But now an international team of researchers claims it was the other way around. They say TB appeared in humans much earlier -- 60,000 years earlier -- and then spread to animals.

The team is under the direction of Sebastien Gagneux of the Swiss Tropical and Public Health Institute. He explains why it is important to study the history of TB.

“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.”

He notes that humans and TB 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 human bodies. Yes, side by side, or one inside the other.”

Scientists know that people have bacteria on them and in them all the time. In fact, the bacteria help keep us alive. Researchers are attempting to learn if tuberculosis bacteria were always harmful to humans.

Scientists want to know why only five to 10 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 good for human health?

“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.”