Tuberculosis Hitched A Ride When Early Humans Left Africa
Dogs often get credit for being humans' constant companions. But dogs have nothing on tuberculosis bacteria.
TB and people have been trapped in a relationship that's been going on for thousands of years — perhaps even tens of thousands of years, scientists said earlier this week.
"The old, traditional view was that tuberculosis emerged during the Neolithic transition when people started to domesticate animals and develop agriculture, which started about 10,000 years ago," says Sebastien Gagneux, an infectious diseases specialist at the Swiss Tropical and Public Health Institute in Basel. "Our work suggest that TB is really much older."
Gagneux and his colleagues have found evidence that TB hitched its cart to the human evolutionary horse more than 70,000 years ago, before our ancestors migrated out of Africa. The researchers reported their findings Sunday in the journal Nature Genetics.
The researchers looked at the genetic code of tuberculosis bacteria in 259 samples to map the early connection between TB and humans.
Our relationship has been a scientific mystery because on the one hand, TB needs people. "The TB bacteria are exclusively human pathogens," Gagneux tells Shots. "They can only survive in humans, not in the environment. There's no known other reservoir."
But on the other hand, TB is extremely efficient at killing us — its only host. Left untreated, approximately half of the people who get sick from it will die. TB has been blamed for roughly 20 percent of all deaths in Europe during the the 19th century. The disease was so widespread in ancient Egypt that signs of the bacteria can still be seen in mummies that date back about 6,000 year ago.
TB's tendency to kill its partner seems to make it unlikely that tuberculosis would last for long. But in fact the exact opposite has occurred.
Tuberculosis appears to be one of the oldest diseases in human history. Gagneux and his team's findings may help explain this paradox.
TB's longevity appears to have its origins in the early part of the TB human courtship some 20,000 to 30,000 years ago. Back then, the bacteria developed the ability to go dormant in its hosts and then to re-emerge decades later.
Lying low could have been a survival strategy that the bacteria developed when humans were still hunter-gatherers, Gagneux says. Humans were so dispersed back then that if TB quickly killed its host, then it would never get an opportunity to spread. "Having this strategy of latency might actually be a handy way to survive in very small populations," Gagneux says.
But now TB's strategy of latency makes the disease incredibly difficult to control. It allows the bacteria to hide out in people and then burst forth later in new environments. "Through this work, we tried to address this apparent discrepancy of latency versus virulence [in TB]," he says.
As with many dysfunctional relationships, there may also be dependency issues. Gagneux and his colleagues think that that throughout history, people may have benefited from their relationship with TB. "Perhaps latent infection with tuberculosis imparted some degree of immunity against more lethal pathogens [that people] encountered in the new environment or in contact with archaic human populations," Gagneux and his team speculate in the current study.
More studies are needed to test that hypothesis. But Gagneux hopes that understanding the long, complicated relationship between TB and humans will eventually help us find a way to break out of it.