By Helen Briggs
BBC News science reporter
The tuberculosis bacterium emerged in East Africa three million years ago and may have spread around the world when early humans left their ancestral home.
Humans may have taken TB with them as they migrated
According to molecular analysis of modern strains, the pathogen is much older than previously thought.
As such, it predates other human afflictions such as the plague.
French researchers hope the work will lead to improved diagnosis and treatment of TB, which kills three million people each year.
TB is re-emerging in areas such as Eastern Europe, south-east Asia, and sub-Saharan Africa, due to the spread of drug-resistant strains of the disease and the rise in HIV.
It is caused by the bacterium Mycobacterium tuberculosis, which attacks the lungs, giving rise to symptoms such as coughing, loss of appetite, fever, and night sweats.
TB has long been a human disease - tissue samples from Egyptian mummies over 4,000 years old show signs of infection.
TB is caused by Mycobacterium tuberculosis
Until now, scientists had believed the disease arose a few tens of thousands of years ago and then spread rapidly around the world.
But investigation of a rare tuberculosis-causing bacterium isolated from patients in East Africa suggests the roots of the disease go back much further.
Molecular analysis indictaes that the East African samples and the commoner strains are all descended from a more ancient bacterial species that emerged in Africa as long as three million years ago.
"Tuberculosis could thus be much older than the plague, typhoid fever, or malaria, and might have affected early hominids," said senior researcher Veronique Vincent of the Institut Pasteur in Paris.
Understanding the ancient origins of the bacterium could help in the development of new drugs against TB, which is rapidly growing resistant to the drugs used to treat it.
"Now we have in our hands bacteria that are quite different from the others and all are able to produce TB," co-worker Cristina Gutierrez told the BBC News website. "Maybe we could find which the essential genes for virulence are."
The research is published in the first edition of a Public Library of Science (PLoS) journal, PLoS Pathogens.