The door opens. You walk into the room. You hear your favorite music. You see your best friends. Your favorite drink is waiting on the bar. Smiling, the hostess approaches, "I did it all for you." Ah, what a dream - the perfect party, the perfect host! While such a fantasy may not always be the lot of the human guest, it is real life for many microbial visitors. Every parasite has its "perfect host," the one organism that is ideally suited to its needs. Of course, this perfect pairing of guest and host did not evolve by accident. Over the course of time, parasites have evolved in tandem with the organisms upon which they live. It is a unique relationship, the host organism providing a complete environment for the parasite. The parasite has so completely adapted itself to the conditions of particular host species that it can live nowhere else. Dirofilaria immitis, or the Canine Heartworm, goes through its entire life cycle within the body of a dog. Hymenolepis diminuta is a tapeworm of rodents. And Enterobius vermicularius is a pinworm that infects humans. In each of these examples, the parasite has become adapted to living in a particular part or parts of its host organism. It feeds, reproduces, and eventually dies inside the bloodstream, organs, or cells of its unwilling host.
Yet how did such organisms evolve? And how did they become so specifically attuned to the physiology of their host species?
In order to find the answers to these questions, it is necessary to turn back the evolutionary clock and to look at the origins of life itself. The most primitive life forms all shared similar structures, the ancestral forms of those structures that today compromise the typical cell. As it still does today, Ribonucleic Acid, or RNA, served the purpose of regulating the life-functions within the cell. A simpler form of DNA, to which it is closely related, RNA was most likely the early basis of life.
RNA serves a multitude of roles in living cells. These include: serving as a temporary copy of genes that is used as a template for protein synthesis (mRNA), functioning as adaptor molecules that decode the genetic code (tRNA) and catalyzing the synthesis of proteins (rRNA). There is much evidence implicating RNA structure in biological regulation and catalysis. Interestingly, RNA is the only biological polymer that serves as both a catalyst (like proteins) and as information storage (like DNA). For this reason, it has be postulated RNA, or an RNA-like molecule, was the basis of life early in evolution." (Murthy, 2002)
In addition to performing the above functions in all cells, RNA is essential to the operation of parasitic microbes. Specifically, it is the RNA's ability to make copies of itself, and in fact to alter those copies according to the genetic make-up of its host, that enables the parasite to function within its host organism. While the precise origin of this RNA "Editing" has long been debated, it is now believed that the ability to edit RNA was present even in the most primitive of life forms. Most likely, it preceded the evolution of actual parasites.
RNA editing seems to be an early evolutionary invention that came on the scene before the appearance of parasitism. Editing may have been inherited from the RNA world, or developed from the mitochondria in response to unknown regulatory demands. In the course of evolution, editing was partially or completely eliminated in many lineages. We hypothesize that it turned out to be useful for the development of parasitic adaptations, as exemplified by the developmental regulation of editing in T. brucei." (Simpson and Maslov, 1994)
Thus in the distant past, the proto-parasite existed as a self-sufficient organism. Structurally, it was in no way appreciably different from the primitive ancestral varieties of any other life form. Its retention of the archaic ability of RNA Editing accorded it the propensity to become a parasite, however this development was not a foregone conclusion.
In fact, parasitism developed not once, but rather many times during the course of evolution. Protozoans diverged very early in the course of their development, some groups going on to become parasite, while others continued to be free-living. Even among creatures of a much higher order, this diverge among related species occurred at a remote period long before the appearance of even the earliest forms of their present day hosts. Recent research has shown a vast amount of genetic difference among the helminthes. Indeed there is greater divergence among the helminthes than there is between fish or mammals, or birds and amphibians, thus revealing a very ancient development. Therefore, the helminthes as a group evolved long before their eventual hosts the vertebrates.
Clearly then, they already possessed the necessary tools for parasitism even before their victims came on the scene. (Evolution of Parasitism) So what might have caused this change in lifestyle?
Again, it is necessary to look back in time, and to the most primitive of organisms. While some classes of organism are exclusively parasitic, others contain both parasitic and free-living members. An example of this is the amoeba. Some amoebas live in the intestinal tracts of animals while others live free in the mud on the bottoms of rivers and streams. Those amoebas that live in the often oxygen-poor environment of a river or stream bottom possess the ability to perform both aerobic and anaerobic respiration. It might well have been just such a free-living amoeba that was accidentally ingested by a passing vertebrate. Finding itself inside the animal's digestive tract, it was already equipped to survive in an oxygen-free environment. Yet another example is to be found in the larval stage of the free-living rhabditids, which stage is similar to the free-living stage of certain kinds of parasitic nematodes. (Evolution of Parasitism)
Again, it is a pre-existing characteristic that fits the organism for parasitic life. The amoeba can survive in the animal's intestinal tract because it can withstand the anoxic atmosphere. While on the other hand, the similarity of the larval form of the parasitic nematodes to that of the rhabditids means that the two organisms can endure similar conditions both inside and outside the body of another organism. And as well, both the above examples serve as reminders that nearly identical forms can live either within another organism or outside.
However, it is once an organism has taken up residence inside another organism, that a second and crucial process comes into play. This is the process of Coevolution.
Coevolution is based relatively simply on the fact that Evolution is a non-stop process. All species are continually changing and developing. Genetic mutations, errors in the copying of DNA and RNA, lead to minute, or even at times, dramatic changes that might be either beneficial or maladaptive. In the normal course of things the maladaptive forms will die out, while the successful adaptations will survive as a result of those organisms that possess them living on to reproduce. The same process of evolution is at work both in host and parasite. As the host itself changes, the environment inside it changes as well. Subtle differences in conditions might mean death a microorganism living inside the body of another animal. If the parasite is to survive, the genetic changes that take place automatically within it must change in such a way as to adapt the organism to the new conditions. If for example, the stomach of the host should become more acidic, the parasitic bacteria within must be resistant to that change. Those with the appropriate genetic make-up will survive, while those without will die. In time a new strain of bacterium will be created. It is simply a chance operation, but one which that because of the extremely high rate of reproduction of microorganisms, allows the species as a whole to respond to all but the most sudden changes.
The longer this process of coevolution continues, the more specifically adapted to a particular species the parasite will become. Returning again to the example of the amoeba, that first amoeba that was swallowed up by a passing fish and found refuge in its intestines was equally capable of being swallowed up by any species of fish, or indeed by any species of vertebrate. If the only adaptation it required to survive inside an animal's digestive tract was the ability to exist in an anaerobic environment, then it could have lived just as easily in the gut of any animal. However, the longer it remains within and reproduces inside of the same animal, the more likely the microorganism's chance mutations will be specifically suited to the conditions of life within that particular animal. The prehistoric ancestor of the trout went through numerous permutations before it became the trout we see today. In the lifetime of a single fish, an amoeba, or some similar organism, runs through countless generations. In that time, and…