Researchers may have discovered the true origin of human hepatitis B virus (HBV) in the genes of a freshwater fish, according to new findings published in the Journal of Virology and reported by the Washington Post.

The scientific discovery, credited to a team of doctoral students at West Virginia University, challenges theories made two years ago that the virus may have first originated in ancient birds.

According to the study, which was recently announced by the U.S. Geological Survey, researchers discovered the hep B-like virus after running genetic sequencing tests on white sucker fish from the Great Lakes region. The ancient virus is apparently more than 82 million years old.

It is still unclear how the hep B-like virus is transmitted between fish, or how it may have passed to humans over time. However, they say the finding helps bring the community one step closer to determining the immune implications of the disease.

Today, nearly 350 million people around the world have chronic HBV, and nearly half a million people die from the virus every year.