“Me” is the common pronoun that we use when we refer to ourselves yet a somewhat more appropriate one would be “us”, because me is not only “me”, it’s also more than a hundred trillion unlikely friends who inhabit the same body as my consciousness does. The entire surface of the skin, the digestive tract and every orifice we have serves as a support for these tiny organisms to cling on and make a living. By number, that is approximately ten times more than the number of cells the human body is made of. It is somewhat disconcerting to know they are there doing what bacteria do best, eat and multiply, but we need not worry, because they are doing just as much for us as we do for them and what’s really neat is that they know their place: they accept us as hosts suppressing that which they do best in the interest of a long lasting relationship. They don’t cross into spaces where they don’t belong, they listen to chemical signals that our bodies issue and take action accordingly functioning very much like an organ, yet they are in fact individual organism, coming from more than a thousand different species all living in a fragile harmony with each other and ourselves.
What each of them do, depends very much on the spot they inhabit: they protect us from invading pathogens, they break down cancer inducing substances and toxins or train our immune system forming a bond with us from our very early days of life. Strictly speaking, the relationship between humans and their symbionts is not obligate, that is, the human body can survive without the aid of these creatures but in practice that would require a life in a completely sterile environment and a very special diet. Even if somehow we would achieve this impossible combination, there is no way to tell when our own immune system, having nothing else to kill, would turn back and kill us instead. This cooperation goes much further than just two organisms tolerating each other while while living out their lives. This is cooperation on the deepest level between two very different organisms towards a single scope: survival. A process called symbiosis.
Symbiosis is an extremely successful strategy. Virtually every macroscopic creature in the world lives in some sort of symbiosis with microscopic and / or macroscopic organisms. Trees for instance would be unable to suck up sufficient water to irrigate their enormous bodies because they lack the root surface needed to do that, so they live in symbiosis with fungi whose extremely fine structure increases the suction surface of the tree. In return, trees provides the fungi with nutrients. Cows and other ruminants cannot in fact digest the food they eat, but they posses a special chamber called rumen where they host bacteria that break down cellulose and sugars transforming them into nutrients that ruminants can process. Many plants are pollinated by insects being unable to multiply on their own, and the list goes on with more and more amazing examples of cooperation between vastly different organisms, but inside our very bodies there there is trace of an even more formidable example of symbiotic relationship …
All the cells in our bodies contain a type of little bodies called mitochondria. These tiny organelles have their own DNA, separate from the DNA of our cells, they have a double walled membrane around them and multiply by division just like bacteria do, completely independently from the hosts cell’s division process. Mitochondria cannot be created by the cell, the first of them needs to be acquired through inheritance, in fact mithocondria is inherited as is, without gene recombination only on the maternal line via the egg.
Mitochondria are not only part of the human cells, they are present in almost every eukaryotic cell (briefly speaking, modern cells that have DNA wrapped in a nucleus) and function as the powerhouse of these cells, producing energy by burning nutrients in the presence of oxygen. It is supposed that they became part of the eukaryotic cell more than a billion years ago, when an intracellular bacteria developed a symbiotic relationship with its host cell (producing energy in exchange for nutrients). This theory is further supported by the fact that the genome of mitochondria is very similar to certain strains of bacteria still alive today suggesting a common ancestry in an independent organism. This weird alliance stands at the foundation of mostly all oxygen breathing organisms in the world.
Chloroplasts (the Oxygen factory in plants) is supposed to have emerged in a very similar manner, when a blue-green algae (Cyanobacteria) was accepted into a host cell establishing a symbiotic relationship a process which gave raise to all plant life on our planet.
And these are not the only such examples. An equally amazing symbiotic like behavior can be observed between cells and viruses. In fact an “endogenous retrovirus” similar to HIV is responsible for suppressing the mother’s immune system during the attachment of the fetus to the uterus’ wall. Without this virus, which became part of the mammalian DNA, mammals could not exist. Thousands of other such endogenous viruses exist in the human genome making up almost eight percent of the the human DNA. These retroviruses have given up indefinite multiplication, which inevitably kills the host cell in exchange for being accepted into the cell and hence be multiplied whenever the cell multiplies. By their nature viruses are not self sustainable creatures and always need a cell to replicate so this process cannot be considered a true symbiotic relationship, nevertheless, these viruses did achieve stability with their host resulting in mutual benefit both parties and thus, in this mostly philosophical paper I will consider it a symbiotic relationship.
