VOA常速英语:TB Origins Found in Africa

The origins of humans have been traced to Africa. And now, so have the origins of tuberculosis. New research shows the evolutionary trees of both humans and TB have grown side-by-side.

TB bacteria originated in Africa at least 70,000 years ago.That’s the finding of a team of researchers led by Professor Sebastien Gagneaux of the Swiss Tropical and Public Health Institute. But why study the history of TB?

Gagneaux said, “At the end of the day, it’s a certain kind of historic question and there have been long discussions about where TB came from originally. That’s on the one hand. On the other hand, the idea is that by learning from the past and how infectious disease evolves over time, this potentially could give us some clue about the future of the TB epidemic.”

To trace the origins of TB researchers relied on genetic material, which is relatively easy to come by.

“The trick is to use the genomic information that we can get from bacteria living today. That’s an approach which has been used for all kinds of other organisms, including humans, themselves. So we actually are learning a lot from what people are doing with human genetics,” he said.

Gagneaux said that the evolutionary trees of humans and TB probably did more than just grow side by side.

“I think that’s a nice way to put it. Maybe you can even say one inside the other. Imagine where the TB bacteria live, which is actually inside of human bodies. Yes, side by side, or one inside the other.”

Humans have bacteria on them and in them all the time. In fact, they help keep us alive. Researchers are trying to determine if tuberculosis bacteria were always harmful to humans.

“That’s also something that we’re trying to address in this work because there’s this striking feature in tuberculosis, which is this phenomena called latency – so-called latent infection -- meaning that people can carry these bacteria. So they’re actually infected without having any symptoms of disease. This latency period can last for several decades. Most of the people, who are actually carrying these bacteria, will actually never develop so-called active tuberculosis,” he said.

Studies are trying to determine why only five to ten percent of the estimated two billion people infected with the bacteria actually come down with active tuberculosis.

Another question is whether the bacteria were at one time beneficial to humans?

“Obviously, there seems to be something special about these five to ten percent of people who are coming down. Maybe that’s just bad luck. We know there are obvious very strong risk factors, such as HIV co-infection or malnutrition. Diabetes is also a factor, which can increase your risk of developing active tuberculosis once you have been infected. But again this idea that maybe carrying these bacteria in this latent form could potentially be beneficial because it might protect against other diseases. Again, that’s a very provocative hypothesis, which we, however, cannot completely neglect.”

Gagneaux said TB left Africa when humans did, about 65,000 to 70,000 years ago. Then, about 10,000 years ago, came the Neolithic Demographic Transition. It’s the time when people started to develop agriculture and domesticate animals. But it was also a time when diseases jumped from domesticated animals to humans for the first time. Gagneaux says for many years it was assumed that TB took the same path – from animals to humans. However, the research shows that TB in humans pre-dates the domestication of animals. It’s particularly adapted to live inside humans and can’t really survive on its own in the environment.

There was another important development during the Neolithic Demographic Transition. Humans started to form settlements that were densely populated. It’s an ideal situation for the spread of tuberculosis through the air from human to human.

“Because of these changes in these human behaviors and numbers, potentially TB might have become more virulent in the sense of causing disease maybe more quickly or maybe a more deadly disease. It’s true that TB is very deadly nowadays. So if you don’t treat it, kills up to 50 percent of people who actually have active tuberculosis,” Gagneaux said.

He added that before settlements, in the hunter-gatherer days, perhaps TB was not as deadly.

“Overly deadly would be a bad strategy for any pathogen because you might just kill off all susceptible hosts and you might end up with nobody else to infect. And so only once through this Neolithic transition -- when more and more people were actually living close by -- TB maybe evolved in a way to be able to become more virulent and take advantage, if you will, of this increasing number of susceptible people to infect.”

After humans left Africa, they started to change in appearance as they adapted to their new geographical locations. TB also evolved and now there are many different strains of bacteria that cause the disease. The strain found in South Africa differs from that found in China, for example. Africa, though, still has the greatest diversity of TB strains.

Researchers hope that knowing the evolutionary history of TB will help in the development of new drug treatments and vaccines. Currently, the number of drug-resistant TB cases is growing. The knowledge may also help predict “future patterns of the disease.”