Tuberculosis (TB) infected early humans in Africa 70,000 years
ago and was carried worldwide by human out-of-Africa migrations,
according to new research into the genome sequences of
Mycobacterium tuberculosis, the bacteria responsible for the
disease.
Douglas Young of the MRC's National Institute for Medical
Research (now part of the Francis Crick
Institute) worked with an international team to
identify changes over that time in the genome. These changes caused
TB to become a more aggressive disease. The work is hoped to lead
to new targets for drug treatment.
Dr Young explained: "The number of tuberculosis bacteria
increased markedly in association with human population expansion
during the Neolithic revolution, perhaps adapting to cause a more
virulent form of disease.
"If we are correct in inferring that TB has moved from a
predominantly latent infection to a more aggressive disease,
identification of underlying changes in the bacterial genome may
uncover novel targets for drug development."
Dr Young and his team used high-throughput DNA sequencing
techniques to analyse the whole genomes of 259 strains of M.
tuberculosis. They then used a series of statistical tools and
methods for analysing biological data to construct a model of how
the bacterium evolved and to set this within a geographic and
historical context.
They suggest that the long history of M. tuberculosis evolving
alongside humans (for 70,000 years) shows that it has been able to
adapt to changing human populations during this time - including
both low and high population densities. Diseases infecting high
density populations tend to be highly infectious and easily
transmitted, relying on large populations to make sure the disease
doesn't become extinct, while diseases infecting low density
populations survive using characteristics such as periods of
latency followed by reactivation. M. Tuberculosis has evolved to
show both patterns.
Dr Young concluded: "A potential implication of this line of
research is the identification of strains of tuberculosis bacteria
that differ in the threat they pose to public health. This may
contribute to future drug discovery and to the design of improved
strategies for disease control."
He added: "The idea that TB has co-evolved with modern humans
throughout our history of migration and population expansion is of
fundamental interest in our efforts to understand the human
condition. Tracking events affecting evolution of the bacteria may
provide novel insights into events in our own prehistory."
The paper, Out-of-Africa migration and Neolithic co-expansion of Mycobacterium
tuberculosis with modern humans, is published in Nature
Genetics.