Endosymbiotic hypothesis

The endosymbiotic theory, known to academics as the serial endosymbiosis theory (SET), is a scientific theory concerning the origins of mitochondria, plastids (e.g. chloroplasts), and nuclei in eukaryotic cells. According to this theory, these organelles arose from free-living bacteria that were taken inside another cell as endosymbionts.

Creation science critique of the edosymbiotic hypothesis

Citing a biology textbook Creation Ministries International writes:

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Sometimes it is stated more cautiously: ‘In the endosymbiont theory, the ancestor of the eukaryotic cell (we will call this organism a protoeukaryote) is presumed to have been a large, anaerobic, heterotrophic prokaryote that obtained its energy by a glycolytic pathway. Unlike present-day bacteria, this organism had the ability to take up particulate matter … . The endosymbiont theory postulates that a condition arose in which a large, particularly complex, anaerobic prokaryote took up a small aerobic prokaryote into its cytoplasm and retained it in a permanent state [emphasis added].’ Whichever way it is stated, it is given an aura of authority and certainty by its frequent repetition in writings on cell biology. Many students find it convincing. However, like many evolutionary ideas, it may look solid from a distance, but gaps appear on close scrutiny. The evidence for the endosymbiont theory revolves around selected similarities between mitochondria and bacteria, especially the DNA ring structure. However, these similarities do not prove evolutionary relationship. There is no clear pathway from any one kind of bacteria to mitochondria, although several types of bacteria share isolated points of similarity. Indeed, the scattered nature of these similarities has left plenty of room for a less-publicized ‘direct evolution’ theory of mitochondrial origin, in which they never had any free-living stage. There is enough diversity among the mitochondria of protozoa to make evolutionists wonder if endosymbiotic origin of mitochondria occurred more than once. Mitochondrial DNA The endosymbiont theory implies that there should be considerable autonomy for mitochondria. This is not the case. Mitochondria are far from self-sufficient even in their DNA, which is their most autonomous feature. Mitochondria actually have most of their proteins coded by nuclear genes, including their DNA synthesis enzymes.

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History

The endosymbiotic theory was first articulated by famed Russian botanist Konstantin Mereschkowski in 1905. Mereschkowski based his theory in part on work done by botanist Andreas Schimper, who had observed in 1883 that the division of chloroplasts in green plants closely resembled that of cyanobacteria. Schimper had himself tentatively proposed (in a footnote) that green plants could have arisen from the symbiosis of two simpler organisms. In the 1920s, Irvin Wallin, a professor at the University of Colorado Medical School, extended the idea of an endosymbiotic origin to the mitochondria. These theories were initially dismissed (or ignored) by the scientific community. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example, studies by Hans Ris) combined with the discovery that plastids and mitochondria contain their own DNA (which had recently been identified as the hereditary material of organisms) led to a resurrection of the idea in the 1960s.

The endosymbiotic theory was advanced and substantiated with microbiological evidence in 1967 by American biologist Lynn Margulis. A young faculty member at Boston University, Margulis authored a theoretical paper entitled The Origin of Mitosing Eukaryotic Cells. The paper, however, was "rejected by about fifteen scientific journals," Margulis recalled(see Best of the Public). It was finally accepted by The Journal of Theoretical Biology and is considered today a landmark in modern endosymbiotic theory. Drawing heavily from the ideas of Mereschkowski and Wallin, Margulis's endosymbiotic theory formulation is the first to rely on direct microbiological observations as opposed to paleontological or zoological observations, which were previously the norm for new works in evolutionary biology.

The possibility that peroxisomes may have an endosymbiotic origin has also been considered, although they lack DNA. Christian de Duve proposed that they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earth's atmosphere. However, it now appears that they may be formed de novo, contradicting the idea that they have a symbiotic origin. It is believed that over millennia these endosymbionts transferred some of their own DNA to the host cell's nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell (called "serial endosymbiosis"). This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between bacteria species, even if they are not closely related. Bacteria can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome.

