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Scientists think that the protobionts are the evolutionary precursors of prokaryotic cells. Protobionts may be originated as an array of microspheres of diverse organic and inorganic compounds enclosed by lipidic membranes. Proteins, carbohydrates, lipids, and other organic substances were the most important autocatalytic organic compounds. Water was a very important factor in the assembly of the protobionts' endoplasm. After this event, several microspheres could self-organize into organelles that were able to perform specific functions; for example, lysosomes, peroxysomes, vacuoles, etc.
Gradually, some segments of the external membrane would invaginate for forming membranous organelles, like endoplasmic reticulum and Golgi apparatus. First protobionts would not have a nucleus membrane (nuclear envelope); consequently, they could be identified like prokaryotes.
Mitochondria and chloroplasts could develop soon after as vagile and self-sufficient protobionts, which specialized in obtaining energy directly from the environment.
Mitochondria would be heterotrophic protobionts (with their own DNA), which obtained energy from the organic molecules that were dissolved in large quantities in the immediate environment (chemiosmotic organisms). Some mitochondria would be engulfed by other larger protobionts. Possibly, the earliest mitochondria could be used as food by other protobionts, but some of them could not be processed as food, but survived living as symbionts into more complex protobionts. Progressively, the functional relationship would be more vital for both mitochondria and protobionts, until they could not omit one to another. This could be the theory about the origin of the first prokaryotic heterotrophic protists (e.g. Archaea and Bacteria).
The same process could have happened with chloroplasts, which would be chemoautotrophic or chemiosmotic protobionts. At present, chemoautotrophic organisms are able to get energy from organic matter from the environment, as well as to transform radiant energy (transported by photons) into chemical energy (in molecular bonds) by the action of chlorophyll. Some protobionts would incorporate chloroplasts to their endoplasm, but by some self-defense mechanism, some chloroplasts also would survive in the endoplasm of more complex protobionts. Comparable with mitochondria, chloroplasts would become a vital part of those protobionts, in which they would live as symbionts. Such protobionts could not live without chloroplasts and the chloroplasts could not survive outside their hosts. The first autotrophic prokaryotic protists could originate from this way (such as cyanobacteria and sulfur bacteria).
From the end of the preceding century, many biologists have been considering that RNA was the earlier nucleic acid in protobionts instead DNA because when the environment was too hot, the enzymes for the synthesis of DNA could not work appropriately, and DNA is unsteady at high temperatures. Scientists think that the Earth was extremely hot when protobionts got assembled. The biologists that think that the RNA was the nucleic acid of the early protobionts assume that when the environmental conditions were more propitious, the molecules of RNA could build DNA molecules. They think that RNA was competent to produce autocatalytic and non-autocatalytic proteins and that some autocatalytic proteins would help to the self-synthesis of RNA molecules. However, with the current knowledge about the physicochemical properties of nucleic acids and considering the impossibility of an inorganic synthesis of RNA (the processes have been always forced by an external operator). That's why I think that both hypotheses, the DNA world and the RNA world, are unrealistic; not only in the range of the early protobionts, but in the whole assortment of organic compounds spontaneously synthesized on Earth. The experimentation suggest that all depended on the synthesis of autocatalytic proteins, which reproduced through a progression equivalent to the reproduction of prions at present, without the involvement of nucleic acids.
logist Nasif Nahle Sabag