Tuberculosis is caused by a species of Mycobacteria that initially lived in water and was pretty harmless. How did this disease leap into human lungs and become so virulent? And how did it spread throughout the globe?
A new study suggests that the first transmission of tuberculosis to humans in the New World may have come from an unlikely source: seals.
For years scientists had a working hypothesis for how human TB spread and evolved. Here’s Nicole Skinner, writing for Scientific American:
The general opinion in recent years has been that TB emerged about 70,000 years ago, and that modern humans first acquired it before leaving Africa. These dates were worked out by measuring how much all known strains of TB bacteria differ from each other, and then using the rate at which genetic differences accumulate—a ‘molecular clock’—to work out how much time was needed for all that diversity to evolve.
This would explain why modern American strains of Mycobacterium tuberculosis so closely resemble those from Europe—inhabitants of the New World would have acquired tuberculosis after the Spanish conquest in 1492. But there’s one glaring discrepancy in the archaeological evidence: in the mid-twentieth century, scientists discovered TB-like lesions on the bones of ancient coastal Peruvians. Their 1,000-year-old skeletons suggest that human tuberculosis existed on the continent long before contact with Europeans.
Now, a team led by palaeogeneticist Johannes Krause at the University of Tübingen in Germany has conducted a new molecular clock calibration using the differences between modern strains and three 1,000-year-old Peruvian strains as a guide. They published their results in Nature yesterday.
Here’s John Timmer, writing for ArsTechnica:
The authors were able to obtain M. tuberculosis genomes from all three samples and used these to reconstruct an evolutionary tree of the bacteria’s history. And, when they did, they got a bit of a surprise: their results showed that the human form of M. tuberculosis is only about 4,000 years old. In a bit of what passes for deadpan humor among scientists, the authors write that the results, “presented us with a challenge to explain how a mammalian pathogen could have reached human populations in the Americas about 10,000 years after inundation of the Bering land bridge.”
Fortunately, their results also came with a solution. Rather than clustering with one of the human lineages, the samples from the Peru skeletons showed up on the pinniped lineage—the strain that infects seals and sea lions. The authors suspect that the seals probably picked up the disease in Africa and then transported it across the Atlantic to South America. From there, it spread around the periphery of the continent to Peru, where the locals picked it up by dining on some pinnipeds.
This study is based on only three skeletons, so we can’t be sure it tells the whole story. At the very least, though, it’s a humbling example of how many untold pathways exist for the spread and evolution of infectious diseases.