Where do intestinal worms like Ascaris lumbricoides come from? Large intestinal worms come to live in the intestine when you swallow infective eggs that have matured in warm moist soil.
But where do they come from? They come from other worms, living in other intestines. These worms mate and females produce eggs. After being ingested, the egg hatches, releasing a larva that migrates through the tissues, arrives in the lungs and returns to the intestine via the bronchi, throat, and stomach.
But where do they come from in the beginning?
Spontaneous generation
In ancient times, and even until the beginning of the 18th century, many people believed that parasites were the product of spontaneous generation: they simply appeared, ready made. The invention of the microscope, of course, made it possible to see the eggs and larvae of many parasites, and the dividing trophozoites of protozoa. Scientists realized that parasites don’t just appear – they reproduce.
Though we tend to look down on a parasitic lifestyle, it’s clearly a good way to live, and through the course of evolution, many organisms have found a way to live entirely - or almost entirely - off the resources of other organisms. In fact, according to Roberts and Janovy, there are more parasitic organisms on Earth today than nonparasitic ones (p. 1).
The first parasites: evolution of eukaryotic cells
The first parasite probably moved into a host two billion years, or longer, ago. The invasion ultimately resulted in the evolution of four of the kingdoms of life we know today: Protista, Fungi, Plantae, and Animalia. Organisms in these four kingdoms are made up of eukaryotic cells – organized cells with a nucleus and other internal organelles, and a cytoskeleton. Eukaryotic cells are believed to have evolved from prokaryotic cells, which lack these structures.
Scientific theory today, supported by molecular studies, tells us that organelles inside eukaryotic cells, including mitochondria (which produce energy for the cell) and plastids (found in plants and algae and involved in photosynthesis) were once free living organisms that somehow came to live inside prokaryotes.
These free living organisms may have been engulfed by the prokaryote, or they may have invaded, but in any case, they did not die and get metabolized as food. Instead, they survived inside the host cell, getting everything they needed from it, and eventually became part of the cell machinery, a necessary part of cell function. The relationship is known as endosymbiosis.
Thus, we owe our existence to parasites.
Human parasites today
Since the last eukaryote common ancestor, countless organisms have evolved with complex structures made up of eukaryotic cells. Many of them, like those first invaders, are parasites. These are organisms that at some point found themselves on, or in, another species and were able to live there. Some human parasites have been our companions through the millennia as we evolved, codiverging as human ancestors split from other species. Others have crossed to us more recently, after evolving with other animals.
Where did parasites come from? They evolved, just like humans, and as they did so, they took advantage of an obvious opportunity: getting everything you need from someone else is a practical and relatively easy way to live.
Sources:
Hoeppli, R. 1959 Parasites and Parasitic Infections in Early Medicine and Science Singapore: University of Malaya Press.
Koonin, Eugene V. 2010 “The Origin and Early Evolution of Eukaryotes in the Light of Phylogenomics." Genome Biology 11:209.
McMenamin, Mark, and Dianna McMenamin. 1994 Hypersea New York: Columbia University Press.
Roberts, Larry S. and John Janovy Jr. 2000 Foundations of Parasitology 6th ed. Boston: McGraw Hill.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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