Scientists have discovered the earliest known evidence of plague, showing that deadly outbreaks occurred around 5,500 years ago -  about two centuries earlier than previously believed.

The disease has affected humans for thousands of years and killed millions, including a large portion of Europe’s population during the 14th-century Black Death. Although rare today, plague still exists and can be treated with antibiotics.

Deep sleep found key to muscle growth, fat burning and brain function

Researchers led by evolutionary geneticist Eske Willerslev of the University of Copenhagen examined remains from four cemeteries near Siberia’s Lake Baikal and found traces of plague-causing bacteria in the teeth of 18 ancient hunter-gatherers.

Carbon dating of the remains showed that the disease caused at least two outbreaks, with the earliest cases appearing about 5,500 years ago. Scientists said the prehistoric plague evolved gradually and likely spread from marmots through consumption of raw organs or contact with infected hides. It could also pass between humans through coughing and sneezing.

Many victims were children aged between 8 and 11. Researchers found several related individuals buried together, including three girls and an aunt and nephew.

The study, published in the journal Nature, suggests ancient plague affected both small nomadic communities and larger populations. Scientists said understanding its evolution may help explain how dangerous pathogens emerge in the future.