Mystery journey
If these findings are correct and M. tuberculosis is less than 6,000 years old, they pose another conundrum. The microbe must have reached the Americas before Europeans arrived, but after the land bridge between North America and Asia disappeared around 11,000 years ago, Krause notes. “How did it get to America, say, from Asia, if there was no longer a land bridge?”
In the search for answers, the researchers decided to look at the genetics of 40 different strains of tuberculosis that infect animals. They found that the ancient strains from Peru were not like human-adapted forms, but were very similar to forms called Mycobacterium pinnipedii, which are adapted to seals and sea lions.
Mycobacterium pinnipedii has been transmitted from seals to humans in zoos. Brown notes that in the past archaeologists have speculated that this might have been a source of TB infection in coastal areas of South America, where seals were hunted.
Tom Gilbert, an evolutionary biologist at the University of Copenhagen, says that the seal-transfer idea is certainly attractive, but he is cautious about the paper’s conclusion. “What’s more likely — that marine mammals gave rise to tuberculosis in humans in South America, or that we simply haven’t sampled enough relevant terrestrial hosts on the continent to spot the true ancestor?”