Traces of hepatitis B extracted from a 4,500-year-old skeleton provide the oldest evidence of a human virus ever found.
The extinct strain shows that the cause of potentially lethal liver disease that affects millions of people worldwide today was active in the Bronze Age.
British scientists who made the “truly remarkable” discovery compared it to finding the first fossils.
Previously the oldest detected human viruses dated back around 450 years and most have been no more than 50 years old.
Evidence of HBV infection was found in 25 of around 300 individuals dating from the Bronze Age to medieval times.
Of these, 12 skeletons yielded enough of the viral genetic code to allow detailed analysis.
One of them, a person from the Bronze Age Beaker culture from Osterhofen, Germany, was almost 4,488 years old.
Dr Terry Jones, from Cambridge University, joint first author of the study, said: “Scientists mostly study modern virus strains and we have mainly been in the dark regarding ancient sequences – until now. It was like trying to study evolution without fossils.
“If we only studied the animals living today it would give us a very inaccurate picture of their evolution. It is the same with viruses.”
Today hepatitis B affects millions of people worldwide. In 2015 an estimated 257 million people were chronically infected with HBV and 887,000 people died from complications associated with the virus, such as liver cancer.
The scientists analysed DNA sequences from 304 people who lived in central and western Eurasia between 200 and 7,000 years ago.
Hepatitis B was detected in 25 skeletons spanning a period of almost 4,000 years.
The findings appear in the latest issue of the journal Nature.
Co-lead author Barbara Muhlemann, a Cambridge PhD student, said: “People have tried to unravel the history of HBV for decades. This study transforms our understanding of the virus and proves it affected people as far back as the Bronze Age.
“We have also shown that it is possible to recover viral sequences from samples of this age which will have much wider scientific implications.”
Another important discovery was proof that viruses can become extinct. The particular HBV strain that was around in the Bronze Age no longer exists.
Little is known about the evolution of hepatitis B, which is thought to have infected the ancestors of modern birds during the dinosaur era.