Paleogeneticist Dr. Johannes Krause (University of Tübingen, Germany) and colleagues were interested in the origin of tuberculosis (TB) in the Americas. Since strains of TB found in the Americas are related to strains found in Europe, prior theories held that Spaniards may have introduced it to the Americas while colonizing South America. The problem with those theories is that pre-Columbian skeletal remains showed signs of TB much earlier.
Dr. Krause was quoted in Scientific American, “Pathogens don’t leave fossils, but they do leave their DNA in the skeleton, teeth and bones of the victims of the disease.” So the research team sequenced M. tuberculosis bacteria that were extracted from 1,000 year old skeletal remains of humans in Peru. They calculated the rate that TB strains evolved from 1,000 years ago to today. What they discovered was that the most recent common ancestor of all current strains of M. tuberculosis had evolved only 6,000 years ago. According to a quote in Scientific American by Dr. Terry Brown (biomolecular archeologist at University of Manchester, UK), “This is a landmark paper that challenges our previous ideas about the origins of tuberculosis, not just in the Americas but in the Old World too.” These calculations suggest that the bacteria arrived in America before the Europeans did but sometime after 11,000 years ago (after the land bridge between Asia and North America had already disappeared). Moreover, the strains from Peru were not like human-adapted strains.
If it was not brought across from the land bridge, as was previously thought, where did it come from? To help solve this puzzle, they also sequenced strains of TB that infect animals and discovered that the strains from Peru were quite similar to Mycobacterium pinnipedii, the strains that infect seals and sea lions. Since people at zoos have contracted this strain from seals, it seems reasonable to speculate that perhaps early seal hunters in the Americas were likewise infected by their prey. Although others suggest that perhaps we may not have sampled enough potential hosts in the Americas to find the real ancestor of human TB in America. It is also not clear whether seal-transmitted TB could be passed from human to human. But it is an intriguing theory.
Bos KI, Harkins KM, Herbig A, Coscolla M, Weber N, Comas I, Forrest SA, Bryant JM, Harris SR, Schuenemann VJ, Campbell TJ, Majander K, Wilbur AK, Guichon RA, Wolfe Steadman DL, Collins Cook D, Niemann S, Behr MA, Zumarraga M, Bastida R, Huson D, Nieselt K, Young D, Parkhill J, Buikstra JE, Gagneux S, Stone AC, Krause J. Pre-Columbian mycobacterial genomes reveal seals as a source of New World human tuberculosis. Nature. In press, 2014. doi:10.1038/nature13591