Evidence

A great body of evidence exists in the fields of molecular biology, biochemistry, and genetics that supports the theory that mitochondria and plastids arose from bacteria.

• New mitochondria and plastids are formed only through a process similar to binary fission. In some algae, such as Euglena, the plastids can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the plastids will not regenerate.
• They are surrounded by two or more membranes, and the innermost of these shows differences in composition from the other membranes of the cell. They are composed of a peptidoglycan cell wall characteristic of a bacterial cell.
• Both mitochondria and plastids contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (in being circular in shape and in its size).
• DNA sequence analysis and phylogenetic estimates suggest that nuclear DNA contains genes that probably came from plastids.
• These organelles' ribosomes are like those found in bacteria (70S).
• Proteins of organelle origin, like those of bacteria, use N-formylmethionine as the initiating amino acid.
• Much of the internal structure and biochemistry of plastids, for instance the presence of thylakoids and particular chlorophylls, is very similar to that of cyanobacteria. Phylogenetic estimates constructed with bacteria, plastids, and eukaryotic genomes also suggest that plastids are most closely related to cyanobacteria.
• Mitochondria have several enzymes and transport systems similar to those of bacteria.
• Some proteins encoded in the nucleus are transported to the organelle, and both mitochondria and plastids have small genomes compared to bacteria. This is consistent with an increased dependence on the eukaryotic host after forming an endosymbiosis. Most genes on the organellar genomes have been lost or moved to the nucleus. Most genes needed for mitochondrial and plastid function are located in the nucleus. Many originate from the bacterial endosymbiont.
• Plastids are present in very different groups of protists, some of which are closely related to forms lacking plastids. This suggests that if chloroplasts originated de novo, they did so multiple times, in which case their close similarity to each other is difficult to explain.
• Many of these protists contain "primary" plastids that have not yet been acquired from other plastid-containing eukaryotes.
• Among eukaryotes that acquired their plastids directly from bacteria (known as Primoplantae), the glaucophyte algae have chloroplasts that strongly resemble cyanobacteria. In particular, they have a peptidoglycan cell wall between the two membranes.
• Mitochondria and plastids are similar in size to bacteria.

Problems

• Neither mitochondria nor plastids can survive in oxygen or outside the cell, having lost many essential genes required for survival. The standard counterargument points to the large timespan that the mitochondria/plastids have co-existed with their hosts. In this view, genes and systems that were no longer necessary were simply deleted, or in many cases, transferred into the host genome instead. (In fact these transfers constitute an important way for the host cell to regulate plastid or mitochondrial activity.) For example, most plastids are not able to produce respiratory proteins necessary for respiration. Like any living cell, plastids would die if energy is not provided to them by respiration.
• A large cell, especially one equipped for phagocytosis, has vast energetic requirements, which cannot be achieved without the internalisation of energy production (due to the decrease in the surface area to volume ratio as size increases). This implies that, for the cell to gain mitochondria, it could not have been a eukaryote, and must have been a bacterium. This in turn implies that the emergence of the eukaryotes and the formation of mitochondria were achieved simultaneously. This may be explained by possibly a very close symbiotic relationship between two types of bacteria which eventually led to gene exchange and engulfing of the mitochondria precursors through partial fusion or engulfing by the host bacteria.
• Genetic analysis of small eukaryotes that lack mitochondria shows that they all still retain genes for mitochondrial proteins. This implies that all these eukaryotes once had mitochondria. This objection can be answered if, as suggested above, the origin of the eukaryotes coincided with the formation of mitochondria. Alternatively, we may postulate extinction of all other descendants of a mitochondrion-free ancestral eukaryote, perhaps due to competition from the symbiotic clade, or oxygen poisoning as levels continued to rise.

These last two problems are accounted for by the hydrogen hypothesis.

See Also

• Theory of Evolution
• Symbiosis
• Cell
• Biology