The plague wiped out millions of people beginning in the sixth century and through medieval times, but a study Thursday suggests the illness may have existed thousands of years earlier.

A DNA analysis of human teeth from Europe and Asia showed the bacterium that causes the plague was detectable some 3,000 years earlier than previously documented.

The study in the journal Cell suggests that this bacterium, known as Yersinia pestis, was common, though it may have caused a slightly different but still devastating kind of illness.

"We found that the Y. pestis lineage originated and was widespread much earlier than previously thought, and we narrowed the time window as to when it developed," said senior study author Eske Willerslev of the Center for GeoGenetics at the University of Copenhagen.

"This study changes our view of when and how plague influenced human populations and opens new avenues for studying the evolution of diseases."

The bacterium is blamed for three massive outbreaks, beginning with the Justinian Plague which started in 541 AD and killed more than 25 million people over the next two centuries.

After that came the Black Death which began in China in 1334 and spread along the great trade routes to Europe where it killed around half the population.

The Third Pandemic, also known as the Modern Plague, emerged in China in the 1850s and killed some 10 million people.

Until now, scientists did not have direct molecular evidence for this bacterium from skeletal material older than 1,500 years.

The new evidence suggests that the plague "may have been responsible for major population declines believed to have occurred in the late 4th and early 3rd millennium BC," said a statement from the University of Cambridge.

The earliest form of the plague was not carried by fleas, an adaptation that the bacterium is believed to have gained in the first millennium BC.

Therefore, the prehistoric plague would not have caused bubonic plague, which led to swollen lymph nodes that formed lumps on the body.

Rather, an early version of the plague likely affected the lungs, causing "desperate, hacking coughing fits just before death," and was transmitted simply by breathing near infected people, according to the University of Cambridge.

The findings also suggest that illnesses caused by the plague may have shaped large-scale human migrations.

"Our study changes the historical understanding of this extremely important human pathogen and makes it possible that other so-called plagues, such as the Plague of Athens and the Antonine Plague, could have been caused by Y. pestis," said co-author Simon Rasmussen of the Technical University of Denmark.

Researchers said their scientific approach could also be used to shed light on other diseases through history, even using ancient material that shows no obvious sign of disease.afp