By contrast, pathogenic organisms will employ a very different strategy to do what they need to do, survive and multiply. When a micro organism enters a host it either becomes symbiotic or it becomes pathogenic. In the latter case, due to the inability of the parties to acquire balance the two will start interfering with each other’s normal life cycle resulting in a fight often to the death of at least one side. If the host fights off the invader, the pathogen killed. If the host fails to fight off the invader, it will die, dragging along the pathogen as well. In either case the pathogen dies. In order to preserve the species, it needs to become contagious to be able to jump hosts. It must either become highly contagious, to be able to spread itself quickly onto other hosts before the host expires, or it needs to learn to stay dormant resisting in this manner the inhospitality of the outer world until it reaches fertile grounds again.
Pathogenic behavior generally arises by chance or circumstance when a micro organism enters a host while not being adapted to symbiotic life with it therefore causing a disease. But almost all pathogenic micro organisms have a reservoir, a host with whom they developed some sort of symbiotic relationship or at least a neutral one. It is in the interest of any such organism that cannot live on its own to develop somewhere along the line a symbiosis with a host, otherwise, it faces extinction because it can only stay in suspended animation (ie spores) for so long.
Not all micro organisms are symbiotic or pathogenic, living out their lives independently of any host. Others, are only able to grow and reproduce inside a host an aspect which is particularly interesting in the context of this story because of their obligate relationship with their host with whom they must negotiate their connection. This is not unlike our very own relationship with our host, Planet Earth.
The peace factor
It is interesting to observe the two different strategies that these organism employ. Of course we cannot assume that they do so by choosing one path or the other deliberately but it is intriguing that so many of them build stable relationships with different kinds of hosts. It is as if there was a built in mechanism that drives organisms towards equilibrium, peace and eventually cooperation.
One could argue that evolution backs the development of cooperation by providing advantage to both parties, but it does not explain the moment of truce. It is after all a very unlikely event that happens when a foreign organism invades a host in search for resources, having nothing else in mind but survival and multiplication, but when it gets there it suddenly gives up multiplication and starts giving back something for the resources he found with the host. At the same time, the host will not obliterate the invader, as it should by nature, but accepts whatever this has to offer. There are many coincidences that need to happen here: the invader gives up a basic built in aspect of its life cycle, it produces a substance which is foreign to the host yet the host will need to recognize that as beneficial and it too stop its physiological urge to eliminate the invader. Often times this process happens gradually where host and symbiont get used to each other over time, but every now and then there is this moment where both give more than they receive without any explanation. It’s like some obscure innate need creates a brief timeout which give the organisms the chance to seal the bond. Some kind of a “peace factor” built into living things like gravity around objects.
Competition is vastly easier from an evolutionary point of view. Very little steps are sufficient to create an advantage big enough for one species to out compete a neighboring species or its own kind for resource and driving it to extinctions. It is easier because small changes are lot closer to the natural tendency of each organism to keep its genetic integrity over generations, still, the greatest leaps of evolution are more often then not, a story of cooperation rather than one of competition.
Since its merger with the eukaryotic cell, mitochondria have lost many of their original DNA, a very natural process, because the cell will provide them with almost all the building blocks that they ever need in exchange for their service, but by doing so, they have lost their ability to live on their own; they can only exist inside these cells. At the same time, by the nature of their service (create energy for the cells), all cells who depend on this type of energy must have them, otherwise they could not function. Due to this strict coexistence, mitochondria, are not considered symbionts any more but more like a little organs of the cell, hence the name organelle. But to these days, they exhibit some very strange properties which makes one wonder weather they are in fact part of the building block of the cell, or they are still some very special employees of them.
It’s distant cousin, Rickettsia, has taken the exact opposite path: it’s kept its very own independence remaining an intracellular parasitic bacteria, which can cause some serious illnesses in humans, by invading their cells, multiplying and eventually killing these host cells until either the infection or the human dies.
Rickettsia will probably never argue that the path it has taken is the more pure path, a noble one, which preserved its identity over more than a billion years versus the traitor which abandoned its kind and married the enemy. Nor will the mitochondria argue that peace is better than war, that living in harmony is worth giving up some of that which makes you who you are. That its even worth loosing one’s identity because only then one can gain a new one, a better one, a superior one.
Incidentally though, a handful of cells, which also chose to drop their identity and to perform a very specific role within yet a more complex organism, do wonder about just that. And they do so with energy produced by their mitochondria. When I think back at this magnificent journey, at all the selfless acts that had to happen so that “I” can exist, a sense of elation invades my brain and its neurons. As self-righteous as it may be, I can’t help thinking that “we”, humans, are more superior than bacteria, not in the sense that we deserve more just because we are more complex, but more like having potential to achieve greater goals than just surviving and spreading.