What do women have in common with termites and carpenter ants?
"We are not made up, as we had always supposed, of successively enriched packets of our own parts. We are shared, rented, occupied. At the interior of our cells, driving them, providing the oxidative energy that sends us out for the improvement of each shining day, are the mitochondria, and in a strict sense
they are not ours." ~ Lewis Thomas
"Over the long term, symbiosis is more useful than parasitism.
More fun, too. Ask any mitochondria." ~ Larry Wall
Introduction
When examining how we know that evolution is true many people may not realize that evidence is abundant within us. It’s not just evidence from the fossil record. Several examples were detailed in my post on unintelligent design, and how our DNA also greatly supports human evolution is discussed in the sections on shared ERVs and human chromosome 2 fusion.
As you may remember from biology, we need to replace millions of cells per day. When a cell divides it produces a new cell and passes its instructions to the daughter cell by duplicating the DNA instructions in its nucleus before dividing. These instructions will be used to make the organelles in the new cell such as the Gogli apparatus, cell membrane, vacuoles, nucleus, lysosomes, etc.
There is one organelle however that is not made this way. It actually has it’s own DNA, different from the nuclear DNA. These are the mitochondria of nearly all eukaryotic cells. Also known as the powerhouses of the cells, they produce the ATP that is used to supply the energy currency to just about everything the cell does. Some cells, like red blood cells, do not have any to save space for carrying oxygen by hemoglobin whereas other cells like muscle cells have thousands per cell.
A few eukaryotes are lacking mitochondria. They obtain their energy through anaerobic pathways - reactions that don’t use oxygen. Examples include intestinal parasites such as Giardia lambda, Entamoeba histolytica and Trichomans tenax. It is unclear if they lost their mitochondria or if they are descendants before mitochondria were acquired (1). More on this in the next section.
How We Know Mitochondria Previously Were Bacteria
As biologists studied these organelles, they noted that unlike all the other cellular organelles, they were unique. They were about the size of bacteria. They have two distinct membranes. The deep folds on the inner mitochondrial membrane resemble the folds called mesosomes found in prokaryotic cells like bacteria. Mitochondria have their own DNA, only 37 genes, and the DNA is in a circular form like bacteria, unlike the cell’s DNA in the membrane nucleus (2). The mitochondrial genes are very similar to bacterial genes like those found in Rickettsia. Of their 1,000 proteins, 40% of the mitochondria are bacterial. When they reproduce they do so by fission end to end like bacteria do instead of being made by the cell. They have unique ribosomes, the structures that assemble amino acids to make proteins. Lastly, they can be affected by antibiotics, which are used to control or eliminate bacterial infections.
All of these characteristics and the DNA findings especially eventually convinced scientists that mitochondria were once free living bacteria that were captured somehow by our very distant past ancestors. How scientists were finally convinced involves a researcher named Lynn Margulis and that is also an interesting story itself. This theory, that mitochondria were once free living bacteria that came to set up shop and become adapted in eukaryotic cells, is called the endosymbiotic or symbiogenesis theory and is credited to her.
Fossil evidence supports that this swallowing of a bacteria by another prokaryote without killing it probably happened about 2.4 billion years, giving rise eventually to the present condition of mitochondria powering eukaryotic cells (3). A similar capture probably occurred with chloroplasts in plants since they have the same bacterial characteristics discussed above and closely match cyanobacteria.
Ancient genes for symbiosis hint at mitochondria's origins
https://www.quantamagazine.org/ancient-genes-for-symbiosis-hint-at-mitochondrias-origins-20220426/?fbclid=IwAR0hLiKTVzTqIwdvx5UOMcdWQr2cC05b96f-FQSGN5oC0Tuf0LE7SowvIdc
Termites, Carpenter Ants, Moms, and Mitochondria
It turns out termites and carpenter ants that eat wood for food (xylophagia) actually cannot digest wood themselves (4). Although there is one group of termite species that do have the ability to make cellulase, many cannot. These species must use gut bacteria to do it for them. Just like we can’t make vitamin K but rely on our gut bacteria to do it for us since our diets don’t supply enough. In the case of these wood eating insects, the adults pass the needed bacteria orally to newborn offspring to establish the gut bacteria they will need to digest and live off wood as food.
In humans, we only inherit mitochondria from our mothers. She passes on all the mitochondria we will inherit and must do this since we depend on mitochondria to reproduce themselves. Any paternal mitochondria that the sperm carries into the egg at fertilization is destroyed by the maternal egg enzymes, and some studies show the paternal mitochondria of the sperm may also auto-destruct after the sperm enters the egg (6). So like termites and carpenter ants, we are dependent on our mothers when it comes to having bacteria in order to live. Our mothers must “seed” our original zygote cell with former bacteria in order for us to survive.
How could it have happened?
A recent study shows how phagocytosis (engulfing to eat) in one or more single celled organisms experienced eventually may have led to endosymbiosis. https://www.quantamagazine.org/ancient-genes-for-symbiosis-hint-at-mitochondrias-origins-20220426/
Conclusion
The origin of mitochondria is another story that is best understood from evolution (5). This theory is most commonly known as the endosymbiotic theory for mitochondria and chloroplast origins. This is another example of how understanding evolution gives us a “why” of life. Evolution is vital even to medicine since we can now understand why mitochondrial diseases are only passed from mother to child and not from the father. Evolution is vital for a complete understanding in medicine. This topic is discussed further at my blog on Evolutionary Medicine.
Addendum (4/2023):
A new form of symbiosis has been discovered. A bacterium supplies unicellular ciliates with energy, but from nitrogen. The bacterium has not co-evolved long enough to loose its independence like what happened with mitochondria.
A single celled alga was just found to have 7 different genomes present indicating multiple past endosymbionic absorptions in the past.
Citations