The criterion for individuality is strong organic interaction, not similarity between parts, and the condition for judging two entities from different time periods as identical is causality. Biological research uses this concept of individuality to provide important insights into understanding the origin and evolution of life.
We call a car an entity, but we don’t call a body of water an entity. When parts come together to form an entity, what are the conditions under which we can call it an entity? First of all, similarity between parts is not a condition for individuality. For example, two identical twins have the same DNA sequence and physical appearance, but they are not identical individuals. Therefore, strong organic interaction between the parts is often suggested as a condition. The parts that make up an entity influence each other in ways that are far stronger than the influence of external entities on the entity. For example, the cells in our bodies are constantly communicating with each other and working in harmony to function as an organism.
We can also ask under what conditions two objects existing at different times are judged to be the same entity. It is the causality between the two objects. The past ‘me’ and the present ‘me’ can be considered the same because there is a strong causality between them. The past ‘me’ and the present ‘me’ are causally connected through the process of cell division and cell replacement. Also, ‘me’ and ‘my descendants’ are causally connected when ‘me’ creates a new individual through cell division. Although “me” and “my descendants” are not the same entity, they are causally connected in a stronger way than between “me” and other entities. From this perspective, we can see that individuality is not just about how things are now, but also about change and continuity over time.
This philosophical question of individuality is also an important topic of research in biology. The unit of life is the cell. Cells have DNA, which contains the unique genetic information of an organism, and they pass their DNA on to the next generation by replicating, multiplying, and reproducing. Cells are divided into eukaryotic cells, which are eukaryotic organisms like humans, and prokaryotic cells, which are prokaryotic organisms like bacteria and archaea. Eukaryotic cells have a membrane-enclosed nucleus in their cytoplasm that contains DNA, while prokaryotic cells do not have a nucleus. The cytoplasm of eukaryotic cells also contains several types of membrane-enclosed organelles, among them mitochondria, which produce the bioenergy needed for cellular activity. Most eukaryotic cells have mitochondria as an essential part of their cellular organization.
In the early 20th century, it was theorized that these mitochondria were originally proto-mitochondria, a type of bacteria. This theory, called symbiogenesis or intracellular symbiosis, explains that the symbiotic relationship between the two prokaryotes led to the creation of eukaryotes with eukaryotic cells. Symbiosis is when different organisms live together, and the assumption of different organisms is also true in “endosymbiosis,” where one organism lives inside the cell of another. The idea of abiogenesis was not recognized by the biological community for some time. The function and approximate structure of mitochondria and examples of internal symbiosis between organisms were already known, but it was not easily believed that mitochondria had once been independent organisms, and the combination of two prokaryotes was not noted in traditional genetics, which views species as evolving and differentiating through mutations and natural selection over the course of generations. However, with the advent of electron microscopy, the theory of symbiogenesis came to the forefront when it was discovered that mitochondria have DNA that is different from the DNA in the cell nucleus and have their own ribosomes that synthesize proteins.
According to this theory, eukaryotes arose as a result of the internal symbiosis of protozoan mitochondria with the cells of archaea. There is some debate as to which came first, the formation of the archaeal nucleus or the start of the internal symbiosis, but the idea is that the archaea became eukaryotic cells with a nucleus in the cytoplasm, and the protozoan mitochondria became mitochondria, a cell organelle, and eukaryotes were born. There are several lines of evidence suggesting that mitochondria were originally a type of bacteria. Like bacteria, new mitochondria can only be made by “binary fission” of existing mitochondria. The mitochondrial membrane has a different type of transporter protein, furin, than the eukaryotic membrane, and cardiolipin, which is present in the cell membrane of bacteria. Mitochondrial ribosomes are also more similar to bacterial ribosomes than eukaryotic ribosomes.
Since mitochondria still replicate and multiply with their own unique DNA, why is the relationship between mitochondria and eukaryotic cells not considered symbiotic? Even if two organisms cannot live apart from each other, if their organic interactions are not so strong that they lose their individuality, they are symbiotic, because the organic interactions between mitochondria and eukaryotic cells are so strong that they cannot be seen as separate entities. The evidence that mitochondria have lost their individuality and become organelles is that eukaryotic cells control the proliferation of mitochondria, and when they replicate and multiply, they also replicate and multiply their mitochondria. In addition, many of the genes in mitochondria have been transferred to the DNA of the cell nucleus, resulting in significantly shorter mitochondrial DNA. The proteins needed for metabolic processes in the mitochondria are synthesized from the DNA in the cell nucleus, and most of the genes that remain in the mitochondrial DNA are responsible for producing bioenergy. Human mitochondria, for example, have a short DNA length 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 life plays an important role in our exploration of the nature of life itself. Studying individuality is more than just of theoretical interest; it provides important insights across a variety of disciplines, including genetics, evolutionary biology, and bioethics. It also plays an essential role in understanding the origin and evolution of life, allowing us to gain a deeper appreciation for the diversity and complexity of life.
