In this blog post, we will examine the origins of this obsession with the boundaries of objects and explore the reasons behind it from philosophical and biological perspectives.
We refer to a car as an object, but we do not refer to seawater as an object. What conditions must be met for a collection of parts to be called an object? First, similarity between parts cannot be a condition for objecthood. For example, identical twins have the same DNA sequence and appearance, but they are not the same object. Therefore, strong organic interactions between parts are often cited as the condition. The parts that make up an individual influence each other in a way that is incomparably stronger than the influence of external entities on the individual. For example, the cells that make up our bodies constantly communicate with each other and work in harmony to function as a single organism.
We can also ask what conditions are necessary for two objects existing at different times to be considered the same individual. The answer is causality between the two objects. The reason we can consider the past “I” and the present “I” to be the same is because of the existence of strong causality. The past “I” and the present “I” are causally connected through the process of cell division, where cells are replaced. Additionally, when “I” creates a new entity through cell division, ‘I’ and “my descendants” are causally connected. Although “I” and “my descendants” are not the same individual, they are connected by a stronger causality than “I” and other individuals. From this perspective, we can see that individuality must consider not only the current state but also changes and continuity over time.
These philosophical questions about individuality are also important research topics in biology. The unit that constitutes a living organism is the cell. Cells contain DNA, which carries the unique genetic information of living organisms, and reproduce and propagate by replicating this DNA, passing it on to future generations. Cells are divided into eukaryotic cells, such as those found in humans, and prokaryotic cells, such as those found in bacteria and archaea. Eukaryotic cells have a nucleus surrounded by a membrane in the cytoplasm, which contains DNA, while prokaryotic cells do not have a nucleus. Additionally, the cytoplasm of eukaryotic cells contains various organelles surrounded by membranes, among which mitochondria are organelles that produce the biological energy necessary for cellular activities. Most eukaryotic cells have mitochondria.
The theory that these mitochondria were originally a type of bacteria called prokaryotic mitochondria was proposed in the early 20th century. This theory, known as the endosymbiotic theory or endosymbiosis, explains that eukaryotic organisms with eukaryotic cells were born as a result of a symbiotic relationship between two prokaryotic organisms. Symbiosis refers to the coexistence of different organisms, and this also applies to “internal symbiosis,” where one organism lives inside another. The endosymbiotic theory was not widely accepted in the biological community for a long time. Although the functions and general structure of mitochondria were already known, and examples of endosymbiosis between organisms existed, it was difficult to believe that mitochondria had once been independent organisms. Additionally, in traditional genetics, which views the process of one organism passing on its traits through generations as driven by mutations and natural selection, leading to the evolution and diversification of species, the merger of two prokaryotic organisms did not receive much attention. However, with the advent of electron microscopes, it was discovered that mitochondria contain DNA different from that of the cell nucleus and possess their own ribosomes for protein synthesis, reigniting interest in the endosymbiotic theory.
According to the endosymbiotic theory, eukaryotic organisms originated from protoplasts that engaged in endosymbiosis within the cells of archaea. While there is debate over whether the formation of the archaea’s nucleus or the onset of endosymbiosis occurred first, the theory posits that archaea developed a nucleus in their cytoplasm to become eukaryotic cells, while protoplasts became mitochondria, organelles within cells, leading to the emergence of eukaryotic organisms. There are several pieces of evidence supporting the idea that mitochondria were originally a type of bacteria. Like bacteria, new mitochondria can only be formed through the “binary fission” of existing mitochondria. The membranes of mitochondria contain purines, a type of transport protein different from those found in the cell membranes of eukaryotic cells, and cardiolipin, a lipid found in the cell membranes of bacteria. Furthermore, the ribosomes of mitochondria are more similar to those of bacteria than to those of eukaryotic cells.
Even though mitochondria still replicate and proliferate with their own DNA, why is the relationship between mitochondria and eukaryotic cells not considered a symbiotic relationship? Even if two organisms cannot survive separately, if their organic interactions are not strong enough to cause them to lose their individuality, they are considered to be in a symbiotic relationship. However, the organic interactions between mitochondria and eukaryotic cells are so strong that they cannot be considered separate organisms. The evidence supporting the view that mitochondria have lost their individuality and become cell organelles includes the fact that eukaryotic cells regulate the proliferation of mitochondria and replicate them along with themselves during cell division. Additionally, a significant portion of mitochondrial DNA has been transferred to the cell nucleus, resulting in a markedly shorter mitochondrial DNA sequence. The proteins required for metabolic processes in mitochondria are synthesized from DNA in the cell nucleus, and most of the genes remaining in mitochondrial DNA are responsible for producing biological energy. For example, human mitochondria have only 37 genes, making their DNA very short.
The concept of individuality raises many interesting questions from both biological and philosophical perspectives. The process of defining and understanding individuality amid the complexity and diversity of living organisms plays an important role in our exploration of the essence of life itself. The study of individuality goes beyond mere theoretical interest, providing important insights across various academic fields such as genetics, evolutionary biology, and bioethics. Furthermore, such research plays an essential role in understanding the origin and evolution of living organisms, enabling us to gain a deeper understanding of their diversity and complexity.
