Sunday, September 20, 2015

A Great Filter – Mitochondria

Mitochondria, small chemical processing organelles located inside almost all cells, from bacteria on up, are usually thought to have originated when one bacteria swallowed another, and instead of dissolving, the internal one continued to survive in the cytoplasm of the larger cell. Dating methods indicate that the initial incorporation occurred about 2 billion years ago.

There has been no evolution to multi-cellular organisms of those primitive bacteria which do not contain mitochondria. This implies that they are necessary for further evolution. Alternatively, there might have been a spurt of multi-cellular organisms without mitochondria, but they became extinct for some reason, perhaps in competition with those that do have mitochondria.
The majority of the chemical processing that the mitochondria do is the preparation of adenosine tri-phosphate (ATP), which is the chemical fuel used by many processes within the cell. Why is it so beneficial for cells to have mitochondria? One guess is that the processes that involve the highest energy levels are disruptive to other protein manufacturing or membrane maintenance or some other cellular functions. It is better for cells to encapsulate this energy production behind the mitochondria’s double membrane structure. There the energy production processes can more safely operate. Rupture of the mitochondrial membranes can lead to cell death.

Another reason might have occurred later in the evolutionary process. Different cells have different numbers of mitochondria in most organisms, and having a simply reproducible unit that can respond to the differing levels of energy requirements in cells would appear to be an advantage. Mitochondria dump ATP back into the cell plasma, but they likely also monitor the levels there, and can respond by dividing into two as many times as necessary to produce the level of ATP that the cell needs to consume for efficient operations. As an example, liver cells can contain a thousand mitochondria, as compared to some other cells which might only have a few.

Thus, the initial generation of mitochondria a couple of billion years ago could represent a Great Filter. The mitochondria we see now have evolved along with the cells for these two billion years, and it is not clear just what the initial bacteria that was able to survive inside the cellular wall of another bacteria was composed of. It is known that mitochondrial DNA is the most diverse group of it, as there are many coding differences between the DNA of mitochondria from widely different organisms. This does not necessarily mean that one bacteria was incorporated into another bacteria multiple times, as simple mutations might have resulted in the coding differences, which are not substantial and only affect a one or a few coding sequences, often concerned with STOP codes, the four amino acid sequence that dictates that DNA replication should end at a certain position.

Bacterial cell walls are intact, but they can connect with other bacteria and exchange DNA, in what is known as bacterial recombination. This process allows bacteria within the group that exchanges DNA to more quickly respond to a change in environment or to more quickly spread a better gene. Since bacterial reproduce by simple replication, this is an early equivalent of the exchange of DNA that occurs in organisms that have two sexes.
In order to do this recombination, the two bacteria involved would have to contact one another, and the cell walls would have to fuse and then split open. They would then remain open for a while so that there could be a swapping of some chromosomes or possibly even parts of chromosomes. If, when the cells contacted, there was a flaw in both the external bacterial membranes at the point of contact, so that one would split without having linked to the other bacteria’s membrane, and then move over the other bacteria’s membrane to enclose it, before closing again. There would have to be a corresponding flaw in the process that the other, smaller, bacteria had for its membrane, which kept it from splitting. Given that bacterial recombination is an uncommon event in the life of a bacteria, and that two opposite types of defects in the process of cell fusing would have to take place, plus some mechanism to restore the external membrane of the largest cell, plus on top of all that, the ability of the smaller bacteria to survive inside the larger one, we are really looking at a rare event. A large number of highly improbable events exist in this postulated sequence of mitochondrial origination, which means there is a low probability of it happening. This is the recipe for a Great Filter. Naturally, there is no knowledge at this point of how mitochondria originated.

The survival of the dual cell might not be very good. Furthermore, at this point there is no evolutionary advantage to having a second bacteria inside the larger one. How the dual bacteria would reproduce is also interesting. If the internal bacteria was able to take advantage of the chemical soup inside the external bacteria and make copies of itself, when the external bacteria went through its budding and reproduction process, at least one copy of the internal one might be transferred. The existence of the internal bacteria does not seem to provide any advantage to the external one’s survival, and furthermore, the external bacteria needs to find nutrients able to satisfy both its original requirements and those of the internal parasite it is now carrying. This seems to be an evolutionary disadvantage. In an environment that was nutrient rich, it might not be a problem that would threaten the survival of the external bacteria, so this is another condition to be placed upon the survival of the incorporation process.

The evolution or modification of the membrane of the interior bacteria would have to occur soon after the incorporation, whereby it started to provide ATP to the external bacteria. This would change the relationship from a parasitic one to a symbiotic one. Once this happens, and if there really is an increased safety element provided by having no ATP generation in the external bacteria, but only in the interior one, it would seem the high road to evolution of mitochondria has been found.

Perhaps in the history of planet Earth, this combination of events only happened once. Maybe bacterial fusion of two bacteria with complementary flaws in their membrane fusion processes could have happened multiple times, but most times it only resulted in an evolutionary disadvantage, and the dual cell soon became extinct, if it did not die immediately after the incorporation. Similarly, perhaps it happened multiple times, but only once in an environment where there was a plethora of nutrients that existed for long enough for a secondary mutation or multiple mutations in the two cells to occur, where the external one gave up ATP production and the internal one specialized in it. There seems to be so many unlikely combinations that labeling mitochondria a potential candidate for a Great Filter is not outlandish.

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