London, Oct 23 (IANS): Plague has been endemic in the human population since the beginning of the Bronze Age, more than 5,000 years ago, new research has found.

The findings based on analysis of ancient DNA suggest that plague was endemic in the human populations of Eurasia at least 3,000 years before the first plague pandemic in historical records -- the Plague of Justinian in 541 AD.

The researchers also found that the ancestral plague would have been predominantly spread by human-to-human contact -- until genetic mutations allowed Yersinia pestis (Y. pestis), the bacteria that causes plague, to survive in the gut of fleas.

These mutations, which may have occurred near the turn of the first millennium BC, gave rise to the bubonic form of plague that spreads at terrifying speed through flea -- and consequently rat - carriers, the study said.

The bubonic plague caused the pandemics that decimated global populations, including the Black Death, which wiped out half the population of Europe in the 14th century.

"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 and how it developed," said senior study author Eske Willerslev, professor at the University of Cambridge.

Researchers analysed ancient genomes extracted from the teeth of 101 adults dating from the Bronze Age and found across the Eurasian landmass from Siberia to Poland.

They found Y. pestis bacteria in the DNA of seven of the adults, the oldest of whom died 5,783 years ago -- the earliest evidence of plague.

Researchers concluded these early strains of plague could not have been carried by fleas.

"The Bronze Age is the edge of history, and ancient DNA is making what happened at this critical time more visible," study co-author Robert Foley from Cambridge University noted.

The plague that stalked populations for much of the Bronze Age must have been pneumonic, which directly affects the respiratory system and causes desperate, hacking coughing fits just before death, the study noted.

Breathing around infected people leads to inhalation of the bacteria, the crux of its human-to-human transmission, the researchers said.

The findings were detailed in the journal Cell.