New analysis of fossilized teeth suggests the bubonic plague first appeared during the Bronze Age, thousands of years before the disease -- dubbed the Black Death -- decimated Europe during the 14th century.

Researchers from universities of Copenhagen and Cambridge surveyed 101 fossilized teeth collected from Central Europe and Eurasia. Seven contained evidence of the plague, the oldest from a man who died 5,783 years ago.

Analysis of the infected seven showed early strains of the plague were not easily transmitted and likely not carried by fleas.

Six of the seven plague samples were without what scientists call the "virulence gene," dubbed ymt. Also missing was a an "activator gene" mutation called pla.

The ymt gene, found in all modern strains, enables the plague bacteria Yersinia pestis to survive the toxic environs of a flea's digestive system. In doing so, it strangles the flea's digestive tract and causes the starving flea to bite the nearest warm body, looking for a meal -- thus, encouraging the spread of the disease.

Meanwhile, the pla mutation allows Y. pestis bacteria to infect different tissues -- thus, turning a lung infection into a full-body disease and enabling the plague to attack the blood and lymph nodes.

Early forms of the plague were endemic to human populations, but without the mutations, the disease was pneumonic only. Still, the pneumonic strain was increasingly on the move.

"The Bronze Age was a period of major metal weapon production, and it is thought increased warfare, which is compatible with emerging evidence of large population movements at the time," said researcher Marta Mirazon-Lahr, a professor at Cambridge's Leverhulme Centre for Human Evolutionary Studiesexplained in a press release. "If pneumonic plague was carried as part of these migrations, it would have had devastating effects on small groups they encountered."

"Well-documented cases have shown the pneumonic plague's chain of infection can go from a single hunter or herder to ravaging an entire community in two to three days," she added.

At some point, however, the ymt and pla mutations turned the plague into an even deadlier force.

The most recent of the seven infected teeth featured both key genetic mutations. The tooth was found in present day Armenia and dated to 951 BCE, suggesting the bubonic plague that eventually spawned the Black Death emerged around the turn of the first century BCE.

The Bible's Books of Samuel tells of a pandemic among the Philistines in 1320 BCE, referencing groin swelling, a sign the plague had begun attacking the lymph nodes. The World Health Organization calls it the first description of bubonic plague.

The latest evidence -- detailed in the journal Cell -- supports the theory that the more deadly version of the bubonic plague got its start in the Middle East.

The timing also fits neatly with what scientists understand about bacterial evolution and human history. Infections can't become too lethal too quickly or they will die out. If bacteria kills a flea before it can spread the disease, the bacteria will die out along with the carrier. Especially lethal strains require denser populations and more frequent interactions among carriers.

These conditions were becoming reality at the end of the Bronze Age as human populations began to move more frequently. Trade was growing and warfare was common.

"As Eurasian societies grew in complexity and trading routes continued to open up, maybe the conditions started to favour the more lethal form of plague," said researcher Robert Foley, also a professor at Leverhulme. "The Bronze Age is the edge of history, and ancient DNA is making what happened at this critical time more visible."