In the 1970s, the symbiosis hypothesis was forcefully reproposed by Lynn Margulis, and within a few years it received the support of an astonishing number of experimental discoveries. Mitochondria and chloroplasts are still carrying fragments of their ancient circular DNA, and have 70s ribosomes which are typical of bacteria, all of which leaves little doubt about their origin. It is practically certain, therefore, that mitochondria and chloroplasts were acquired by symbiosis during cellular evolution, and paleontology even allows us to establish that this happened around 1500 million years ago, because it is only after that period that geological strata show fossilized cells that are large enough to contain intracellular organelles.
All this, however, tells us nothing about the cells that acquired orgenelles by symbiosis, and on this point biologists are divided in two opposing camps. Some maintain that the cells which engulfed bacteria were themselves bacteria, an hypothesis which leads to two precise conclusions: (1) the first living cells were bacteria, and (2) eukaryotes are chimeras of bacteria (the bacterial theory of life).
Other biologists are convinced that the acquisition of bacteria by symbiosis required characteristics that do not exist in bacteria, which means that both the direct ancestor of eukaryotes and the common ancestor of bacteria and euraryotes could not have been bacteria. As anyone can see, the disagreement is about two major points: (1) the nature of the first cells, and (2) the origin of eukaryotes. It is important therefore to understand if the first cells which appeared on Earth were bacteria, as maintained by the bacterial theory of life, or cells which had the potential to generate both bacteria and ancestral eukaryotes.

Three primary kingdoms

All living cells contain ribosomal RNAs, and it seems that these nucleic acids have changed very little in the course of evolution because their structures are similar in all organisms.