by Mark Rose
The latest tapeworm research suggests a time frame for human migration out of Africa, and also reveals that people gave the parasite to domestic animals such as pigs--not the other way around.
According to Eric P. Hoberg, a zoologist with the U.S. Department of Agriculture, and his colleagues, tapeworms probably jumped from predators to humans between 2 and 2.5 million years ago, when hominids inhabited savannah environments in sub-Saharan Africa and were likely hunting or scavenging the same prey favored by hyenas and lions. Of the three tapeworms that infect people today, the researchers linked one, Taenia solium, most closely to a species that uses hyenas and African hunting dogs as its hosts. The other two, T. saginata and T. asiatica, are linked most closely to a species with lions as its host. Because T. solium and the pair T. saginata/T. asiatica are only distantly related, it appears that two tapeworms independently made the jump to hominids: the ancestor of T. solium, and a single parent species of both T. saginata and T. asiatica.
The amount of genetic difference between T. saginata and T. asiatica suggests that they split from a single human-adapted species by 160,000 years ago, says Hoberg. Worm-carrying human populations may have migrated out of Africa at or before this time, leading to the separation of parasite populations that evolved into the two species. Cattle eventually became the intermediate host in T. saginata, pigs in T. asiatica. Because domestication took place much later, around 10,000 years ago, and because neither animal comes from a sub-Saharan progenitor, the transmission of tapeworms had to be from humans to cattle and pigs, not the reverse. It has long been thought that prohibitions on eating pork in some religions stem from their role as tapeworm hosts, hence being "unclean." The new study suggests pigs may have gotten the blame deserved by our own ancestors.