Tuberculosis (TB), a bacterial infection, is currently the most deadly single pathogen in the world. How did this bacterium become a global scourge? A recent paper published in PNAS posits that when early humans began to utilize fires, they generated conditions ideal for the emergence of TB. If this hypothesis is correct, it could have serious implications for the study of emergent infectious diseases and how they interact with cultural and technological advances.

Evolutionary data, including whole genome sequencing, suggests that TB first became a human pathogen tens of thousands of years ago during the Neolithic period on the continent of Africa. It’s thought to have come from land mammals (especially bovines). There is also data suggesting that, in the Americas, TB hopped into infected humans from sea mammals. For the purposes of this paper, however, the authors chose to focus on the Africa-based ancestors of modern TB.

For an environmental pathogen to become an endemic human disease, it must first undergo a series of profound evolutionary transformations. The pathogen must adapt to the biological environment of their human hosts and must be able to readily move among them. The authors of the new paper discuss ways in which fire may have promoted these necessary transformations.

In its current evolutionary state, TB causes only latent infections in most humans—it’s typically not an active infection, and the transmission rate is low. Any precursor to modern tuberculosis would probably have needed to overcome a low transmission rate in order to become an endemic human infection. The authors suggest that this is where fire comes in. Its use would have created a sudden boom in transmission opportunities, which would have helped TB to overcome its low transmission rate.

In building their argument, the authors note that the earliest convincing data for human control of fire predates estimates for the emergence of the ancestors for modern tuberculosis.

How could fire influence transmission? The authors suggest that it enabled new contexts for human social interactions and thus transmission. Fires provided social focal points for early humans, particularly when food was being prepared or consumed or after daylight hours. The maintenance of a fire would have required increased cooperation and social interaction, all of which increased the probability of infectious disease transmission.

Additionally, the inhalation of smoke from fires, particularly in enclosed or indoor spaces, likely made the pulmonary tissues of early humans especially sensitive to infection. In support of this idea, the authors cite modern epidemiological evidence, which indicates that exposure to smoke from tobacco increases the risk of TB. They suggest that when early humans were tending fires, it was likely under very low ventilation conditions, which would have a similar effect to smoking on pulmonary tissues.

To test these ideas, the authors developed a mathematical model showing how the ancestor of modern TB could have evolved and adapted to reach the minimum transmission rate that would lead its modern status as endemic. They found that the controlled use of fire increased this probability by several degrees of magnitude.

Though this paper isn’t necessarily conclusive, it does make a carefully constructed argument that fire helped foster TB’s rise, using a comprehensive patchwork of evolutionary, epidemiological, and anthropological data. The paper shows how a careful synthesis of findings from different fields can provide new ways of looking at our evolutionary past and the development of human relationships with pathogens.

PNAS, 2016. DOI: 10.1073/pnas.1603224113 (About DOIs).