It is believed that everything is very harmonious in the human body, but even now there are completely foreign elements in our cells.
Mitochondria float in the mucus of your cells and tirelessly make molecules that move your body. But before, these mitochondria did not depend on your body: they were bacteria in the outside world. Therefore, at a fundamental level, you are the result of symbiosis, the result of the interdependence of two life forms.
Mitochondria produce energy for our body. Floating in the cell cytoplasm inside the cell, but outside the cell nucleus, they work to produce adenosine triphosphate, or ATP. ATP is used as a source of energy, which is generated when a substance is broken down into its individual components. You need energy all the time, and the mitochondria provide it. These organelles are so important that they even participate in the fertilization process. Mitochondria float in unfertilized eggs, waiting for a sperm.
Mitochondria have their own DNA, which is called mitochondrial, or mtDNA. And mitochondria can reproduce themselves completely independently of any deoxyribonucleic processes occurring inside the cell nucleus. Since mitochondria have their own DNA and do not need fertilization when it comes to reproduction, you are a clone of your mother. In your cells, her mtDNA, and her mother's mtDNA, and her mother's mother's mtDNA, and so on. That's why mtDNA is of such interest to genetic scientists. Sometimes it can mutate, but everything happens differently from normal DNA inside the cell nucleus. The fact that mitochondria have separate DNA is, of course, strange. But this circumstance pales in comparison with the shape of this DNA. Mitochondrial DNA is round. This is a spiral that is twisted into a ring. Mitochondria seem even weirder when you look at the barrier between them and the rest of the cell. Mitochondria differ from other organelles in that they have a double shell. Circular DNA, double cell walls, and the fact that mtDNA is completely different from the DNA of the cell nucleus, makes mitochondria look more like not human cells, but some bacteria that have donned their own diving suits, consisting of double cell membranes, and plunged into our cells. Scientists now believe that this is exactly what happened in the process of evolution. This process is explained in detail by the modern endosymbiotic theory, which clearly proves that our cell could at some point "shelter" a parasitic bacterium, but did not absorb it, but began to use it as an energy source.