Research on DNA from 400 to 10,500 years old shows that the parent strain of the hepatitis B virus emerged at least 12,000 years ago in Eurasia.
- By studying the genomes of the hepatitis B virus from 137 Eurasians and Native Americans between 10,500 and 400 years old, the researchers found that they all came from the same parent strain.
- This parent strain appeared at least 12,000 years ago in Eurasia, before the appearance of agriculture.
Each year around the world, 350 million people contract the hepatitis B virus (HBV). Nearly a million of them do not survive this acute viral infection of the liver, which can become chronic and increase the risk of cirrhosis or hepatocellular carcinoma.
Transmitted during contact with contaminated bodily fluids (sexual fluids, blood, mother-child transmission) or by direct or indirect contact with an infected person, the hepatitis B virus is actually as old as humanity. Recent studies have shown that HBV has been infecting humans for millennia. However, past mutations and its dispersal pathways were until now largely unknown.
In the review Science, researchers from the Max-Planck Institute (Germany) shed light on the evolutionary history of HBV. For this, they analyzed the genomes of the virus from the human remains of 137 Eurasians and Native Americans between 10,500 and 400 years old. They then discovered that all these strains come from a single parent strain, which appeared at the end of the Pleistocene, at least 12,000 years ago. These findings highlight pathways of dissemination and changes in viral diversity that reflect well-known human migrations and demographic events, as well as unexpected patterns and connections to the present.
A parent strain that infected early Native Americans
Current strains of the hepatitis B virus are classified into nine genotypes, two of which are found primarily in populations of Native American ancestry. According to this new work, these two strains are descended from an HBV lineage that diverged towards the end of the Pleistocene and was carried by some of the earliest inhabitants of the Americas. “Our data suggest that all known HBV genotypes are descended from a strain that infected the ancestors of early Americans and their closest Eurasian relatives around the time these populations diverged”explains Denise Kühnert, co-supervisor of the study.
The study also shows that the virus was present in large parts of Europe 10,000 years ago, before the spread of agriculture on the continent. “Many human pathogens are thought to have arisen after the introduction of agriculture, but it is clear that HBV was already affecting prehistoric hunter-gatherer populations”says Johannes Krause, director of the Department of Archaeogenetics at the Max Planck Institute for Evolutionary Anthropology and co-supervisor of the study.
These early strains of HBV carried by hunter-gatherers were later superseded by newer strains that were likely spread by early farmers on the continent. These new viral lineages continued to prevail throughout Western Eurasia for nearly 4,000 years. The predominance of these strains lasted until about 5,000 years ago.
strains inherited from prehistory
The study also highlights the sudden decline in HBV diversity in western Eurasia during the second half of the 2nd millennium BCE, a time of major cultural change, including the collapse of large state societies of the Bronze Age in the Eastern Mediterranean region. “We believe this reflects the epidemiological changes seen around 1,200 BCE, when most of the major Bronze Age societies in the eastern Mediterranean collapsed.”says Denise Kühnert.
All old HBV strains found in Western Eurasia after this time belonged to new viral lineages that are still prevalent in the region today. However, it seems that a variant linked to the earlier prehistoric diversity of the region has persisted until today. This prehistoric variant evolved into a rare genotype that appears to have emerged recently during the HIV pandemic, for reasons that remain to be understood.
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