Tapeworms that plague humanity originated in carnivores such as lions and hyenas, and jumped to humans after we began eating their prey animals on the African savannah.
This contradicts the long-held view that humans acquired tapeworms from our domesticated livestock, cattle and swine, following the agricultural revolution around 10,000 years ago.
In a 2001 paper by Eric Hoberg and colleagues working at the United States Department of Agriculture, genetic analysis of a wide variety of tapeworm species found that the human-infecting varieties were most closely related to those that infect modern lions and hyenas (1).
Tapeworms are a large group from the so-called ‘flatworm’ family (known technically as the platyhelminthes), which are relatively simple creatures with no body cavity, heart, circulatory system or lungs. They are evolutionarily simpler than the unrelated but misleadingly similarly named ‘roundworms’ (also called nematodes), which also parasitize humans and animals.
The main tapeworm species that infect humans are Taenia saginata, Taenia asiaticus (both beef tapeworms), and Taenia solium (the pig tapeworm).
Like most tapeworms, these parasites have a relatively simple life cycle involving a definitive predator host in which the adults live, and an intermediate host in which juveniles reside as tissue cysts called cysticerci. When the predatory definitive host eats (undercooked) meat infected with cysticerci, the cysts hatch and adult worms can grow to meters in size in the definitive host’s intestines, producing eggs which pass in the faeces and are eaten by the herbivorous intermediate hosts, whereupon cysticirci form in the tissues and the life cycle is completed.
Humans act as the definitive host for T. saginata and T. asiatica by eating undercooked beef infected with tapeworm cysticerci, and likewise for T. solium by ingestion of undercooked infected pork. The adult worms cause relatively little disease other than mild gastrointestinal discomfort, and can be asymptomatic. They produce eggs at a prodigious rate that are continuously shed into the environment through faeces.
But humans can also act as intermediate hosts following ingestion of tapeworm eggs from the contaminated environment. Cysticirci can then form anywhere in the body, including major internal organs such as the heart and brain, causing heart failure in the former and epilepsy in the latter. Cysticircosis, as it is known, can be fatal.
Scientists traditionally adopted an ‘anthropophilic’ view in which humans acquired T. saginata and T. asiatica from ancestors of cows, and T. solium from ancestors of pigs, following the domestication of these species. But Hoberg’s work turns this on its head: one of the closest relatives to T. saginata and T. asiatica is T. simbae. As you might guess from the name, this is a tapeworm whose definitive host is the African lion (‘simba’ is Swahili for lion). T. simbae cycles between antelope and lions in a typical predator-prey lifecycle. Similarly, the closest relative of T. solium is T. hyaenae, which cycles between the predatory hyena and various prey species including Impala and the Sable antelope.
The implication is that humanity acquired these tapeworms following a shift in diet from herbivory to omnivory; we started eating the same big-game African prey animals as lions and hyenas did, and thus became another definitive host for the worms. Indeed, this may well have pre-dated the origin of our species, Homo sapiens, going back as far as 2 million years to the origin of the genus Homo.
It is thought that climatic change brought our ape ancestors out from the forests into the open plains, where they could have incorporated big-game meat into their previously mainly herbivorous diet. Perhaps relying on scavenging from kills by hyenas and lions at first, species such as Homo habilis (the first tool-user, or ‘handy man’) and Homo erectus subsequently developed the skills needed to hunt prey for themselves, and our digestive physiology evolved accordingly.
According to this view, humans only recently infected our domesticated livestock with the tapeworms we had acquired from African game much earlier, T. saginata and T. asiatica entering cattle as the new intermediate host and T. solium cycling through pig meat.
Furthermore, given that humans can act as both definitive and intermediate host for T. solium, Hoberg reflects that cannibalism amongst our ancestors may have enhanced the transmission of this parasitic tapeworm, cycling between intestinal adults and tissue cysts in the same species.
So pigs and cows should be blaming us for infecting them with cysticircosis and not the other way round! A fantastic finding, and one discussed in Alice Roberts’ latest superb BBC series on human evolution, Origins Of Us.
(1) E P Hoberg, N L Alkire, A de Queiroz, and A Jones.Out of Africa: origins of the Taenia tapeworms in humans. Proc Biol Sci. 2001 268(1469): 781–787.