Endosymbiotic Theory Definition

Endosymbiotic theory is the unified and widely accepted theory of how organelles arose in organisms, differing prokaryotic organisms from eukaryotic organisms. In endosymbiotic theory, consistent with general evolutionary theory, all organisms arose from a single common ancestor. This ancestor probably resembled a bacteria, or prokaryote with a single strand of DNA surrounded by a plasma membrane. Throughout time, these bacteria diverged in form and function. Some bacteria acquired the ability to process energy from the environment in novel ways. Photosynthetic bacteria developed the pathways that enabled the production of sugar from sunlight. Other organisms developed novel ways to use this sugar is oxidative phosphorylation, which produced ATP from the breakdown of sugar with oxygen. ATP can then be used to supply energy to other reactions in the cell.

Both of these novel pathways led to organisms that could reproduce at a higher rate than standard bacteria. Other species, not being able to photosynthesis sugars or break them down through oxidative phosphorylation, decreased in abundance until they developed a novel adaptation of their own. The ability of endocytosis, or to capture other cells through the enfolding of the plasma membrane, is thought to have evolved around this time. These cells now had the ability to phagocytize, or eat, other cells. In some cells, the bacteria that were ingested were not eaten, but utilized. By providing the bacteria with the right conditions, the cells could benefit from their excessive production of sugar and ATP. One cell living inside of another is called endosymbiosis if both organisms benefit, hence the name of the theory. Endosymbiotic theory continues further, stating that genes can be transferred between the host and the symbiont throughout time.

This gives rise to the final part of endosymbiotic theory, which explains the variable DNA and double membranes found in various organelles in eukaryotes. While the majority of cell products start in the nucleus, the mitochondria and chloroplast make many of their own genetic products. The nucleus, chloroplasts, and mitochondria of cells all contain DNA of different types and are also surrounded by double membranes, while other organelles are surrounded by only one membrane. Endosymbiotic theory postulates that these membranes are the residual membranes from the ancestral bacterial endosymbiont. If a bacteria was engulfed via endocytosis, it would be surrounded by two membranes. The theory states that these membranes survived evolutionary time because each organism retained the maintenance of its membrane, even while losing other genes entirely or transferring them to the nucleus. Endosymbiotic theory is supported by a large body of evidence. The general process can be seen in the following graphic.

Serial endosymbiosis

Endosymbiotic Theory Evidence

The most convincing evidence supporting endosymbiotic theory has been obtained relatively recently, with the invention of DNA sequencing. DNA sequencing allows us to directly compare two molecules of DNA, and look at their exact sequences of amino acids. Logically, if two organism share a sequence of DNA exactly, it is more likely that the sequence was inherited through common descent than the sequence arose independently. If two unrelated organisms need to complete the same function, the enzyme they evolve does not have to look the same or be from the same DNA to fill the same role. Thus, it is much more likely that organisms who share sequences of DNA inherited them from an ancestor who found them useful.

This can be seen when analyzing the mitochondrial DNA (mtDNA) and chloroplast DNA of different organisms. When compared to known bacteria, the mtDNA from a wide variety of organisms contains a number of sequences also found in Rickettsiaceae bacteria. Fitting with endosymbiotic theory, these bacteria are obligate intracellular parasites. This means they must live within a vesicle of an organism that engulfs them through endocytosis. Like bacterial DNA, mtDNA and the DNA in chloroplasts is circular. Eukaryotic DNA is typically linear. The only genes missing from the mtDNA and those of the bacteria are for nucleotide, lipid, and amino acid biosynthesis. An endosymbiotic organism would lose these functions over time, because they are provided for by the host cell.

Further analysis of the proteins, RNA and DNA left in organelles reveals that some of it is too hydrophobic to cross the external membrane of the organelle, meaning the gene could never get transferred to the nucleus, as the cell would have no way of importing certain hydrophobic proteins into the organelle. In fact, chloroplasts and mitochondria have their own genetic code, and their own ribosomes to produce proteins. These proteins are not exported from the mitochondria or chloroplasts, but are needed for their functions. The ribosomes of mitochondria and chloroplasts also resemble the smaller ribosomes of bacteria, and not the large eukaryotic ribosomes. This is more evidence that the DNA originated inside of the organelles, and is separate completely from the eukaryotic DNA. This is consistent with endosymbiotic theory.

