Tapeworms tell tales of deeper human past

People have been blaming the wrong episode in history for endowing our species with tapeworms, according to parasitologists.

Adult tapeworms injected with ink show the intricate branches of the worm uterus. The species that people catch from raw beef (above) has more of these branches than the tapeworm associated with raw pork (below). CDC

For decades, most scientists assumed that humanity picked up tapeworms while domesticating cattle and pigs some 10,000 years ago, explains Eric P. Hoberg of the Department of Agriculture in Beltsville, Md.

After a new analysis of the tapeworm family tree, however, Hoberg and his colleagues push the wormy leap to people much further back in history. The closest relatives of our tapeworms spend their early stages in grazing animals and their adulthood in disparate African carnivores, the researchers say in the April 22 Proceedings of the Royal Society of London B. Therefore, the team suggests, human ancestors first picked up tapeworms when they started eating more of the same worm-bearing meat as big cats and hyenas did.

Such an analysis demonstrates “how elegant these parasites are and what kind of stories they can tell us,” Hoberg says. He adds that as far as he knows, this study is the first to link human tapeworm infections with increased meat scavenging and hunting.

The tapeworm work represents “a piece of human history that we haven’t had parasite evidence for before this,” says longtime parasite historian Daniel Brooks of the University of Toronto. He points out that other studies have traced parasite-people associations to other historical watersheds. For example, his own work on pinworms suggests that the pests hitchhiked right up the primate family tree to modern humans. More recently, the sheep liver fluke probably developed a taste for people as a consequence of livestock domestication.

Hoberg and his colleagues base their argument for an African origin of human tapeworm infections on a family tree that they built by comparing the body structures of 35 tapeworm species.

The three modern Taenia tapeworms that pass their adult years in the human intestine spend their youth in either cattle or pigs. However, if these tapeworms originally came from early domesticated animals, they would sprout from the same branch as do tapeworms in wild relatives of cows and pigs.

They don’t, says Hoberg. The closest tapeworm relatives to two of the human pests, Taenia saginata and Taenia asiatica, turn up on a genealogical branch with a tapeworm that matures in lions. The third human pest, Taenia solium, ends up on a distant twig next to a hyena-maturing species. That pattern held in each statistical method of tree building that the researchers tried.

Hoberg and his colleagues found enough genetic evidence to estimate when T. saginata and T. asiatica diverged from a common ancestor. From that they estimate that people picked up tapeworms between 1.7 million and 170,000 years ago, long before livestock domestication.

“The conclusion fits well with what we see” in ancient carnivore evolution, says paleontologist Lars Werdelin of the Swedish Natural History Museum in Stockholm. Roughly a million years ago, he says, hyenas’ ancestors changed anatomically as if to compete more avidly for carcasses. Their competitor could have been our ancestor, he says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.