Tuberculosis, an often fatal bacterial infection of the lungs, was a scourge in the days before antibiotics. It's caused by a species of Mycobacteria, most of which live harmlessly in watery environments. Understanding how some of these have managed to make the leap to human lungs has turned out to be rather complicated. Further evidence of this comes from a study published Wednesday that suggests that infectious strains of the bacteria managed to cross the Atlantic before the first European strains did—carried in the lungs of seals.
Getting things wrong about the history of tuberculosis seems to be a regular pastime of the people who study infectious diseases. Originally, due to some genetic similarities, people had proposed that we had picked it up from farm animals. But a careful study of evolutionary trees recently showed that it's likely that cows actually picked up tuberculosis from us, rather than the other way around.
Similarly, the study of the strains found in the Americas had suggested that all of the bacteria present here had been derived from the European version. Which suggested that, along with other lovely gifts like smallpox, the disease was brought to the New World by the first European settlers.
Problems with this idea also cropped up, however. Tuberculosis infections can leave marks on the victims' bones, and some American skeletons that date from pre-Columbian times had these markings—suggesting the disease was here first. The new study, performed by a large international consortium, is an attempt to bring some clarity to this situation.
In order to do so, the researchers gathered samples from nearly 70 skeletons that showed signs of tuberculosis infection, taken from a mix of pre- and post-contact sites. From that large collection, however, they were only able to obtain M. tuberculosis DNA from three. All three come from the same region of coastal Peru—which suggests that there was something distinctive about the preservation conditions there—and date from about a thousand years ago—significantly before European contact.
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 process is apparently still going on. The authors' data also suggests that the pinniped strain managed to make it to Australia about 700 years ago.)
Three skeletons from one region are not enough to give us a clear picture of the disease's history in the Americas. But the results do unambiguously tell us that the disease arrived here well before European humans and that the culprit (or at least one of the culprits) was a seal. Still, obtaining samples from some other locations in the Americas will be essential for a clear picture.
After all, if there's one thing that the history of M. tuberculosis research tells us, these conclusions are probably wrong in one detail or another.