Lastly, the position and structure of these organelles lends to the endosymbiotic theory. The mitochondria, chloroplasts, and nuclei of cells are all surrounded in double membranes. All three contain their DNA in the center of the cytoplasm, much like bacterial cells. Although less evidence exists linking the nucleus to any kind of extant species, both chloroplasts and mitochondria greatly resemble several species of intracellular bacteria, existing in much the same manner. The nucleus is thought to have arisen through enfolding of the cell membrane, as seen in the graphic above. Throughout the world, there are various endosymbiont bacteria, all of which live inside other organisms. Bacteria exist almost everywhere, from the soil to inside our gut. Many have found unique niches within the cells of other organisms, and this is the basis of endosymbiotic theory.

  • Endosymbiont – An organism that lives with another organism, cause both organisms to receive benefits.
  • Cyanobacteria – Still extant, cyanobacteria are photosynthetic bacteria whose ancestors probably became the chloroplasts of plant cells.
  • Proteobacteria – The bacterial ancestor to the mitochondria organelle.
  • Eukaryote – An organism with membrane bound organelles, thought to have evolved from endosymbiotic interactions.


1. Some people refute the theory that similar DNA is due to common descent, a cornerstone of endosymbiotic theory. They say that similar sequences of DNA can arise through convergent evolution, or pressure from similar sources. Why is this improbable?
A. Genetic recombination and mutations are the only things that are known to give rise to new DNA.
B. There are so many DNA bases, the combinations are endless.
C. Similar evolutionary circumstances are unlikely to need similar proteins.

Answer to Question #1
A is correct. The only documented cases of new genes arising come from mutations of DNA that is already present. As mutations take a long time to add functional material to a genome, it is much more likely that existing proteins will be modified to fit the task. While there are only 4 DNA base molecules, a single mutation in DNA can cause a large functional change in a protein, which can lead to novel chemical pathways. To build an entirely new gene from scratch that is identical to that found in a similar species would mean the process of mutation and selection would have to take place almost identically in both populations, for thousands of DNA base molecules in a specific order. Since mutations are entirely random, this is nearly impossible. However, proteins with similar functions are often created from seemingly unrelated proteins in different groups of animals.

2. A bacterial cell is ingested by a human. The bacteria travels to the intestine, where it is endocytosed by an intestinal cell. The bacteria is not destroyed, and lives in an endosymbiotic relationship with the human cell. Did endosymbiotic theory just happen before our eyes?
A. Yes
B. No
C. This may be the first step

Answer to Question #2
C is correct. Many organisms in nature exist in endosymbiotic relationships with another species. Endosymbiotic theory specifies that over time these endosymbionts lose the ability to live independently of the host, and the host becomes reliant on the endosymbiont. Further, the genome of the endosymbiont will be greatly reduced, as the host will provide the majority of the proteins it needs to function.

3. Mitochondria and chloroplasts divide separately from the normal cell cycle. During mitosis, they are distributed more or less evenly to each new cell. Does this support endosymbiotic theory?
A. Yes, the separate cycle suggests a different lineage.
B. No, the mitochondria and chloroplasts are simply needed for energy at all times.
C. Maybe? Both seem like good answers.

Answer to Question #3
C is correct. While this may seem unimportant, this remains a disputed area in endosymbiotic theory. Some critics suggest that the differing reproductive rates of mitochondria and chloroplasts is simply to preserve the flow of energy that the cell needs during division. If the flow of energy was lost because all of the mitochondria were dividing, the entire cell would have to stop in the middle of division. Proponents of endosymbiotic theory point to the fact that the DNA is different in these organelles as well, which suggests that they never reproduced in a eukaryotic fashion.