In this blog post, we will look at where the obsession with the boundaries of objects comes from and why from a philosophical and biological perspective.
We say that a car is an object, but we don’t say that seawater is an object. What are the conditions for a collection of parts to be called an object? First of all, similarity between parts cannot be a condition for objecthood. For example, identical twins are identical in DNA base sequence and appearance, but they are not identical individuals. Therefore, strong organic interactions between parts are often suggested as a condition. The parts that make up an individual affect each other in a way that is incomparably stronger than the external existence affects the individual. For example, each cell that makes up our body constantly communicates with each other and functions as a single organism in harmony.
One may also ask about the conditions under which two objects existing at different times are judged to be the same entity. It is the causal relationship between the two objects. The reason why the past “me” and the present “me” can be seen as the same is because of the strong causal relationship between them. The past “me” and the present “me” are causally connected through the process of cell division, in which cells are replaced. In addition, when “I” create a new entity through cell division, “I” and “my descendants” are causally connected. Although “I” and “my descendants” are not the same entity, they are connected by a stronger causal relationship than between “I” and other entities. From this perspective, it can be seen that individuality should not only consider the current state but also take into account changes over time and continuity.
This philosophical question of individuality is also an important research topic in biology. The unit that makes up living organisms is the cell. Cells contain DNA that contains the unique genetic information of living organisms, and they pass on their DNA to future generations through the process of replicating and multiplying. Cells are divided into eukaryotic cells of eukaryotic organisms such as humans and prokaryotic cells of prokaryotic organisms such as bacteria and archaea. Eukaryotic cells have a membrane-enclosed nucleus in their cytoplasm and DNA inside it, whereas prokaryotic cells do not have a nucleus. In addition, the cytoplasm of eukaryotic cells contains various membrane-enclosed organelles, among which mitochondria are organelles that produce the bioenergy 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, called the symbiotic theory or the theory of intracellular symbiosis, explains that eukaryotes with eukaryotic cells were born as a result of a symbiotic relationship between two prokaryotes. Symbiosis refers to the coexistence of different organisms, and the assumption of different organisms is also true of “internal symbiosis,” in which different organisms coexist within the cells of one organism. The theory of symbiotic evolution was not recognized by the biological community for a while. The function and general structure of mitochondria, as well as the cases of internal symbiosis between organisms, were already known, but it was difficult to believe that mitochondria were once independent organisms. And the combination of the two prokaryotes did not attract attention in traditional genetics, which views the process of a single organism passing from one generation to the next as one in which mutations and natural selection occur, and as a result, species evolve and diverge. Then, with the advent of electron microscopes, it was discovered that mitochondria have their own DNA, different from the DNA of the cell nucleus, and their own ribosomes to synthesize proteins, which brought the theory of symbiotic evolution to the fore.
According to the theory of symbiotic origin, eukaryotes are believed to have originated from the internal symbiosis of archaeal mitochondria within the cells of archaea. There is some debate as to whether the formation of the archaeal nucleus or the beginning of internal symbiosis came first, but archaea became eukaryotic cells with the appearance of nuclei in their cytoplasm, and the archaeal mitochondria became organelles called mitochondria, which is how eukaryotes were born. There are many reasons why mitochondria were originally a type of bacteria. Like bacteria, new mitochondria are created only through “mitosis” of existing mitochondria. The membrane of mitochondria contains purines, a different type of transport protein than the transport proteins in eukaryotic cell membranes, and cardiolipin, which is found in the cell membranes of bacteria. In addition, the ribosomes of mitochondria are more similar to those of bacteria than those of eukaryotic cells.
Why is the relationship between mitochondria and eukaryotic cells not seen as a symbiotic relationship, even though mitochondria still replicate and proliferate with their own DNA? Even if two organisms cannot live apart from each other, if the organic interaction is not strong enough to lose their individuality, they are considered to be in a symbiotic relationship, because the organic interaction between mitochondria and eukaryotic cells is so strong that the two cannot be seen as different individuals. The evidence that mitochondria have lost their individuality and become cell organelles is that eukaryotic cells regulate the proliferation of mitochondria and when they replicate and proliferate, they also replicate and proliferate the mitochondria. In addition, a large portion of the genes in the mitochondria have been transferred to the DNA of the cell nucleus, and the length of the DNA in the mitochondria has been significantly shortened. Proteins required for the metabolic process in mitochondria are synthesized from DNA in the cell nucleus, and most of the genes remaining in the DNA of mitochondria are responsible for producing bioenergy. For example, the DNA of human mitochondria is short, with only 37 genes.
The concept of individuality raises many interesting questions from a biological and philosophical perspective. The process of defining and understanding individuality in the complexity and diversity of living organisms plays an important role in exploring the nature of life itself. Studying individuality goes beyond theoretical interest and provides important insights across various academic fields, including genetics, evolutionary biology, and bioethics. In addition, such research plays an essential role in understanding the origin and evolution of life and allows us to gain a deeper understanding of the diversity and complexity of life.
