Marine mammals may have carried TB bacterium across the ocean
Seals brought tuberculosis to South America long before Columbus sailed to the New World, a new study shows.
An analysis of tuberculosis DNA recovered from three 700- to 1,000-year-old Peruvian skeletons reveals that the strain of TB in the ancient bones doesn’t match the strain brought to the New World by European explorers. Instead, it closely resembles one that infects seals in the Southern Hemisphere, an international group of researchers reports August 20 in Nature.
Researchers had long thought that Mycobacterium tuberculosis, the bacterium that causes TB, originated in cattle as M. bovis, jumped to humans after dairy cows were domesticated and then came to the Americas with Europeans.
Several pre-Columbian skeletons in the New World, however, have been found with spine and rib deformities suggestive of TB infections, says Christina Warinner, a molecular anthropologist at the University of Oklahoma in Norman who was not involved in the new study. Those skeletons sparked speculation that TB was even more ancient than previously thought, perhaps migrating out of Africa as humans colonized the globe. In that scenario, scientists reasoned that TB would have crossed the Bering land bridge with the first people to settle the Americas.
The new study concludes that all animal strains of TB are actually derivatives of a human-infecting subspecies that originated in Africa, notes study coauthor Johannes Krause, a paleogeneticist at the University of Tübingen in Germany. “Africa really seems to be the cradle of tuberculosis, not just humankind,” he says. He and his colleagues used genetic data from many types of mycobacteria to construct a tuberculosis family tree. The human-infecting strains formed seven major branches. All of the animal-infecting TBs, including the seal one, were related to M. tuberculosis from one of those branches.
Using the Peruvian skeletons as a time stamp, Krause and colleagues calculated that TB mutates 10 times as rapidly as indicated by previous estimates that had used humans’ migration out of Africa as a calibration. Using the new mutation rate, the researchers determined that TB first infected humans about 4,000 to 4,400 years ago in Africa. That’s a much younger date — by at least 5,000 years — than previously suggested, but Krause is confident. “Ours is the first directly calibrated date,” he says, “Everything else is an estimation or assumption.” Traders probably spread the disease around the Old World along with their wares.
The date had the researchers scratching their heads about how an African strain could get to South America, says study coauthor Anne Stone, an anthropological geneticist at Arizona State University in Tempe. The Bering land bridge had been drowned 1,000 years or more before the bacterium jumped into humans, so TB couldn’t have come to the New World with the first Americans. And the ancient Peruvian strain clearly arrived before Europeans. Instead, says Stone, “It swam to the Americas.”
She and her colleagues speculate that people in Africa passed TB to mice, hyraxes or some other animal, which then infected seals. Seals gave the disease back to humans in South America.
All the human skeletons belonged to the Chiribaya culture and were excavated from sites near Peru’s southwestern coast. Seal-bone tools found at the sites indicate that these people probably had contact with seals. But the ancient Peruvians’ TB was slightly different than the M. pinnipedii that infects modern seals. The researchers estimate that the bacterium may have started to infect humans at least 100 years before those ancient Peruvians died. The timing suggests the bacterium may have adapted to humans, enabling person-to-person transmission.
Pre-Columbian skeletons from Illinois also show signs of TB infection, Warinner says. She wonders whether the seal bacterium caused an epidemic throughout the Americas, or if people in Illinois caught the disease from a different animal source. “They weren’t getting it from seals,” she says.
The modern-day seal bacterium is not known to infect people, and the strain that infected ancient Peruvians is no longer around. European strains now cause most TB infections in the Americas, says Eric Rubin, a microbiologist at the Harvard School of Public Health who was not involved in the study. The seal version may have petered out because pre-Columbian American populations were too sparse to sustain its spread, or because it wasn’t virulent enough to compete with the European strains.
Editor's note: This story was updated on August 26, 2014, to correct the age of the skeletons.
K.I. Bos et al. Pre-Columbian mycobacterial genomes reveal seals as a source of New World human tuberculosis. Nature. Published online August 20, 2014. doi:10.1038/nature13591.
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