Origins of TB may lie in humans, not animals

The origins of human tuberculosis have been traced back to African hunter-gatherers living 70,000 years ago, scientists claim.
The study by an International team of scientists suggests that Tuberculosis (TB) did not originate in animals some 10,000 years ago, spreading to humans, as commonly believed.
Tuberculosis remains one of deadliest infectious diseases of humans, killing 50 per cent of individuals when left untreated, researchers said.
Even today, TB causes 1-2 million deaths every year mainly in developing countries. Multidrug-resistance is a growing threat in the fight against the disease.
Researchers led by Sebastien Gagneux from the Swiss Tropical and Public Health Institute (Swiss TPH) has now identified the origin in time and space of the disease.

Using whole-genome sequencing of 259 Mycobacterium tuberculosis strains collected from different parts of the world, they determined the genetic pedigree of the deadly bugs.

This genome comparison, published in the journal Nature Genetics, indicates that TB mycobacteria originated at least 70,000 years ago in Africa.
The researchers compared the genetic evolutionary trees of mycobacteria and humans side-by-side. And to the researcher's surprise, the phylogenetic trees of humans and the TB bacteria showed a very close match.
"The evolutionary path of humans and the TB bacteria shows striking similarities," said Gagneux.
This strongly points to a close relationship between the two, lasting tens of thousands of years.

Humans and TB bacteria not only have emerged in the same region of the world, but have also migrated out of Africa together and expanded all over the globe, researchers said.
The migratory behaviour of modern humans accompanied with changes in lifestyle has created favourable conditions for an increasingly deadly disease to evolve.
"We see that the diversity of tuberculosis bacteria has increased markedly when human populations expanded," said Gagneux.
Human expansion in the so called Neolithic Demographic Transition (NDT) period combined with new human lifestyles living in larger groups and in village-like structures may have created conditions for the efficient human-to-human transmission of the disease, Gagneux suggests. This may also have increased the virulence of the bacteria over time.

The results indicate further that TB is unlikely to have jumped from domesticated animals to humans, as seen for other infectious diseases.