Fire brought warmth and comfort to early humans but may also have triggered the emergence of deadly tuberculosis, Australian researchers suggest.

Key points: Tuberculosis first emerged in humans in Africa then spread to animals

Tuberculosis first emerged in humans in Africa then spread to animals Mystery about how a benign microorganism became a killer

Mystery about how a benign microorganism became a killer Hypothesis suggests fire increased human interaction and smoke made lungs more susceptible to disease

Smoke-damaged lungs, as well as the closeness of humans around a campfire, could have created the ideal conditions for tuberculosis to mutate from a harmless soil bacterium into our number one bacterial killer, according to the researchers' data model.

The model, published today in the Proceedings of the National Academy of Sciences, showed controlled use of fire would have increased the likelihood of tuberculosis emerging by several orders of magnitude.

Mathematical biologist Associate Professor Mark Tanaka of the University of New South Wales has had a long-standing interest in the evolution of disease-causing microorganisms such as tuberculosis, but a sudden insight led him to think about the role of fire in catapulting tuberculosis into the medical limelight.

"When fire was controlled around 400,000 years ago in human populations, probably cultural practices changed, so there would have been increases in social interactions, increases in population density and increased time for interaction into the night," said Dr Tanaka.

This increased physical contact — which makes the spread of the disease more likely — was coupled with wood smoke, which also increases the risk of contracting tuberculosis because of damage to the lungs.

The rise of tuberculosis

Dr Tanaka and his colleagues from the University of New South Wales and Monash University said archaeological and molecular evidence indicated tuberculosis arose in humans in Africa and then spread to animals.

Just when this happened is controversial — while some research puts the date more recently at 6,000 years, others point to a date of 70,000 years.

While the date of the earliest use of fire is also subject to debate, Dr Tanaka suggested that it probably came before tuberculosis.

In the study, researchers used a mathematical model to explore how a benign soil-dwelling microorganism like Mycobacterium tuberculosis might develop into a transmissible pathogen.

"In the absence of fire, this [mutation process] would be ... extremely unlikely because the bacterium ... lives in the soil and water, and doesn't transmit between people because it's not adapted to humans," Dr Tanaka said.

But adding fire to the mix significantly increases the risk of the bacteria hitting the mutation jackpot.

"You get multiple sporadic cases, and most of them fail in the sense that they fail to evolve and so there are multiple failed chains of transmission, but eventually the right mutations come along and the whole thing is triggered."

Health consequences of technological developments

Paleopathologist Dr Piers Mitchell from the Department of Archaeology and Anthropology at University of Cambridge said the change from harmless soil bacteria to deadly disease would not have happened overnight.

"People seem to have been using fire for hundreds of thousands of years, so it's clearly not a case of 'fire this week, tuberculosis the next week'," Dr Mitchell said.

But he said the idea that controlled fire enabled the emergence of tuberculosis was certainly plausible, and fitted with the hypothesis that human technological developments could have health consequences.

"It makes sense if our invention of technologies changes our environment, and some bits of that will be good and some bits may, in unexpected ways, be bad," Dr Mitchell said.