A Worm's View of Human Evolution
I was surprised recently to discover how much a few worms have to say about human evolution. There are many different ways of knowing in science, many different pathways to uncovering evidence, and some of them only the most ingenious among us would imagine. So it is with a project recently described in the Proceedings of the Royal Society of London that focuses on the anatomy, phylogeny and genetic variability of various species of tapeworms to uncover new facts about the habits of hominids, our human ancestors.
Eric Hoberg of the U.S. Department of Agriculture, Nancy Alkire of the University of Colorado, and Alan Queiroz and Arlene Jones, both of the Natural History Museum in London, realized that the tight adaptational relationship between a particular species of tapeworm and its host means that tapeworms can reveal a great deal about the animals in whose guts they live. Human beings are vulnerable to infestation by any of three species of taeniid tapeworms: Taenia saginata, T. asiatica and T. solium. All three are host-specific, meaning they can't survive as adults outside a human gut. Since tapeworms must be ingested to pass from individual to individual or from species to species, the research team realized that the genetic and host differences among tapeworm species can be used to understand something about the changing dietary habits of a host species.
Tapeworms have their own charm and at the very least must be considered to have found a clever way to make a living. They co-opt the work of their host species, who unwittingly provide food and housing to the parasites at various stages in their lives. The beauty of the system is that the host species actually infects itself as it goes about its daily tasks; the tapeworm need do nothing except be at the right place at the right time. The complicated life cycle of a tapeworm is magnificently adapted to its parasitic existence. For taeniids, this always involves taking advantage of a predator-prey relationship, in which a carnivore harbors the adult tapeworm and an herbivore hosts the infective larvae. With the exception of the three taeniid tapeworms that are specific to humans, adult tapeworms in this study commonly live in the intestines of carnivores such as lions, hyenas or African wild dogs. When the adult worms mature, they shed eggs, which pass out of the host's body in its feces. The eggs are then ingested by an intermediate host, which is often a particular species of herbivore, such as a domestic cow, some antelope or a pig (domestic or wild). In the body of the intermediate host, the tapeworm eggs develop into larvae that live in the host's flesh. The larvae then pass into the definitive host when it eats the infected intermediate host, either raw or inadequately cooked. Once in the definitive host's gut, the tapeworm larvae mature into adults and shed eggs, completing the cycle.
When did hominids first become definitive hosts for tapeworms? If we knew the answer to this question, we'd know when our ancestors began to eat animal flesh regularly enough for the human-specific tapeworms to evolve. If we knew which species were the intermediate hosts of tapeworms that are closely related to the human-specific tapeworms, we'd have a good idea of which prey species our ancestors ate. Finally, if we knew which other definitive hosts carry the tapeworm species most closely related to ours, we might learn something of the style of eating and obtaining meat practiced by our ancestors.
As a paleoanthropologist who spent decades trying to track the lifestyle and origins of meat-eating, hunting and scavenging among early hominids, I regret that I never conceived of such an interesting and sound approach to these questions. Fortunately Hoberg and his colleagues did. Among parasitologists and others who appreciate the humble tapeworm, the conventional wisdom has been that humans were first exposed to tapeworms that lived in domestic animals, either in companion carnivores such as dogs or in food animals such as cattle and swine. It is important to realize that a domestic animal is distinctly different from a tame individual of a wild species. A dog or a sheep is a domestic animal; a wolf or a mountain goat is not, even when it is brought up from birth by humans. A tame animal does not pass that tameness onto its offspring; taming is not a heritable, genetic change, and there is no simple way to discover when a hominid first tamed another species. In contrast, domestication is genetic and fundamental and changes the lives of those involved. Species become domesticated because humans control their reproduction and survival over generations. By picking and choosing which animals live and reproduce and which are eaten before they reproduce, people act as powerful selective agents. The process of domestication is actually a manipulation of the genetic makeup of a formerly wild species by humans, with the result that the evolved domesticate has desirable genetic attributes such as docility, high meat yield, abundant milk, thick wool or the like. Too, there are often bony changes in domesticated species that permit them to be recognized in the fossil record.