Spread it with fire
Tuberculosis (TB), a bacterial infection, is currently the most deadly pathogen in the world. How did this bacteria become a global plague? A recent article published in PNAS states that when early humans began using fires, they created conditions ideal for tuberculosis to develop. If this hypothesis is correct, it could have serious implications for the study of emerging infectious diseases and how they interact with cultural and technological advancements.
Evolutionary data, including whole genome sequencing, suggest that TB first became a human pathogen tens of thousands of years ago during the Neolithic period on the African continent. It is thought to come from land mammals (especially cattle). There is also data suggesting that TB in the Americas may have entered infected humans through marine mammals. However, for the purposes of this article, the authors chose to focus on the African-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 move easily between them. The authors of the new paper discuss ways fire may have fostered these necessary transformations.
In its current evolutionary state, TB causes only latent infections in most people – it is not usually an active infection and the rate of transmission is low. Any precursor to modern tuberculosis would likely have to overcome a low transmission rate to become an endemic human infection. The authors suggest that this is where fire comes into play. Its use would have caused a sudden increase in transmission capabilities, which would have helped TB overcome the 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 of modern tuberculosis.
How can fire affect transmission? The authors suggest that it enabled new contexts for human social interactions and thus transmission. Fires were social points of interest for early humans, especially when food was being prepared or consumed or after daylight. Keeping a fire going would require more collaboration and social interaction, increasing the chance of infectious disease transmission.
In addition, inhalation of smoke from fires, particularly in closed or indoor spaces, likely made the lung tissues of early humans especially susceptible to infection. In support of this idea, the authors cite modern epidemiological evidence showing that exposure to tobacco smoke increases the risk of tuberculosis. They suggest that when early humans were making fires, it was likely under very low ventilation conditions, which would have a similar effect to smoking on lung tissue.
To test these ideas, the authors developed a mathematical model that shows how the ancestor of modern TB could have evolved and adapted to reach the minimum transmission rate that would lead to its modern status as endemic. They found that the controlled use of fire increased this probability by several degrees.
While this paper is not necessarily conclusive, it does make a carefully constructed argument that fire promoted the rise of tuberculosis, drawing on a vast 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.
PNAS2016. DOI: 10.1073/pnas.1603224113 (About DOIs).