Term Paper: Search of the Perfect Host

Pages: 9 (2829 words)  ·  Bibliography Sources: 1+  ·  Level: College Senior  ·  Topic: Genetics  ·  Buy This Paper


[. . .] Each adaptive mutation brings it more in tune with the biology of its host. Even slight variations may make real differences in the survival rates of such short-lived life forms. The amoeba that can live only moments longer can reproduce that much more often than its counterparts. In time, a parasite is created that is perfectly attuned to every nuance of its host's body. Any noteworthy change in environment would have devastating consequences for the parasite species.

The effect upon a parasite of long-term residence within the body of a specific species is the same as the effect upon a larger and more complex organism of longtime residence in a specific locality and environment. Ducks and geese on South Sea islands evolved to become flightless. During most of their long occupation of these isolated spots of land, they had no natural predators. Thousands of miles of open ocean protected them from the carnivores to which they had once been accustomed. There was no longer any need to waste valuable energy on flight. Those birds that whose flight muscles chanced to grow smaller, and who flew less frequently, grew bigger and more powerful than their rivals. They reproduced more often, and soon came to dominate the population. Nevertheless, such specific adaptations to a particular environment could prove dangerous. Flightlessness was beneficial as long as there were no predators within reach of the fowls' island home. The arrival of human beings on these islands spelled the doom of these creatures. It was impossible for them to readapt before being destroyed. The same can be said of the isolated human populations that were exposed to new diseases. The Native Americans had no experience of smallpox, and so died in the millions when they were exposed to infected European explorers. Such human and animal catastrophes are, on a different scale, the same as those that would befall a microorganism suddenly exposed to an environment with new and unfamiliar hazards.

However, the specialization of ducks, or geese, or humans is nothing compared to the specialization found among parasite species. Trillions of generations of microbial and invertebrate reproduction have allowed these creatures to take specificity to heights impossible for more complex animals. Particular species of helminthes and nematodes can only live inside certain species of vertebrates. The roundworm Ascaris lumbricoides is the largest of the nematodes that parasatize human beings. Its entire life cycle has been adapted to life within the human body. Found in many parts of the world, the adult lives inside the lumen of the human small intestine. The adult female lays upwards of 240,000 eggs, which are passed out, with the feces. After a short period of time, these eggs become embryos, at which stage they are now capable of causing infection in human beings. A person accidentally ingests the embryos. The eggs hatch into larvae in the human intestine. Through the intestinal mucosa, the larvae make their way to the bloodstream, and from there to the lungs. It is in the lungs that the second major stage of development occurs. Many parasites exist in different parts of the host' body at different points in their life cycle, and Ascaris lumbricoides is no exception. After a couple of weeks spent maturing in the lungs, the larvae climb up through the bronchial tubes and into the throat where they are swallowed and returned to the small intestine. There they mature and the females eventually lay eggs, thus completing the cycle. (Parasites and Health: Ascariasis, 2002)

As can be seen in the preceding example, Ascaris lumbricoides is especially adapted to life inside the human body. Its life cycle specifically requires that it move from one organ to another. It uses the various parts of the human body as highways and nurseries. No stage of its growth can occur in any place other than that specific spot required by the parasite at that particular point in its life. Conditions must be just right. The physiology of another animal might be different enough that it would thwart the larvae in their attempt to find the route into the lungs, or into the bloodstream. Precise internal environments vary among the vertebrates, meaning that the intestines of a fish might not present such a welcoming home to this particular roundworm. In fact this species could not possibly parasitize any marine or aquatic creature, as an essential part of its life cycle involves the depositing of its eggs on moist, shady ground. The eggs require that location and environment in order to embryonate. Even the composition of tissues and bodily fluid differ slightly from species to species. Ascaris lambricoides is uniquely adapted to life under human conditions.

Clearly such a complex relationship between the roundworm and human beings and their environment can only have developed over a very long period of time. Much evolutionary change has occurred since an unfortunate land animal first swallowed the embryonic eggs of a worm-like parasite. The parasite's pre-existing adaptations enabled it to survive inside that animal's digestive tract. It grew and reproduced, its eggs passing out with the animal's feces. Other animals browsed on plants that grew on the spot where the excrement had been deposited. If the new animal's physiology was similar enough to the first animal's the parasites survived. If not, they died. But, there were always enough generations, always enough genetic mutations to allow for that chance change that made it possible for the roundworm to exist in an ever so slightly different environment. As all of the vertebrates are broadly similar in design, the roundworm, like other nematodes, was able to accommodate itself to a host of other creatures. The size of a roundworm species is suited to the size of the animal it inhabits. While most are microscopic and can fit inside the intestinal tract of any vertebrate with which it is compatible, there are also the giants of the roundworm realm. One species that lives in the intestines of whales can grow to lengths of thirteen meters. (Parasites and Health: Ascariasis, 2002) Again, this is another example of adaptations to specific conditions. It is like the large elephants that live on the savannah, and the smaller ones that inhabit the forest. In fact it is even like the whale itself as such enormous size would never have been possible on land.

Thus, all parasites have evolved to respond to specific conditions. Accidental genetic mutations long ago created creatures that chanced to be able to live inside the bodies of other creatures. Long periods of cohabitation - a sort of symbiosis - engendered conditions of coevolution. Random mutations within the host produced changes that necessitated a response from the parasite. Evolutionary processes allowed the parasite to adapt to the changed conditions. Microorganisms adjusted themselves to the entire life cycle and environment of their hosts. They used such bodily functions of the host as excretion to further propagate themselves. Other and different animals consumed the parasite eggs. And in time, the parasites evolved into new species well adapted to their new surroundings. Parasite specificity is a natural response to long residence in isolated environments. Parasitism itself is the result of chance genetic and physical occurrences, the result of successful adaptations well suited to a now vanished primordial earth. The conditions that produced these characteristics are gone, but the opportunities they afforded still exist, and so the distant descendents of those first parasites are still with us today.


Evolution of Parasitism." (No Date). McGill University. http://martin.parasitology.mcgill.ca/jimspage/biol/evolut.htm

Murthy, Venkatesh L. (July 14, 2002). "An RNA Base Structure Primer." RNABase. http://www.rnabase.org/primer/

Parasites and Health: Ascariasis." (April 24, 2002). Identification and Diagnosis of Parasites of Public Health Concern. Centers for Disease Control and Prevention.

Simpson, Larry and Maslov, Dmitri A. (June 24, 1994). "RNA Editing and the Evolution of Parasites." Science.… [END OF PREVIEW]

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