Controlled fire use in early humans might have triggered the evolutionary emergence of tuberculosis

Mycobacterium tuberculosis cdc.gov

When did tuberculosis arrive as a major lung pathogen of humans? Rebecca Chisholm, James Trauer, Darren Curnoe and Mark Tanaka, all from the University of New South Wales in Sydney, have presented a very nice story purporting to demonstrate how human cultural innovation can lead to social innovation and, in turn, to diseased respiratory systems that might have provided the perfect environment for a benign soil mycobacterium to turn nasty and become transmissible.

They hypothesise that the tubercle bacillus, Mycobacterium tuberculosis, evolved from any number of possible mycobacterial candidates which would have been ubiquitous in soil and vegetation. Several of these benign non-tubercular mycobacteria can, nevertheless, cause opportunistic infections today in humans if they are immunologically compromised or suffer from inflammatory lung diseases. Sometime between 17,000 and 70,000 years ago, they consider (with a preference for the deeper historical marker) such mycobacteria could have occasionally established themselves in human respiratory systems, make the evolutionary transition to pathogenicity, and, following a period when such infections were rare events and difficult to pass from person to person, transmissibility secondarily evolved.

The controlled use of fire for heating, cooking, ground clearance and signalling etc. undoubtably arose before the suggested dates for mycobacterial infection. It dates way back to Homo heidelburgensis. A large proportion of biomass smoke constituents, they point out, are in the inhalable size range, and many of these constituents incite chronic inflammatory and destructive changes in the respiratory system. Early hunter–gatherer communities burning biomass fuels in poorly ventilated spaces (like caves and in the centre of tent or hut-like structures) were, therefore, vulnerable to a wide range of unprecedented adverse health effects. Many pulmonary conditions like chronic obstructive pulmonary disease are associated with TB infection and an impairment of local immunity.

This is how they sum up their research: “Here, we have argued that the extensive changes to human ecology and unprecedented physiological consequences brought about by the controlled use of fire in the Pleistocene created ideal conditions for the emergence of TB. It is possible that during this period of significant ecological and social change, range extensions leading to the consumption of novel food sources and altered energy requirements increased exposure of early humans to the natural reservoir of ancestral Mycobacterium tuberculosis, likely the soil. This increased exposure brought about an increasing number of infections and stuttering transmission chains, both of which provided new opportunities for within-host adaptive evolution. Coupled with increasing host susceptibility to mycobacterial infection attributable to biomass smoke-induced lung damage and the increased opportunities for transmission brought about by the developing social culture that fire use encouraged, we hypothesize that this precursor species evolved transmissibility relatively quickly, almost guaranteeing it’s emergence as a specialized human pathogen.” In other words, start with the controlled use of fire in closed places and the ubiquity of mycobacteria in the environment, add in smoke-induced lung damage and social crowding to aid transmission, and cultural change provides the perfect context for the emergence of a new and deadly pathogen.