Some parasites that infect people have come to us from other species, crossing over after close contact of some sort: they are souvenirs of that close contact. Other parasite species, however, have been with us for hundreds of thousands of years. They are human parasites that have evolved and codiverged with humans. They are “old friends” or “heirloom parasites.”
Heirloom parasites of humans
Human heirloom parasites include, among others:
- Enterobius spp., or pinworm
- Ancylostoma duodenale, or Old World hookworm
- Ascaris lumbricoides, the large intestinal roundworm
- Trichuris trichiura, or whipworm
Some of the Plasmodium species (parasites that cause malaria) and other protozoans may be old friends as well, but prehistoric evidence of these organisms is much harder to find.
Why are heirloom parasites important?
Humanity’s heirloom parasites are of great interest to science for several reasons:
- They may make important contributions to human health.
The hygiene hypothesis suggests that parasites help to “train” the human immune system to respond appropriately to things in the environment. The surge of allergies and autoimmune disease in developed countries today may be at least partly due to the loss of our old friends.
- They tell us about our own history.
Paleoparasitology can reveal what early humans were eating, what parasites infected them, and where they were living. For example, the presence of hookworm and whipworm eggs in the people of pre-Columbian North and South America hints that theories of human migration across the Bering land bridge from Asia 13,000 years ago are at best incomplete. These parasites could not have made that journey with the migrants because they require a period of development in warm moist soil, and they don’t infect other species. Some prehistoric people must have come to North America via a faster and warmer route.
- They tell us about the evolution of our species.
We’ve coevolved with our heirloom parasites: when we evolved, they evolved as well in order to stay with us. Molecular studies of our old friends and closely related parasites that live in other species can reveal events where they, and we, split off from ancestors.
Getting rid of human parasites.
When humans moved out of Africa, millions of years ago, they started leaving their heirloom parasites behind. Some of those parasites couldn’t survive because their required alternate hosts were no longer available. Some couldn’t stand the colder climates in temperate regions because, like hookworm and whipworm, they needed time in a warm environment.
Some, like pinworm and Ascaris lumbricoides, did go along. Pinworm continued to do well because it is passed directly from person to person. Ascaris proved hardy enough to withstand cold winters.
It’s only been in the last century that people in developed countries, who have access to relatively safe food and clean water, good sanitation, and modern medical care, have succeeded in getting rid of our heirloom parasites almost entirely. People in the undeveloped tropics still experience many parasitic infections.
Sources:
Araújo, Adauto, Luiz F. Ferreira, Karl J. Reinhard, and Scott L. Gardner. 2008 “Parasites as Probes for Prehistoric Human Migrations?" (galley proofs) Natural Resources, School of Papers in Natural Resources, University of Nebraska – Lincoln.
Araújo, Adauto and Luiz Fernando Ferreira. 2000 “Paleoparasitology and the Antiquity of Human Host-parasite Relationships.” Memórias do Instituto Oswaldo Cruz On-line. Suppl. 1 Nov.
Cox, F. E. G. 2002 “History of Human Parasitology.” Clinical Microbiology Reviews, 15(4)
Gonçalves, Marcelo Luiz Carvalho, Adauto Araújo, and Luiz Fernando Ferreira. 2003 “Human Intestinal Parasites in the Past: New Findings and a Review.” Memórias do Instituto Oswaldo Cruz, 98(Suppl.1).
Rook, Graham A. W. 2008 “Review Series on Helminths, Immune Modulation and the Hygiene Hypothesis: The Broader Implications of the Hygiene Hypothesis.” Immunology 126.
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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