Human curiosity has no limits. He has always needed to appease this need to know for everything around him, whether through science or faith. One of the great doubts that humanity has pursued is the origin of life. As a human being, wondering about existence, how it has become today, is a fact.
Science is no exception. There are many theories related to this idea. The theory of evolution o serial endosymbiosis theory are clear examples. The latter postulates how the current eukaryotic cells that make up the formation of animals and plants were generated.
Prokaryotic and eukaryotic cells
Before you begin, keep in mind what is a prokaryotic cell and a eukaryotic cell.
They all have a membrane that separates them from the outside. The main difference between these two types is that in prokaryotes there is no presence of membrane organelles and their DNA is free inside. The opposite happens to eukaryotes, which are full of organelles and the genetic material is limited to a region inside a barrier known as the nucleus. We must keep this data in mind, because endosymbiotic theory is based on the explanation of the appearance of these differences.
Also known as Serial Endosymbiosis Theory (SET), was postulated by American evolutionary biologist Lynn Margulis in 1967, to explain the origin of eukaryotic cells. It was not easy and he was repeatedly refused publication, because at that time he dominated the idea that eukaryotes were the result of gradual changes in the composition and nature of the membrane, so this new theory did not fit with the prevailing belief.
Margulis sought an alternative idea of the origin of eukaryotic cells, establishing that it was based on the progressive union of prokaryotic cells, where one cell phagocytes with others, but instead of digesting them, those that make them part. This would have given rise to the various organelles and structures of present-day eukaryotes. In other words, he talks about endosymbiosis, one cell is introduced into another, Obtain mutual benefits through a symbiotic relationship.
The theory of endosymbiosis describes this gradual process in three successive major incorporations.
1. First incorporation
In this step, a cell that uses sulfur and heat as an energy source (thermoacidophilic archaea) binds to a swimming bacteria (spirochete). This symbiosis would initiate the ability of certain eukaryotic cells to move thanks to the plague (such as sperm) and the appearance of the nuclear membrane, Which gave the DNA more stability.
Archaobacteria, although prokaryotic, are a different domain from bacteria and have been described in evolution as being closer to eukaryotic cells.
2. Second incorporation
An anaerobic cell, to which more and more oxygen in the atmosphere was toxic, needed help to adapt to the new environment. The second incorporation that is postulated is the union of aerobic prokaryotic cells within the anaerobic cell, explaining the appearance of peroxisome and mitochondrial organelles. The former have the ability to neutralize the toxic effects of oxygen (mainly free radicals), while the latter derive their energy from oxygen (respiratory chain). With this step, it would appear in the animal eukaryotic cell and fungi (fungi).
3. Third incorporation
The new aerobic cells, for some reason, performed endosymbiosis with a prokaryotic cell that had the ability to photosynthesize (get energy from light), giving rise to the organelle of plant cells, the chloroplast. With this latest addition, it is given the origin of the plant kingdom.
In the last two additions, the introduced bacteria would benefit from the protection and absorption of nutrients, while the host (eukaryotic cell) would gain the ability to use oxygen and light respectively.
Proof and contradictions
Today, the endosymbiotic theory is partially accepted. There are points where they were in favor, but others which give rise to a lot of doubts and discussions.
The clearest thing is that mitochondria and chloroplasts have their own circular double stranded DNA inside freely, regardless of nuclear. Something striking, because they remind prokaryotic cells for their configuration. In addition, they behave like a bacterium, as they synthesize their own proteins, use ribosomes from the 70s (not ribosomes from the 80s like eukaryotes), develop their functions across the membrane and replicate their DNA, and perform binary fission to divide (and no mitosis).
The proof is also found in its structure. The mitochondria and the chloroplast have a double membrane. This could be due to its origin, the interior being the membrane itself that surrounded the prokaryotic cell and the outer vesicle during phagocytosis.
The biggest point of criticism is the first incorporation. There is no evidence to show that this junction between cells existed, and without samples it is difficult to maintain. The appearance of other organelles is also not explained eukaryotic cells, such as the endoplasmic reticulum and the Golgi apparatus. And the same goes for peroxisomes, which have neither their own DNA nor a double layer of membranes, so there are no samples as reliable as in mitochondria or chloroplast.