Known as the endosymbiont theory, it is an evolutionary biology hypothesis that attributes the development of higher life to the endosymbiosis of prokaryotes. The idea was first discussed by the botanist Schimper at the end of the 19th century. Meanwhile, many research results speak in favor of the theory.
What is the endosymbiont theory?
In the course of evolution, according to the endosymbiont theory, two organisms should have become interdependent, so that neither partner could survive without the other. The botanist Schimper first published the idea of the endosymbiont theory in 1883, and his work was intended to explain the origin of chloroplasts. The Russian evolutionary biologist Konstantin Sergeyevich Merezhkovsky revisited the endosymbiont theory in the early 20th century. However, the theory did not become well known until 1967, when it was taken up by Lynn Margulis. In simplified summary, the theory states that unicellular organisms were taken up by other unicellular organisms during evolution. This uptake is said to have made possible the development of cellular components of higher organisms. In this way, according to supporters of the theory, more and more complex life has developed in the course of evolution. Human cell components thus originally go back to unicellular organisms. According to the theory, eukaryotes therefore first arose because prokaryotic precursor organisms entered into symbioses. In particular, chemotrophic and phototrophic bacteria are thought to have been taken up by otherwise prokaryotic cells of the archaea in an act of phagocytosis. Instead of digesting them, the prokaryotic cells stored them inside, where they became endosymbionts. These endosymbionts are thought to have eventually evolved into cell organelles in host cells. Host cell and organelle within each correspond to eukaryotes. The cell organelles of mitochondria and plastids still bear features to this effect. Since eukaryotes also exist without these described organelles, these components must either have been lost phylogenetically or the theory does not apply.
Function and task
The endosymbiont theory names the development of mitochondria and plastids in prokaryotic organisms. The protozoa are thought to have entered into endosymbiosis with other cells and to have continued to live in the host cells. To date, science sees amoeboid protozoa ingest cyanobacteria and continue to live within them. Observations like these seem to support the endosymbiont theory. In the course of evolution, two organisms are thought to have become interdependent, according to the endosymbiont theory, so that neither partner could survive without the other. The resulting endosymbiosis is said to have caused the organelles to each lose parts of the genetic material they no longer needed. The individual protein complexes in the organelles are thus thought to have been composed partly of nuclear-encoded and partly of mitochondrial-encoded units. According to genome analyses, plastids originate from cyanobacteria, while mitochondria are associated with aerobic proteobacteria. Endosymbiosis between eukaryotes and prokaryotes is what scientists call primary endosymbiosis. On the other hand, if cell organelles have arisen from the uptake of a eukaryote with previously experienced primary endosymbiosis event, we are talking about secondary endosymbiosis. Primary plastids are located in two envelope membranes, which, according to the theory, are the same as the membranes of the respective ingested cyanobacterium. Three types of primary plastids, and thus three lineages of autotrophic organisms, are thought to have arisen in this way. Unicellular algae of the Glaucocystaceae, for example, contain plastids of the cyanobacterium, as do red algae. Green algae as well as higher plants contain the most developed plastids, the chloroplasts. Secondary plastids have three or four enveloping membranes. Secondary endosymbioses between green algae and eukaryotes are now known, so Euglenozoa and Chlorarachniophyta may have taken up primary endosymbionts independently.
Diseases and ailments
If the endosymbiont theory is correct, as the current state of research suggests, all complexes of plant, animal, and thus human cells originated in a fusion of prokaryotes. Humans would thus owe life itself to prokaryotes.However, prokaryotes in contact with humans are also responsible for numerous diseases. In this context, reference should be made, for example, to the disease value of the Proteobacteria, which are particularly relevant in the endosymbiont theory. Many bacteria from this division are considered pathogens. This is true, for example, of Helicobacter pylori, which is a rod-shaped bacterium that colonizes the human stomach. With a prevalence of 50 percent, Helicobacter pylori infection is often referred to as one of the most common chronic bacterial infections worldwide. More than 30 million people are infected with the bacterium, but only between ten and 20 percent of all infected people develop symptoms. These symptoms primarily include peptic ulcers, which can affect the stomach or duodenum. Infections with the bacterium are, on the whole, blamed for a whole range of gastric diseases, especially those that manifest themselves in increased secretion of gastric acid. Consequently, in addition to ulcers of the stomach and duodenum, the bacterium may probably also be involved in type B gastritis. Testing for bacterial infection with the proteobacterium is now part of the standardized diagnosis of gastric diseases. Apart from the diseases mentioned, chronic infection with the bacterium is now classified as a risk factor for gastric carcinoma. The same is true for MALT lymphoma. There also appears to be a link between infection and diseases such as idiopathic chronic urticaria (hives), chronic immune thrombocytopenia, iron deficiency anemia, and Parkinson’s disease. Helicobacter pylori has only been discussed here as an example. Numerous other prokaryotes are associated with disease value and are considered pathogens of humans, animals, and plants.
