity. Applying the concepts of average or mode, a small number of highly complex lineages may skew the overall average upward, but this does not imply a general evolutionary direction for all organisms.
From a taxonomic perspective, organisms are classified into three domains: bacteria, archaea, and eukaryotes. Even today, bacteria and archaea maintain their prokaryotic form, account for a vast number of species and individuals, and serve as vital components of Earth’s ecosystems. In contrast, humans are merely a single species belonging to an extremely small lineage within the eukaryotes. Therefore, when considering the state of the majority of organisms, the increase in complexity centered on humans cannot be viewed as representative of the evolutionary direction of life as a whole.
If life were trending toward greater complexity, bacteria, too, should have continuously evolved into more complex forms. However, most bacteria continue to thrive today, maintaining their ecological niches without a significant increase in complexity. This cannot serve as evidence that the type of evolution seen in humans is necessarily superior or the direction all life should pursue.
The case of tapeworms is even more intriguing.
In the process of adapting to a parasitic lifestyle within the bodies of animals, including humans, tapeworms have evolved in a direction where the organs necessary for movement and sensory organs have been simplified. In other words, complexity has not increased but rather decreased. In contrast, other worms have retained their legs while adapting to various environments, diversifying into different species. Thus, evolution is not an end in itself—whether through an increase or decrease in complexity—but rather a process of adapting to the environment, resulting in the expansion of species diversity.
Judging organisms with low complexity as inferior may be an anthropocentric way of thinking. Bacteria play pivotal roles in Earth’s ecosystems—such as nitrogen fixation, organic matter decomposition, and material cycling—and possess remarkable adaptability, enabling them to survive in extreme environments like deep-sea hydrothermal vents, highly acidic conditions, and high-salinity environments. Even today’s science and technology cannot fully replicate or replace the myriad biological capabilities possessed by bacteria.
Organisms with low complexity have the advantage of being able to adapt relatively quickly to environmental changes due to their simple structure. In contrast, organisms with complex tissues and organs have many physiological functions to maintain and internal variables to regulate, making them relatively vulnerable to sudden environmental changes. Of course, complex organisms also respond to environmental changes through various adaptive strategies, but complexity is not always advantageous for survival.
I do not intend to portray bacteria as particularly superior organisms. Rather, I wish to emphasize that humans cannot be viewed as absolutely superior beings either, and that each organism has evolved into its current form as a result of adapting to different environments. There is no inherent reason why complexity must increase, and organisms have continuously expanded their diversity through the process of adapting to and diversifying in response to environmental changes.
To argue that evolution proceeds in a specific direction toward greater complexity, one must be able to predict the future course of evolution to some extent based on complexity. However, predicting the direction of evolution—including that of entire ecosystems—is, in practice, extremely difficult. This is because ecosystems are complex systems in which countless organisms and environmental factors interact with one another, and even small changes can lead to unexpected, far-reaching consequences.
I would like to ask both Dawkins and Gould what exactly they mean by “biological progress,” as mentioned in ‘Darwin’s Table’. Generally, this term brings to mind the development of civilization, high intelligence, or complex communication abilities; however, such criteria are likely a limited, anthropocentric perspective. Rather than asserting that one organism is more “advanced” than another, it is more appropriate to understand that they have developed different characteristics as a result of adapting to different environments.
True “progress” cannot be defined by the superiority of a specific species; it must be viewed from the perspective of the entire ecosystem. In the early days of Earth, when only simple life forms such as bacteria existed, the structure of the ecosystem was likely simple and vulnerable to changes in the external environment. Subsequently, as diverse organisms emerged and species diversity increased, the ecosystem’s food webs and material cycles became more complex, and its ability to absorb external shocks and recover also grew. From this perspective, biological progress can be understood not as the complexity of a specific organism, but as a process in which the stability and resilience—that is, the restorative capacity—of the entire ecosystem improves.
In conclusion, the history of life can be viewed not as a process in which complexity increased in a single direction, but rather as a process in which various species emerged and coexisted while adapting to the environment. As diversity accumulates, the ecosystem’s network becomes denser, and its ability to respond to external shocks improves. If we reexamine the meaning of “progress” from the perspective of the ecosystem as a whole—moving beyond an anthropocentric viewpoint—the debate between Dawkins and Gould could lead to a richer and more productive discussion.
