DNA evidence of human tuberculosis from the bones of a woman and an infant excavated more than a decade ago at a 9,000-year-old village submerged off Israel's coast suggests the disease appeared in humans much earlier than thought. Bone damage apparently produced by some type of infection created a dilemma that puzzled excavation director and anthropologist Israel Hershkovitz, head of the Dan David Laboratory for the Search and Study of Modern Humans at Tel Aviv University in Israel.
     Thanks to a genetic analysis of the skeletons directed by Helen Donoghue and Mark Spigelman, both of University College London, Hershkovitz now knows that his team unearthed the earliest known cases of human tuberculosis. A roughly 25-year–old mother had apparently passed on the bacterial infection to her 1-year–old child, after which they both died and were buried together. Other instances of human tuberculosis that have been confirmed by ancient DNA analyses date to no more than about 5,500 years ago in Egypt and Sweden.
     The Atlit-Yam site was located within marshland, the graves were encased in clay, eventually covered by thick layer of sand and later by salt water, protecting the bones from decay. Examination of DNA from the Israeli skeletons supports the idea, based on earlier studies of genetic variation in different strains of modern tuberculosis bacteria, that bovine tuberculosis evolved after human tuberculosis did, Hershkovitz and his colleagues conclude in a report.
     Infection-related bone damage is difficult to pin on any specific disease, notes biological anthropologist George Armelagos of Emory University in Atlanta. "The genetic analysis of the Atlit-Yam skeletons really opens up our understanding of the human form of tuberculosis by showing that it was not derived from cattle but evolved well before animal domestication," Armelagos says. According to one longstanding hypothesis, tuberculosis initially infected people who drank the milk of domesticated cattle that carried a unique strain of the bacterium. New DNA data from the two Atlit-Yam skeletons "give us the best evidence yet that in a community with domesticated animals but before dairying, the infecting strain of tuberculosis was actually the human pathogen,"
     Hershkovitz estimates that human tuberculosis first evolved around 10,000 years ago, when agriculture's emergence led to densely populated settlements that acted as petri dishes for infection. Tuberculosis may have infected small numbers of people before that, but the bacteria could not have spread widely in small bands of nomadic hunter-gatherers, the Israeli anthropologist argues. Hershkovitz suspects that tuberculosis epidemics led to the demise of early farming communities and their distinctive cultural practices around 8,000 years ago. A new wave of agricultural settlements, which featured the first examples of pottery making, soon followed.
     In Armelagos' view, human tuberculosis could have originated as early as 20,000 years ago. Confirmation of the bacterium's evolutionary age will depend on finding late Stone Age skeletons that show signs of infection, and then successfully extracting DNA from them. Earlier this year, another research team reported that a 500,000-year–old Homo erectus skull found in Turkey displayed bone damage that probably resulted from tuberculosis. Both Hershkovitz and Armelagos regard that claim as unsubstantiated.
     Dr Simon Mays, skeletal biologist at English Heritage's Centre for Archaeology, said: "This does predate the other earliest convincing cases of TB from Italy by about 6,000 years. It tells us that the human form of TB is quite ancient." Ultimately, Dr Donoghue's team hope their work will help others find more effective treatments for TB. About nine million new cases of TB, and nearly two million deaths from TB, are estimated to occur around the world every year.

Sources: PlosOne, Science News (14 October 2008), BBC News (15 October 2008)